**1. Introduction**

Optoelectronic mixers (OEM) are photodetectors which detect an optical signal and internal‐ ly mix it with an electrical signal to obtain an electrical base-band (low frequency) signal. OEM devices have applications in optical communications and sensors such as laser assisted detection and ranging (LADAR) systems. Optoelectronic mixers can simplify signal process‐ ing in an optoelectronic system by combining the photodetection and mixing functions, leading to reduced component count. An optoelectronic mixing device which also amplifies the detected signal would further benefit the system.

In this work, a symmetric gain optoelectronic mixer based on a lattice-matched indium galli‐ um arsenide (In0.53Ga0.47As) / indium phosphide (InP) symmetric heterojunction phototran‐ sistor structure is investigated for chirped-AM laser detection and ranging systems (LADAR) operating in the "eye-safe" 1.55 μm wavelength range. The symmetric currentvoltage (I-V) characteristics of this device allows for it to be operated without the application of a DC bias voltage.

### **1.1. LADAR and the need for optoelectronic mixing devices**

The requirements and constraints of the application, LADAR, determine the specifications of the SG-OEM device. Therefore, a basic review of the application is necessary.

Two types of LADAR systems exist, pulse and continuous wave systems, both of which op‐ erate in similar manner to their RADAR equivalents [1]. In pulsed LADAR, a laser pulse is transmitted, and the time-of-flight of the return signal is measured. The alternative is to modulate the intensity of a continuous-wave laser with a chirped-FM signal. In order to avoid confusion with optical wavelength modulation, this method has also been called chirped-AM LADAR [2]. The frequency difference (fIF) between the reference (LO) and re‐ turn (RF) signals is related to target distance by:

$$f\_{\rm IF} = 2\,\Delta F \frac{D}{cT} \tag{1}$$

MSMs. The DC responsivity of the InGaAs MSM optoelectronic mixers was reported to be

The symmetric MSM Schottky photodetectors do not have a gain mechanism. Incorporating gain to the optoelectronic mixer would allow the following transimpedance amplifier's gain

There are three possible candidate structures, based on the avalanche photodiode (APD), the heterojunction phototransistor (HPT) and the modulated barrier diode (MBD). The avalan‐ che photodiode suffers from several drawbacks, including excess noise, and high sensitivity to temperature, voltage bias and defects in the semiconductor material. HPTs and MBDs, on the other hand, can provide high gain with low noise. The basic HPT and MBD structures are shown in Figure 1. MBDs in particular are low noise devices, which have higher gain for lower incident optical powers. A standard asymmetric heterojunction HPT or MBD requires a DC bias to achieve the associated high gain. In a typical system, the DC biased device is used to detect the incoming optical signal at RF frequency. This signal may need to be am‐ plified electronically. However, only low gains are possible due to the frequency. The next stage employs a mixer circuit to obtain the IF signal from the difference of the RF and LO

**(a) (b)**

The heterojunction phototransistor is a transistor with its emitter made of a wider bandgap material than the base. This improves carrier injection efficiency, and also ensures absorp‐ tion is limited to the base and the base-collector depletion region. The basic HPT is a two terminal device. A number of modifications to the basic HPT structure have been investigat‐ ed to improve performance. A base bias can be provided, either optically or by an electrical

**Figure 1.** Basic structures of (a)heterojunction phototransistor; and (b) modulated barrier diode.

N-InAlAs

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I-InAlAs

p-InGaAs layer

i-InGaAs absorption layer

InP

n-InGaAs

to be reduced, increasing bandwidth and improving the system's noise performance.

approximately 0.34 A/W [4,5].

**1.2. Phototransistors as optoelectronic mixing devices**

signals. The IF signal may need further amplification.

N-InAlAs emitter p-InGaAs base

n-InGaAs collector

InP

where ΔF the difference between the start and end frequencies of the chirp, T the chirp peri‐ od, c the speed of light and D the distance to target. The chirp may cover frequencies rang‐ ing from hundreds of MHz to several GHz. In contrast, the mixing product is in the range of tens of kHz to several MHz, as a function of the chirp period T and distance D.

