**2. Device Structure and Simulation**

contact [6]. The base composition can be graded to establish an electric field which enhances electron transport [7,8].It was demonstrated that symmetric-area heterojunction phototran‐ sistors have a larger bandwidth than asymmetric area HPTs [9]. It should be noted that while Milano *et al* predicted a rather pessimistic bandwidth, improvements in material growth, device design and fabrication techniques have improved the maximum bandwidth

HPT responsivity typically increases with increasing optical power. This has been attributed to recombination at the base-emitter heterojunction.It is desirable to have gain independent from the optical power, or have larger gain at lower optical power levels. Leu *et al* have demonstrated an approach to improve the gain dependence on optical power, by adjusting the doping profile of the emitter and base layers of InP emitter/InGaAs base HPTs [12]. By using a high-low emitter doping, that is reducing the emitter doping in a thin layer at the emitter-base junction, they eliminated the quantum well trapping the electrons at this inter‐ face. Thus, the recombination currents were reduced, and the ideality factor of the transistor

HPTs have been demonstrated for optoelectronic mixing applications, where the LO signal

The modulated barrier diode, also known as the Camel diode, is a non-Schottky majority carrier diode in which the carrier transport is controlled by a potential barrier in the bulk of the semiconductor. The application of MBDs as photodetectors was first demonstrated by A.Y. Cho and co-workers [15,16], who also showed its application in a picosecond sampling system [17]. The gain of the MBD is due to the hole trapping at the heterostructure interface. As holes accumulate in this quantum well, the barrier height will be lowered, resulting in an increased electron current, thus providing gain. As a majority carrier device, the MBD has fast intrinsic response [15,17]. In contrast with the HPT, the MBD device has higher respon‐ sivity at lower optical power levels [15,16]. The MBD has been used in a front-end photore‐ ceiver, integrated with an FET [18], and a monolithically integrated phototransceiver in which it was integrated with an LED [19]. In the first case, the MBD and FET shared a com‐ mon structure, and circuit utilized the MBD's gain and response speed. In the second case, the MBD's increasing gain with lower optical power was utilized to improve optical trans‐

Symmetric Gain OptoElectronic Mixers (SG-OEMs) for chirped-AM LADAR operating in the "eye-safe" 1.55 μm wavelength have been investigated by our research group at the Uni‐ versity of Maine. These devices are based on symmetric heterojunction phototransistors.

The first generation SG-OEMs used indium aluminum arsenide (In0.52Al0.48As)/ indium galli‐ um arsenide (In0.53Ga0.47As) heterostructures grown on InP substrates [20,21].The device structures were designed and simulated using the TCAD-Sentaurus tools from Synopys.

These simulations prediced mixing responsivities up to 100 A/W for these devices.

improved, leading to a flattening of the gain vs. incident power characteristics.

of HPTs to the tens of GHz range [10,11].

94 Optoelectronics - Advanced Materials and Devices

was provided electrically [10,13] or optically [14].

ceiver performance.

**1.3. Symmetric gain optoelectronic mixers**

A schematic of the InP based symmetric gain optoelectronic mixer is shown in Figure 2. The targeted operating wavelength is 1.55 μm, therefore the base is In0.53Ga0.47As, which has a bandgap of approximately 0.74eV at 300K and is lattice matched to the InP substrate. The base is doped with acceptor atoms to obtain a p-type region. The n-type emitter/collector layers in the structure are made of InP. Highly doped n-type InP/In0.53Ga0.47As layers are used for ohmic contact formation with the metal electrodes. The schematic in Figure 2 also shows highly doped interface layers at the emitter-base and collector-base interfaces. The device, as shown, is configured for top illumination.

**Figure 2.** Schematic of an InP/ InGaAs symmetric gain optoelectronic mixer, top illumination configuration.

The design parameters investigated in this work are the base and emitter/collector layer thicknesses and doping levels, as identified in Table 1. The base width wB is the primary pa‐ rameter that will determine the responsivity of the optoelectronic mixer. Increasing the base thickness will extend the carrier path and decrease transistor gain. This will lead to a de‐ crease in the dark and optical currents. However, a trade off has to be made between light absorption, which is directly proportional to base thickness, and the recombination of light generated carriers in the base, which is inversely proportional to base thickness. The respon‐ sivity, R, is proportional to:

$$R \propto \frac{\left(\_{1} \cdot e^{\cdot a w\_{g}}\right)}{d^{\cdot 2}} \tag{2}$$

**Parameter Size [μm]**

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Inner mesa width 16 Outer mesa width 30 Top contact window width 12 Top contact metal width 14 Bottom contact window width 2 Bottom contact metal width 4

The switch to InP layers was proposed due to the film stoichiometry and resulting lattice mismatch issues experienced with InAlAs films [20]. The first task in this project was to de‐ termine how the switch to InP would impact predicted device performance. Figure 3 com‐ pares the simulated I-V characteristics for two structures based on Figure 2. The layer thicknesses and doping densities are given in Table 3. InP\_A is the structure shown in the figure, while in InAlAs\_A all of the InP film layers are replaced by InAlAs, as reported in [20,21]. Both the dark current (i.e., no incident light) and the current with an incident optical

ner mesa and has the same width, 16μm. A transparent electrode was assumed. The incident optical power on the detector is 160 pW/μm. The figure illustrates the behavior of a device

The simulation predicts that the In0.52Al0.48As/ In0.53Ga0.47As based device will have larger dark and optical currents than the InP/ In0.53Ga0.47As one over the bias range. The optical cur‐ rent of the In0.52Al0.48As/ In0.53Ga0.47As based structure is 2.19 nA/μm at 2 V, compared to 1.64

Parameter Value wB 800 nm NA 2.5x1016 cm-3 wE/C 390 nm ND 1x1016 cm-3 wi 10 nm NDi 5x10-18 cm-3

**Table 3.** Layer thickness and doping values for the simulations presented in Figure 3.

are displayed. The light is set to be incident on the device's in‐

**3. DC Simulations: Dark Current and Responsivity**

**3.1. Comparison of InAlAs/InGaAs and InP/InGaAs SG-OEMs**

**Table 2.** SG-OEM horizontal dimensions

power density of 1 mW/cm2

for a bias voltage sweep from 0 V to 5V.

where wB is the thickness of the base region and α is the absorption coefficient. The base thickness and doping will also impact the base narrowing due to the growth of the reverse biased collector-base junction depletion region with increasing reverse bias, known as the Early effect. When the device is sufficiently reverse biased, the collector-base depletion re‐ gion will reach the base-emitter depletion region, shorting the device. This is known as punch-through breakdown, and should be avoided.


**Table 1.** Design parameters investigated for the symmetric gain optoelectronic mixer

Emitter/collector doping impact device performance in several ways. If they are highly dop‐ ed, most of the depletion region will be in the base, significantly reducing the effective base thickness. This will provide higher transistor gain, but will also result in punch through breakdown of the device at low voltages. If these layers are lightly doped, then the series resistance will increase, reducing the available current from the device. The effect of the in‐ terface layers on device performance are also investigated in this work.

The work reported here covers device design, simulation and optimization using the 2D/3D TCAD-Sentaurus device simulator package from Synopsys, and device modeling. Parame‐ ters investigated for device optimization include the highly doped emitter-base interface layers, the base thickness and the doping of each layer. The horizontal dimensions of the standard device are summarized in Table 2. The simulation results are discussed in section 3 and the device model is presented in section 4.


**Table 2.** SG-OEM horizontal dimensions

rameter that will determine the responsivity of the optoelectronic mixer. Increasing the base thickness will extend the carrier path and decrease transistor gain. This will lead to a de‐ crease in the dark and optical currents. However, a trade off has to be made between light absorption, which is directly proportional to base thickness, and the recombination of light generated carriers in the base, which is inversely proportional to base thickness. The respon‐


where wB is the thickness of the base region and α is the absorption coefficient. The base thickness and doping will also impact the base narrowing due to the growth of the reverse biased collector-base junction depletion region with increasing reverse bias, known as the Early effect. When the device is sufficiently reverse biased, the collector-base depletion re‐ gion will reach the base-emitter depletion region, shorting the device. This is known as

*<sup>d</sup>* <sup>2</sup> (2)

*<sup>R</sup>* <sup>∝</sup> (1 - *<sup>e</sup>*

sivity, R, is proportional to:

96 Optoelectronics - Advanced Materials and Devices

punch-through breakdown, and should be avoided.

wB In0.53Ga0.47As base thickness

wE/C InP emitter/collector thickness

and the device model is presented in section 4.

NA In0.53Ga0.47As base acceptor doping density, p-type

ND InP emitter/collector donor doping density, n-type wi InP emitter/collector-base interface layer thickness

**Table 1.** Design parameters investigated for the symmetric gain optoelectronic mixer

terface layers on device performance are also investigated in this work.

NDi InP emitter/collector-base interface layer donor doping density, n-type

Emitter/collector doping impact device performance in several ways. If they are highly dop‐ ed, most of the depletion region will be in the base, significantly reducing the effective base thickness. This will provide higher transistor gain, but will also result in punch through breakdown of the device at low voltages. If these layers are lightly doped, then the series resistance will increase, reducing the available current from the device. The effect of the in‐

The work reported here covers device design, simulation and optimization using the 2D/3D TCAD-Sentaurus device simulator package from Synopsys, and device modeling. Parame‐ ters investigated for device optimization include the highly doped emitter-base interface layers, the base thickness and the doping of each layer. The horizontal dimensions of the standard device are summarized in Table 2. The simulation results are discussed in section 3

**Symbol Parameter**

### **3. DC Simulations: Dark Current and Responsivity**

#### **3.1. Comparison of InAlAs/InGaAs and InP/InGaAs SG-OEMs**

The switch to InP layers was proposed due to the film stoichiometry and resulting lattice mismatch issues experienced with InAlAs films [20]. The first task in this project was to de‐ termine how the switch to InP would impact predicted device performance. Figure 3 com‐ pares the simulated I-V characteristics for two structures based on Figure 2. The layer thicknesses and doping densities are given in Table 3. InP\_A is the structure shown in the figure, while in InAlAs\_A all of the InP film layers are replaced by InAlAs, as reported in [20,21]. Both the dark current (i.e., no incident light) and the current with an incident optical power density of 1 mW/cm2 are displayed. The light is set to be incident on the device's in‐ ner mesa and has the same width, 16μm. A transparent electrode was assumed. The incident optical power on the detector is 160 pW/μm. The figure illustrates the behavior of a device for a bias voltage sweep from 0 V to 5V.


**Table 3.** Layer thickness and doping values for the simulations presented in Figure 3.

The simulation predicts that the In0.52Al0.48As/ In0.53Ga0.47As based device will have larger dark and optical currents than the InP/ In0.53Ga0.47As one over the bias range. The optical cur‐ rent of the In0.52Al0.48As/ In0.53Ga0.47As based structure is 2.19 nA/μm at 2 V, compared to 1.64 nA/μm at 2 V for the InP/ In0.53Ga0.47As based structure. The dark current is also larger for the InAlAs based device. This latter result initially seems counter-intuitive, as In0.52Al0.48As has a larger bandgap than InP, as indicated in Table 4. Table 4 lists the material parameters for the three semiconductor materials, as calculated by TCAD Sentaurus for these composi‐ tions at 300K. This behavior can be attributed to two separate mechanisms. First, InP and InAlAs have different conduction band offsets with InGaAs. Second, the Early effect, i.e. base narrowing, is more prominent in the InAlAs based devices.