The primary advantage of chirped LADAR over pulsed LADAR is the ability to use semi‐ conductor lasers as the transmitter source, leading to lower cost, power and weight. An ad‐ ditional advantage for a LADAR-on-chip implementation is that by using an optoelectronic mixer device, as described below, the microwave bandwidth return signal can be converted into a low frequency electrical signal that can be read-out using CMOS technology.

A typical photodetector in an optoelectronic system would be DC biased, and convert the RF modulation of the optical signal to an electrical signal at the RF frequency. In a chirped-AM LADAR system this RF signal output is then electronically mixed with LO signal. Due to the small available optical power, below 1 nW in some applications, the RF signal output of the detector may need to be amplified with a wide band amplifier before the electronic mixing. This amplifier can only have a low gain, due to the wide bandwidth nature of the RF signal. An alternative is to mix the photodetector output with the LO signal, then ampli‐ fy the low frequency signal.

Signal processing of a chirped-AM LADAR system is simplified if the photodetector is used as an optoelectronic mixer (OEM) [2]. An optoelectronic mixer is a photodetectorwhose re‐ sponsivity is modulated with the LO signal. The OEM output contains the difference (IF), sum, LO and RF signals. The mixed output signal is low-pass filtered to isolate the IF signal, which is then amplified. Due to the frequency difference, tens to hundreds of kHz vs. hun‐ dreds of MHz, much higher gains are possible in the following transimpedance amplifier.

A symmetric I-V characteristic photodetector can be used as an optoelectronic mixer. Sym‐ metric I-V characteristics refer to having equal absolute magnitude current for equal abso‐ lute magnitude voltage, I(-V) = -I(V), with I(0) = 0. This allows driving the OEM directly with the LO signal, without a DC bias. The output of the detector will thus contain the LO, RF, IF and sum frequencies. This output can be low pass filtered and the IF signal amplified. As this IF signal's bandwidth can be up to six orders of magnitudes smaller than the carrier fre‐ quencies, much higher gains can be used at the trans-impedance amplifier (TZA) following the OEM. Due to the lack of a DC bias, sensitivity to background light is reduced, as the re‐ sponse from background light averages to zero. An additional 3 dB signal processing gain is also obtained. The metal-semiconductor-metal (MSM) Schottky photodetector is such a sym‐ metric device [2-5]. Chirped-FM LADAR with GaAs MSM optoelectronic mixers, operating in the 800-850 nm wavelength range, have been reported [2,3]. Eye-safe operation requires operating wavelengths in the 1.3 μm to 1.55 μm. This has motivated to development of In‐ GaAs MSM optoelectronic mixers for operation at 1550 nm [4,5]. These InGaAs MSMs have been reported to have dark currents two orders of magnitude larger than GaAs MSMs [5], affecting noise level, and require larger RF power to achieve similar performance to GaAs MSMs. The DC responsivity of the InGaAs MSM optoelectronic mixers was reported to be approximately 0.34 A/W [4,5].

The symmetric MSM Schottky photodetectors do not have a gain mechanism. Incorporating gain to the optoelectronic mixer would allow the following transimpedance amplifier's gain to be reduced, increasing bandwidth and improving the system's noise performance.

#### **1.2. Phototransistors as optoelectronic mixing devices**

*<sup>f</sup> IF* =2∆*<sup>F</sup> <sup>D</sup>*

tens of kHz to several MHz, as a function of the chirp period T and distance D.

into a low frequency electrical signal that can be read-out using CMOS technology.

fy the low frequency signal.