isotype heterojunction compared to that in the InAlAs/InGaAs case, as predicted by TCAD Sentaurus simulations. The electron concentrations at this interface for both structures is shown in Figure 4. The InP based device is on the left, and the InAlAs based device is on the right. The top layer (above the line at 2.3 μm) is the InGaAs contact layer, and below it is the wider bandgap layer. In the InP/InGaAs structure, the 2DEG induces a depletion layer on either side of it (denoted by the white lines), about 7.5 nm in total, larger than that in the

Base width narrowing also contributes to the larger InAlAs/InGaAs SG-OEM current. The

where xB is the effective base width, xdf is the depletion region width of the forward biased heterojunction and xdr is the depletion region width of the reverse biased heterojunction. The change of the forward biased junction width due to the bias voltage is relatively small compared to the reverse biased junction, and can be considered to be its 0V bias value. From TCAD Sentaurus simulations, the effective base width at 1V for the InP based structure is predicted to be 719.17 nm, and 710.19 nm for the InAlAs based structure. Considering the magnitude of this difference, it can be concluded that the dominant reason for the smaller

**Figure 4.** Comparison of the electron concentration in the InP/InGaAs contact layer n++/N++isotype heterojunction (left) with that in the theInAlAs/InGaAs contact layer n++/N++isotype heterojunction (right). The black line at 2.3 μm designates the metalurgical boundary between the n++InGaAs layer (top in the figure) and the wider bandgap N++

dark current in the InP based devices is the conduction band edge discontinuity.

B B df dr x w – x x = - (3)

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InAlAs based device, which is about 2 nm.

effective base width is defined as:

layer.

**Figure 3.** Dark and optical currents versus bias voltage for InP/ In0.53Ga0.47As and In0.52Al0.48As/ In0.53Ga0.47As based sym‐ metric gain optoelectronic mixers with the same layer thickness and doping.


**Table 4.** Material parameters used by TCAD Sentaurus in the device simulations

The different conduction band offsets results in a significantly larger two-dimensional elec‐ tron gas (2DEG) concentration at the InGaAs side of the InP/InGaAs contact layer n++-N++ isotype heterojunction compared to that in the InAlAs/InGaAs case, as predicted by TCAD Sentaurus simulations. The electron concentrations at this interface for both structures is shown in Figure 4. The InP based device is on the left, and the InAlAs based device is on the right. The top layer (above the line at 2.3 μm) is the InGaAs contact layer, and below it is the wider bandgap layer. In the InP/InGaAs structure, the 2DEG induces a depletion layer on either side of it (denoted by the white lines), about 7.5 nm in total, larger than that in the InAlAs based device, which is about 2 nm.

nA/μm at 2 V for the InP/ In0.53Ga0.47As based structure. The dark current is also larger for the InAlAs based device. This latter result initially seems counter-intuitive, as In0.52Al0.48As has a larger bandgap than InP, as indicated in Table 4. Table 4 lists the material parameters for the three semiconductor materials, as calculated by TCAD Sentaurus for these composi‐ tions at 300K. This behavior can be attributed to two separate mechanisms. First, InP and InAlAs have different conduction band offsets with InGaAs. Second, the Early effect, i.e.

**Figure 3.** Dark and optical currents versus bias voltage for InP/ In0.53Ga0.47As and In0.52Al0.48As/ In0.53Ga0.47As based sym‐

The different conduction band offsets results in a significantly larger two-dimensional elec‐ tron gas (2DEG) concentration at the InGaAs side of the InP/InGaAs contact layer n++-N++

**Eg [eV]** 0.718721 1.33587 1.48159 **χ0 [eV]** 4.5472 4.4 4.2711 **ε<sup>r</sup>** 13.9061 12.4 12.3948 **Nc [cm-3]** 2.5396x1017 5.66x1017 5.7814x1017 **Nv [cm-3]** 7.5107x1018 2.03x1019 9.4152x1018

**In0.53Ga0.47As InP In0.52Al0.48As**

base narrowing, is more prominent in the InAlAs based devices.

98 Optoelectronics - Advanced Materials and Devices

metric gain optoelectronic mixers with the same layer thickness and doping.

**Table 4.** Material parameters used by TCAD Sentaurus in the device simulations

Base width narrowing also contributes to the larger InAlAs/InGaAs SG-OEM current. The effective base width is defined as:

$$\mathbf{x\_B = \ \mathbf{w\_B - \ x\_{df} - \ x\_{dr}}} \tag{3}$$

where xB is the effective base width, xdf is the depletion region width of the forward biased heterojunction and xdr is the depletion region width of the reverse biased heterojunction. The change of the forward biased junction width due to the bias voltage is relatively small compared to the reverse biased junction, and can be considered to be its 0V bias value. From TCAD Sentaurus simulations, the effective base width at 1V for the InP based structure is predicted to be 719.17 nm, and 710.19 nm for the InAlAs based structure. Considering the magnitude of this difference, it can be concluded that the dominant reason for the smaller dark current in the InP based devices is the conduction band edge discontinuity.

**Figure 4.** Comparison of the electron concentration in the InP/InGaAs contact layer n++/N++isotype heterojunction (left) with that in the theInAlAs/InGaAs contact layer n++/N++isotype heterojunction (right). The black line at 2.3 μm designates the metalurgical boundary between the n++InGaAs layer (top in the figure) and the wider bandgap N++ layer.

A photodetector's noise current is proportional to its dark current. Therefore, the InP based SG-OEM should have better noise performance. The I-V curves in Figure 3 also show that the InP/ In0.53Ga0.47As based structure is less susceptible to the Early effect and punchthrough breakdown. This is illustrated by the fact that the InP/ In0.53Ga0.47As based structure has a flatter current curve and does not have the sudden current increase of the In0.52Al0.48As/ In0.53Ga0.47As based structure at 4.5 V, which is due to the device approaching punchthrough breakdown as the base width decreases with the Early effect.

ter/collector layers of structure InP\_B are doped slightly lower than the base layer, therefore most of the depletion region extends into these layers instead of the base. Thus, the InP\_B device has a larger effective base width, which increases the optical current by allowing more electron-hole pairs to be generated, and decreases the dark current by inducing more

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**Figure 6.** Dark and optical currents versus bias voltage for two InP/ In0.53Ga0.47As based symmetric gain optoelectronic mixers with the same layer thickness and doping. Structure InP\_A (with the interface layer) and structure InP\_B (with‐

Figure 7 shows the responsivity versus the bias voltage for structure InP\_A and structure InP\_B. Structure InP\_B is predicted to have larger responsivity than structure InP\_A throughout the bias range. This agrees reasonably well with the dark and optical currents plotted in Figure 6, as the responsivity is directly proportional to the difference of optical and dark currents. Structure B has a responsivity of 12.95 A/W at 2 V. This value is about 1.5 times of the one of structure A, which is 8.194 A/W at 2 V. The currents and responsivity plots displayed above illustrate the fact that structure B (without the interface layers) is a

SG-OEM structures with base widths ranging from 500 nm to 1 μm were simulated with and without the highly doped interface layers. Structure A devices, with the interface layers, are more susceptible to punch-through breakdown, as can be seen from their dark current characteristics shown in Figure 8. In contrast, the structure B devices were better behaved, as shown in Figure 9. The highly doped emitter/base interface layer in the Structure A devices forces the depletion region to extend mostly into the base layer, resulting in an early punch-

better candidate for the symmetric gain optoelectronic mixer design.

recombination at the base.

out the inter face layer).

through breakdown.

#### **3.2. Base – Emitter/Collector Interface Layers**

Our prior work on InAlAs/InGaAs SG-OEMs predicted that using a highly doped interface layer in InAlAs based devices would improve their performance [20,21]. This phenomenon was investigated for InP based devices as well. Figure 5 shows two nearly identical device structures, where the only difference is the presence or absence of the said highly doped in‐ terface layers. The structure InP\_A has the interface layers while structure InP\_B does not.Figure 6 shows the predicted performance of the two structures.

**Figure 5.** Schematic of the InP / In0.53Ga0.47As heterostructure based symmetric gain optoelectronic mixers for investi‐ gating the effect of base-emitter interface layers. Structure InP\_A has the interface layers while structure InP\_B does not.

Structure InP\_B, without the interface layer, is predicted to have a larger optical current than structure InP\_A at low bias voltages. Figure 6 also shows that structure B is less suscep‐ tible to the Early effect, and has lower dark current. The larger optical current and the lower dark current of structure InP\_B is due to structure InP\_B having a larger effective base thick‐ ness than structure InP\_A. In structure InP\_A, the highly doped (10 nm, 1018 cm-3) emitter interface layers force practically all of the depletion region to extend into the base. The emit‐ ter/collector layers of structure InP\_B are doped slightly lower than the base layer, therefore most of the depletion region extends into these layers instead of the base. Thus, the InP\_B device has a larger effective base width, which increases the optical current by allowing more electron-hole pairs to be generated, and decreases the dark current by inducing more recombination at the base.

A photodetector's noise current is proportional to its dark current. Therefore, the InP based SG-OEM should have better noise performance. The I-V curves in Figure 3 also show that the InP/ In0.53Ga0.47As based structure is less susceptible to the Early effect and punchthrough breakdown. This is illustrated by the fact that the InP/ In0.53Ga0.47As based structure has a flatter current curve and does not have the sudden current increase of the In0.52Al0.48As/ In0.53Ga0.47As based structure at 4.5 V, which is due to the device approaching punch-

Our prior work on InAlAs/InGaAs SG-OEMs predicted that using a highly doped interface layer in InAlAs based devices would improve their performance [20,21]. This phenomenon was investigated for InP based devices as well. Figure 5 shows two nearly identical device structures, where the only difference is the presence or absence of the said highly doped in‐ terface layers. The structure InP\_A has the interface layers while structure InP\_B does

**Figure 5.** Schematic of the InP / In0.53Ga0.47As heterostructure based symmetric gain optoelectronic mixers for investi‐ gating the effect of base-emitter interface layers. Structure InP\_A has the interface layers while structure InP\_B does

Structure InP\_B, without the interface layer, is predicted to have a larger optical current than structure InP\_A at low bias voltages. Figure 6 also shows that structure B is less suscep‐ tible to the Early effect, and has lower dark current. The larger optical current and the lower dark current of structure InP\_B is due to structure InP\_B having a larger effective base thick‐ ness than structure InP\_A. In structure InP\_A, the highly doped (10 nm, 1018 cm-3) emitter interface layers force practically all of the depletion region to extend into the base. The emit‐

through breakdown as the base width decreases with the Early effect.

not.Figure 6 shows the predicted performance of the two structures.

**3.2. Base – Emitter/Collector Interface Layers**

100 Optoelectronics - Advanced Materials and Devices

not.

**Figure 6.** Dark and optical currents versus bias voltage for two InP/ In0.53Ga0.47As based symmetric gain optoelectronic mixers with the same layer thickness and doping. Structure InP\_A (with the interface layer) and structure InP\_B (with‐ out the inter face layer).

Figure 7 shows the responsivity versus the bias voltage for structure InP\_A and structure InP\_B. Structure InP\_B is predicted to have larger responsivity than structure InP\_A throughout the bias range. This agrees reasonably well with the dark and optical currents plotted in Figure 6, as the responsivity is directly proportional to the difference of optical and dark currents. Structure B has a responsivity of 12.95 A/W at 2 V. This value is about 1.5 times of the one of structure A, which is 8.194 A/W at 2 V. The currents and responsivity plots displayed above illustrate the fact that structure B (without the interface layers) is a better candidate for the symmetric gain optoelectronic mixer design.

SG-OEM structures with base widths ranging from 500 nm to 1 μm were simulated with and without the highly doped interface layers. Structure A devices, with the interface layers, are more susceptible to punch-through breakdown, as can be seen from their dark current characteristics shown in Figure 8. In contrast, the structure B devices were better behaved, as shown in Figure 9. The highly doped emitter/base interface layer in the Structure A devices forces the depletion region to extend mostly into the base layer, resulting in an early punchthrough breakdown.

**Figure 7.** Responsivity versus bias voltage for two InP/ In0.53Ga0.47As based symmetric phototransistors with the same layer thickness and doping. Structure InP\_A (with the interface layer) and structure InP\_B (without the interface layer).

**Figure 9.** Dark current of structure InP\_B as a function of the base thickness. The base thickness ranges from 600 nm

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**Figure 10.** Responsivity of structure InP\_A as a function of the base thickness. The base thickness ranges from 500 nm

to 900 nm.

to 1000 nm.

**Figure 8.** Dark current of structure InP\_A as a function of base thickness. The base thickness ranges from 500 nm to 1000 nm.

**Figure 9.** Dark current of structure InP\_B as a function of the base thickness. The base thickness ranges from 600 nm to 900 nm.

**Figure 7.** Responsivity versus bias voltage for two InP/ In0.53Ga0.47As based symmetric phototransistors with the same layer thickness and doping. Structure InP\_A (with the interface layer) and structure InP\_B (without the interface

**Figure 8.** Dark current of structure InP\_A as a function of base thickness. The base thickness ranges from 500 nm to

layer).

102 Optoelectronics - Advanced Materials and Devices

1000 nm.

**Figure 10.** Responsivity of structure InP\_A as a function of the base thickness. The base thickness ranges from 500 nm to 1000 nm.

The responsivities of the devices were extracted using simulations with an incident light power of 1 mW/cm2 , corresponding to an incident optical power of 1.6 nW/μm. Figure 10 shows the DC responsivity of Structure InP\_A devices with bases thickness ranging from 500 nm to 1000 nm, with steps of 100 nm. Devices with base thickness below 800 nm show punch-through breakdown effects, where the responsivity increases rapidly as the base nar‐ rows, then falls down rapidly when the device punches through.

doping profile 3 shows no improvement over the InGaAs MSMs [4,5], having an average re‐ sponsivity of 0.36 A/W over the bias range. The device with doping profile 2 presents a good compromise for the end application, with responsivities above 10 A/W for most of the bias range. For example, the predicted responsivity at 2V is 12.95 A/W. This represents a factor of

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**Figure 12.** Responsivity of structure InP\_B as a function of the doping profiles given in table 5. Base thickness is 800

The equivalent circuit model of the SG-OEM is based on the equivalent circuit model of a heterojunction phototransistor. The equivalent circuit model is shown in Figure 13. This model is based on the conventional hybrid-π model.The resistance rTrepresents the equiva‐ lent series resistance of the top metal-semiconductor contact, the contact layers and the top emitter/collector layer.The resistance rBrepresents the equivalent series resistance of the bot‐ tom metal-semiconductor contact, the contact layers and the bottom emitter/collector layer. Cμ and Cπrepresent the junction diffusion capacitances of the base – emitter and base – col‐ lector junctions, respectively.rμ and rπ are the diffusion resistances of these two junctions. The resistance ro represents the Early effect. The current source Idark represents the dark cur‐ rent of the optoelectronic mixer. Iopt represents the photocurrent due to absorption in the base, which is amplified by transistor action. Photon absorption in the InGaAs contact layers is ignored in this analysis as it is substantially smaller than in the base layer. This model can be used for both DC analysis and AC small signal analysis of the device performance. The

38 improvement over the InGaAs MSMs.

nm.

**4. Device Model**

Figure 11 shows the DC responsivity of four InP/ In0.53Ga0.47As SG-OEMs based on structure InP\_B, with base thickness from 600 nm to 900 nm. Similar to structure InP\_A devices, the re‐ sponsivity decreases with increasing base thickness. However, the punch-through behaviour does not occur under 5 V, which agrees with the dark current curves presented in Figure 9.

**Figure 11.** Responsivity of structure InP\_B as a function of the base thickness. The base thickness ranges from 600 nm to 900 nm.

The doping dependence of the responsivity was investigated using a matrix of emitter/collec‐ tor and base layer doping densities. The two extremes and the best case scenario are summar‐ ized below, in table 5. Doping profile 1 results in rapid punch-through of the SG-OEM. While a traditional homojunction bipolar junction transistor (BJT) has an emitter layer that is heavily doped compared to the base, the wider bandgap of the InP layer compared to InGaAs results in increased injection efficiency. Therefore, the collector / emitter layer doping levels can be re‐ duced in comparison to the base, making doping profiles 2 and 3 practical.

Figure 12 shows responsivity as a function of doping profile for structure B devices,. These devices were simulated for a base width of 800 nm. The device with doping profile 1 exhib‐ its punch-through effects rapidly, reaching its peak responsivity of 81.25 A/W at 3V. The rapid decline in responsivity past 3V is due to punch-through breakdown. The device with doping profile 3 shows no improvement over the InGaAs MSMs [4,5], having an average re‐ sponsivity of 0.36 A/W over the bias range. The device with doping profile 2 presents a good compromise for the end application, with responsivities above 10 A/W for most of the bias range. For example, the predicted responsivity at 2V is 12.95 A/W. This represents a factor of 38 improvement over the InGaAs MSMs.

**Figure 12.** Responsivity of structure InP\_B as a function of the doping profiles given in table 5. Base thickness is 800 nm.

### **4. Device Model**

The responsivities of the devices were extracted using simulations with an incident light

shows the DC responsivity of Structure InP\_A devices with bases thickness ranging from 500 nm to 1000 nm, with steps of 100 nm. Devices with base thickness below 800 nm show punch-through breakdown effects, where the responsivity increases rapidly as the base nar‐

Figure 11 shows the DC responsivity of four InP/ In0.53Ga0.47As SG-OEMs based on structure InP\_B, with base thickness from 600 nm to 900 nm. Similar to structure InP\_A devices, the re‐ sponsivity decreases with increasing base thickness. However, the punch-through behaviour does not occur under 5 V, which agrees with the dark current curves presented in Figure 9.

**Figure 11.** Responsivity of structure InP\_B as a function of the base thickness. The base thickness ranges from 600 nm

The doping dependence of the responsivity was investigated using a matrix of emitter/collec‐ tor and base layer doping densities. The two extremes and the best case scenario are summar‐ ized below, in table 5. Doping profile 1 results in rapid punch-through of the SG-OEM. While a traditional homojunction bipolar junction transistor (BJT) has an emitter layer that is heavily doped compared to the base, the wider bandgap of the InP layer compared to InGaAs results in increased injection efficiency. Therefore, the collector / emitter layer doping levels can be re‐

Figure 12 shows responsivity as a function of doping profile for structure B devices,. These devices were simulated for a base width of 800 nm. The device with doping profile 1 exhib‐ its punch-through effects rapidly, reaching its peak responsivity of 81.25 A/W at 3V. The rapid decline in responsivity past 3V is due to punch-through breakdown. The device with

duced in comparison to the base, making doping profiles 2 and 3 practical.

rows, then falls down rapidly when the device punches through.

, corresponding to an incident optical power of 1.6 nW/μm. Figure 10

power of 1 mW/cm2

104 Optoelectronics - Advanced Materials and Devices

to 900 nm.

The equivalent circuit model of the SG-OEM is based on the equivalent circuit model of a heterojunction phototransistor. The equivalent circuit model is shown in Figure 13. This model is based on the conventional hybrid-π model.The resistance rTrepresents the equiva‐ lent series resistance of the top metal-semiconductor contact, the contact layers and the top emitter/collector layer.The resistance rBrepresents the equivalent series resistance of the bot‐ tom metal-semiconductor contact, the contact layers and the bottom emitter/collector layer. Cμ and Cπrepresent the junction diffusion capacitances of the base – emitter and base – col‐ lector junctions, respectively.rμ and rπ are the diffusion resistances of these two junctions. The resistance ro represents the Early effect. The current source Idark represents the dark cur‐ rent of the optoelectronic mixer. Iopt represents the photocurrent due to absorption in the base, which is amplified by transistor action. Photon absorption in the InGaAs contact layers is ignored in this analysis as it is substantially smaller than in the base layer. This model can be used for both DC analysis and AC small signal analysis of the device performance. The circuit parameters were calculated theoretically and extracted from Sentaurus TCAD twodimensional simulations.

The equivalent resistances rT and rB model the metal-semiconductor junction, the degener‐ ately doped InP and InGaAs contact layers, the isotype heterojunction between these contact layers and the quasi-neutral regions (QNRs) of the emitter and collector. Of these compo‐ nents, the quasi-neutral region resistance and the isotype heterojunction dominate rT and rB. The resistance of the contact layers and the quasi-neutral region can be predicted by using

where wlayer is the layer thickness, d the width of the layer, q elemental charge, n the free electron density, and μn the mobility of electrons in the layer. The unit of the contact resist‐

The second contributor to the voltage drop at the contact layers is the highly doped InP/ InGaAs isotype heterojunction interface. The carrier conduction at the highly doped InP/ InGaAs isotype heterojunction interface can be analyzed based on the band diagram shown in Figure 14. The conduction band edge is similar to that of a rectifying metal – semiconduc‐ tor contact. Such a contact can have one of three conduction mechanisms: thermionic emis‐ sion, thermionic-field emission and field emission. It was determined that field emission dominated the current conduction between the InP and InGaAs layers, due to the very high

**Figure 14.** Band diagram of the isotype heterojunction formed by the highly doped InP and InGaAs contact layers.

In order to verify the assumptions made above, the contact regions of the original device were modeled seperately in TCAD-Sentaurus and a set of simulations were carried out. The results were then compared with the theoretical calculations. The structures shown in Figure 15 were simulated to verify the calculations for the top and bottom contact resistances. Fig‐

*qμnnd* <sup>Ω</sup>.cm (4)

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the conductivity of the semiconductor layers, and can be formulated as:

ance as defined by Equation 4 is Ω.cm.

doping densities of both layers.

r = *wlayer*


**Table 5.** The base and emitter/ collector doping profiles for the responsivity doping dependence study

**Figure 13.** (a) Equivalent circuit model of the SG-OEM device structure.

The equivalent resistances rT and rB model the metal-semiconductor junction, the degener‐ ately doped InP and InGaAs contact layers, the isotype heterojunction between these contact layers and the quasi-neutral regions (QNRs) of the emitter and collector. Of these compo‐ nents, the quasi-neutral region resistance and the isotype heterojunction dominate rT and rB. The resistance of the contact layers and the quasi-neutral region can be predicted by using the conductivity of the semiconductor layers, and can be formulated as:

circuit parameters were calculated theoretically and extracted from Sentaurus TCAD two-

Profile 1 1x1016 cm-3 5x1016 cm-3 Profile 2 2.5x1016 cm-3 5x1015cm-3 Profile 3 5x1016 cm-3 5x1015cm-3

**Table 5.** The base and emitter/ collector doping profiles for the responsivity doping dependence study

**Figure 13.** (a) Equivalent circuit model of the SG-OEM device structure.

Base Doping Collector / Emitter Doping

dimensional simulations.

106 Optoelectronics - Advanced Materials and Devices

$$\mathbf{r} = \frac{w\_{\text{layer}}}{q\mu\_n nd} \begin{bmatrix} \Omega. \text{cm} \end{bmatrix} \tag{4}$$

where wlayer is the layer thickness, d the width of the layer, q elemental charge, n the free electron density, and μn the mobility of electrons in the layer. The unit of the contact resist‐ ance as defined by Equation 4 is Ω.cm.

The second contributor to the voltage drop at the contact layers is the highly doped InP/ InGaAs isotype heterojunction interface. The carrier conduction at the highly doped InP/ InGaAs isotype heterojunction interface can be analyzed based on the band diagram shown in Figure 14. The conduction band edge is similar to that of a rectifying metal – semiconduc‐ tor contact. Such a contact can have one of three conduction mechanisms: thermionic emis‐ sion, thermionic-field emission and field emission. It was determined that field emission dominated the current conduction between the InP and InGaAs layers, due to the very high doping densities of both layers.

**Figure 14.** Band diagram of the isotype heterojunction formed by the highly doped InP and InGaAs contact layers.

In order to verify the assumptions made above, the contact regions of the original device were modeled seperately in TCAD-Sentaurus and a set of simulations were carried out. The results were then compared with the theoretical calculations. The structures shown in Figure 15 were simulated to verify the calculations for the top and bottom contact resistances. Fig‐ ure 16 shows the simulated I-V characteristics for the top and bottom emitter/collector qua‐ si-neutral region and contact layer models depicted in Figure 15. The simulation was done under dark conditions, with the bias voltage being swept from 0 to 5 V.

**Figure 15.** Structures for series resistance extraction of InP / In0.53Ga0.47As HPT based SG-OEMs. Original structure is on the left, top contact layers are on the right top and bottom contact layers are on the right bottom.

The dark current of the top emitter/collector region shows a linear trend with increasing bias voltage and the top contact layer series resistance rT can be calculated from the I-V data pre‐ sented in Figure 16 using:

$$r\_{eq} = \frac{\Delta V}{\Delta I} \tag{5}$$

**Figure 16.** Dark current versus bias voltage for top and bottom contact layers of InP / In0.53Ga0.47As HPT based SG-

The frequency response related parameters are the junction capacitances Cμ and Cπ. These

where *N <sup>A</sup>* and *N <sup>D</sup>* are the doping densities of base and emitter/collector, respectively, *ε r,base* is the relative permitivity of the InGaAs base and *ε* r,*E/C* that of the InP emitter/collector, *V bi* is the built-in barrier, *V <sup>R</sup>*is the bias voltage and *q* is unit charge. The total capacitance of the SG-OEM device is dominated by the junction capacitance of the reverse biased junction.

Equivalent capacitance of the SG-OEM was extracted for both the full structure and a single base-emitter/collector heterojunction, as shown in Figure 17. The device total capacitance is the capacitance seen between the two terminals of the SG-OEM, which includes the two

A set of AC bias simulations were carried out on the two structures displayed in Figure 17. The simulations were set at dark condition and the bias voltage was swept from 0 to 5V. A small signal simulation was applied at each voltage point and the corresponding capaci‐ tance was modeled and calculated. The simulated total capacitance of the original structure and the junction capacitance of the base-emitter heterojunction are plotted in Figure 18 as a

2(*<sup>N</sup> Ar*,*base* <sup>+</sup> *NDr*,*<sup>E</sup>* /*<sup>C</sup>*)(*Vbi* <sup>+</sup> *<sup>V</sup> <sup>R</sup>*) (6)

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can be calculated form the junction capacitance formula for a heterostructure:

<sup>C</sup> <sup>=</sup> *qN <sup>A</sup>NDr*,*baser*,*<sup>E</sup>* /*<sup>C</sup>*

base-emitter/collector junction capacitances in series amd the base transit time.

OEMs.

function of the bias voltage.

where req is the equivalent resistance (rT or rB), ΔV is the voltage difference between two points and the ΔI is the corresponding current difference on the I-V curve shown in Figure 16.The value of rT, for this structure, is calculated to be 2.97x10-2 Ω−cm. This value is close to the sum of the theoretically calculated quasi-neutral region resistance (6.9x10-3Ω−cm) and isotype heterojunction field emission equivalent resistance(1.03x10-2Ω−cm). Therefore, it can be concluded that the top contact series resistance is dominated by the quasi-neutral layer resistance and the field emission equivalent resistance of the isotype heterojunction formed by the InGaAs/InP contact layers. The I-V curve of the bottom contact layer, on the other hand, shows a non-linear saturating trend as the voltage increases. The current saturation is induced by the narrowing of the contact layer after the mesa etch step. The increase of the current is limited by the narrow corner region of the InGaAs contact layer. The equivalent resistance is predicted to be approximately 0.57 Ω−cm, assuming the contact layer is etched mid-way and the current starts to crowd in the narrowing contact layer. This resistance will depend on accurate control of the inner mesa etch step in the device fabrication process and can be an issue at high current levels.

ure 16 shows the simulated I-V characteristics for the top and bottom emitter/collector qua‐ si-neutral region and contact layer models depicted in Figure 15. The simulation was done

**Figure 15.** Structures for series resistance extraction of InP / In0.53Ga0.47As HPT based SG-OEMs. Original structure is on

The dark current of the top emitter/collector region shows a linear trend with increasing bias voltage and the top contact layer series resistance rT can be calculated from the I-V data pre‐

where req is the equivalent resistance (rT or rB), ΔV is the voltage difference between two points and the ΔI is the corresponding current difference on the I-V curve shown in Figure 16.The value of rT, for this structure, is calculated to be 2.97x10-2 Ω−cm. This value is close to the sum of the theoretically calculated quasi-neutral region resistance (6.9x10-3Ω−cm) and isotype heterojunction field emission equivalent resistance(1.03x10-2Ω−cm). Therefore, it can be concluded that the top contact series resistance is dominated by the quasi-neutral layer resistance and the field emission equivalent resistance of the isotype heterojunction formed by the InGaAs/InP contact layers. The I-V curve of the bottom contact layer, on the other hand, shows a non-linear saturating trend as the voltage increases. The current saturation is induced by the narrowing of the contact layer after the mesa etch step. The increase of the current is limited by the narrow corner region of the InGaAs contact layer. The equivalent resistance is predicted to be approximately 0.57 Ω−cm, assuming the contact layer is etched mid-way and the current starts to crowd in the narrowing contact layer. This resistance will depend on accurate control of the inner mesa etch step in the device fabrication process and

<sup>∆</sup> *<sup>I</sup>* (5)

*req* <sup>=</sup> <sup>∆</sup> *<sup>V</sup>*

the left, top contact layers are on the right top and bottom contact layers are on the right bottom.

sented in Figure 16 using:

108 Optoelectronics - Advanced Materials and Devices

can be an issue at high current levels.

under dark conditions, with the bias voltage being swept from 0 to 5 V.

**Figure 16.** Dark current versus bias voltage for top and bottom contact layers of InP / In0.53Ga0.47As HPT based SG-OEMs.

The frequency response related parameters are the junction capacitances Cμ and Cπ. These can be calculated form the junction capacitance formula for a heterostructure:

$$\mathbf{C} = \sqrt{\frac{qN\_A N\_{Dr, \text{user}, E/\text{C}}}{2\{N\_{gr, \text{base}} + N\_{Dr, E/\text{C}}\}(V\_{\text{bi}} + V\_R)}}\tag{6}$$

where *N <sup>A</sup>* and *N <sup>D</sup>* are the doping densities of base and emitter/collector, respectively, *ε r,base* is the relative permitivity of the InGaAs base and *ε* r,*E/C* that of the InP emitter/collector, *V bi* is the built-in barrier, *V <sup>R</sup>*is the bias voltage and *q* is unit charge. The total capacitance of the SG-OEM device is dominated by the junction capacitance of the reverse biased junction.

Equivalent capacitance of the SG-OEM was extracted for both the full structure and a single base-emitter/collector heterojunction, as shown in Figure 17. The device total capacitance is the capacitance seen between the two terminals of the SG-OEM, which includes the two base-emitter/collector junction capacitances in series amd the base transit time.

A set of AC bias simulations were carried out on the two structures displayed in Figure 17. The simulations were set at dark condition and the bias voltage was swept from 0 to 5V. A small signal simulation was applied at each voltage point and the corresponding capaci‐ tance was modeled and calculated. The simulated total capacitance of the original structure and the junction capacitance of the base-emitter heterojunction are plotted in Figure 18 as a function of the bias voltage.

voltage. The simulated total capacitance of the SG-OEM structure and that of a single baseemitter/collector heterojunction are plotted in Figure 18 as a function of the bias voltage.

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The AC simulation results show that both the device total capacitance and the base-emitter junction capacitance decrease with increasing bias voltage, as would be expected. The capac‐ itance of the reverse biased heterojunction decreases with increasing VR as given in Equation 6. This is due to the increase of the depletion region width with increasing reverse bias volt‐ age, which leads to a decrease in the junction capacitance. The total capacitance is dominat‐ ed by the reverse-biased junction capacitance, which has a smaller value than the forward biased junction. At 0V bias, the capacitance of a single heterojunction was calculated to be 4 fF/μm using Equation 6, with *ε r,base =* 13.906 for the InGaAs layer and *ε* r,*E/C*= 12.4 for the InP layer. The TCAD Sentaurus simulation gives 4.434 fF/μm, as shown in Figure 18.At 0V bias, both heterojunction capacitances are equal. Therefore, the equivalent capacitance seen look‐ ing into the SG-OEM device, which is the series equivalent capacitance of the two hetero‐

Symmetric gain optoelectronic mixers based on InP/ In0.53Ga0.47As heterostructures are promising candidates use in the receivers of chirped-AM LADAR systems. These devices can reduce LADAR system component count and complexity, and improve their perform‐ ance. Two dimensional device simulations were used to optimize device structure parame‐ ters, including base width and doping density, and emitter/collectorlayer doping density. It was determined that highly doped interface layers caused an increase in dark current and device capacitance and also lowered the base punch through breakdown voltage. Therefore,

This work was partially supported by the US Army Research Laborayory (Dr. Neal Bambha) under the auspices of the U.S. Army Research Office Scientific Services Program adminis‐

junctions, is half of the capacitance of a single heterojunction.

the optimized device design does not contain such an interface layer.

tered by Battelle (Delivery Order 0812, Contract No. W911NF-07-D-0001)

Electrical and Computer Engineering, University of Maine, Orono, Maine, USA

**5. Conclusion**

**Acknowledgements**

**Author details**

Wang Zhang and Nuri W. Emanetoglu\*

**Figure 17.** Structures used for extracting the equivalent capacitances of InP / In0.53Ga0.47As HPT based SG-OEMs. The full SG-OEM structure is on the left, and a single base-emitter junction is on the right.

**Figure 18.** Total device capacitance and capacitance of a single reverse-biased base-emitter/ collect junction of InP / In0.53Ga0.47As HPT based SG-OEMs.

A set of AC simulations were carried out on the two structures displayed in Figure 17, with NA = 2.5x1016 cm-3 and ND = 5x1015 cm-3. The simulations were carried out for dark conditions and the DC bias voltage was swept from 0 to 5V, with a small signal perturbation applied to the bias voltage. The simulated total capacitance of the SG-OEM structure and that of a single baseemitter/collector heterojunction are plotted in Figure 18 as a function of the bias voltage.

The AC simulation results show that both the device total capacitance and the base-emitter junction capacitance decrease with increasing bias voltage, as would be expected. The capac‐ itance of the reverse biased heterojunction decreases with increasing VR as given in Equation 6. This is due to the increase of the depletion region width with increasing reverse bias volt‐ age, which leads to a decrease in the junction capacitance. The total capacitance is dominat‐ ed by the reverse-biased junction capacitance, which has a smaller value than the forward biased junction. At 0V bias, the capacitance of a single heterojunction was calculated to be 4 fF/μm using Equation 6, with *ε r,base =* 13.906 for the InGaAs layer and *ε* r,*E/C*= 12.4 for the InP layer. The TCAD Sentaurus simulation gives 4.434 fF/μm, as shown in Figure 18.At 0V bias, both heterojunction capacitances are equal. Therefore, the equivalent capacitance seen look‐ ing into the SG-OEM device, which is the series equivalent capacitance of the two hetero‐ junctions, is half of the capacitance of a single heterojunction.