92 Optoelectronics - Advanced Materials and Devices

where ΔF the difference between the start and end frequencies of the chirp, T the chirp peri‐ od, c the speed of light and D the distance to target. The chirp may cover frequencies rang‐ ing from hundreds of MHz to several GHz. In contrast, the mixing product is in the range of

The primary advantage of chirped LADAR over pulsed LADAR is the ability to use semi‐ conductor lasers as the transmitter source, leading to lower cost, power and weight. An ad‐ ditional advantage for a LADAR-on-chip implementation is that by using an optoelectronic mixer device, as described below, the microwave bandwidth return signal can be converted

A typical photodetector in an optoelectronic system would be DC biased, and convert the RF modulation of the optical signal to an electrical signal at the RF frequency. In a chirped-AM LADAR system this RF signal output is then electronically mixed with LO signal. Due to the small available optical power, below 1 nW in some applications, the RF signal output of the detector may need to be amplified with a wide band amplifier before the electronic mixing. This amplifier can only have a low gain, due to the wide bandwidth nature of the RF signal. An alternative is to mix the photodetector output with the LO signal, then ampli‐

Signal processing of a chirped-AM LADAR system is simplified if the photodetector is used as an optoelectronic mixer (OEM) [2]. An optoelectronic mixer is a photodetectorwhose re‐ sponsivity is modulated with the LO signal. The OEM output contains the difference (IF), sum, LO and RF signals. The mixed output signal is low-pass filtered to isolate the IF signal, which is then amplified. Due to the frequency difference, tens to hundreds of kHz vs. hun‐ dreds of MHz, much higher gains are possible in the following transimpedance amplifier.

A symmetric I-V characteristic photodetector can be used as an optoelectronic mixer. Sym‐ metric I-V characteristics refer to having equal absolute magnitude current for equal abso‐ lute magnitude voltage, I(-V) = -I(V), with I(0) = 0. This allows driving the OEM directly with the LO signal, without a DC bias. The output of the detector will thus contain the LO, RF, IF and sum frequencies. This output can be low pass filtered and the IF signal amplified. As this IF signal's bandwidth can be up to six orders of magnitudes smaller than the carrier fre‐ quencies, much higher gains can be used at the trans-impedance amplifier (TZA) following the OEM. Due to the lack of a DC bias, sensitivity to background light is reduced, as the re‐ sponse from background light averages to zero. An additional 3 dB signal processing gain is also obtained. The metal-semiconductor-metal (MSM) Schottky photodetector is such a sym‐ metric device [2-5]. Chirped-FM LADAR with GaAs MSM optoelectronic mixers, operating in the 800-850 nm wavelength range, have been reported [2,3]. Eye-safe operation requires operating wavelengths in the 1.3 μm to 1.55 μm. This has motivated to development of In‐ GaAs MSM optoelectronic mixers for operation at 1550 nm [4,5]. These InGaAs MSMs have been reported to have dark currents two orders of magnitude larger than GaAs MSMs [5], affecting noise level, and require larger RF power to achieve similar performance to GaAs

*cT* (1)

There are three possible candidate structures, based on the avalanche photodiode (APD), the heterojunction phototransistor (HPT) and the modulated barrier diode (MBD). The avalan‐ che photodiode suffers from several drawbacks, including excess noise, and high sensitivity to temperature, voltage bias and defects in the semiconductor material. HPTs and MBDs, on the other hand, can provide high gain with low noise. The basic HPT and MBD structures are shown in Figure 1. MBDs in particular are low noise devices, which have higher gain for lower incident optical powers. A standard asymmetric heterojunction HPT or MBD requires a DC bias to achieve the associated high gain. In a typical system, the DC biased device is used to detect the incoming optical signal at RF frequency. This signal may need to be am‐ plified electronically. However, only low gains are possible due to the frequency. The next stage employs a mixer circuit to obtain the IF signal from the difference of the RF and LO signals. The IF signal may need further amplification.

**Figure 1.** Basic structures of (a)heterojunction phototransistor; and (b) modulated barrier diode.

The heterojunction phototransistor is a transistor with its emitter made of a wider bandgap material than the base. This improves carrier injection efficiency, and also ensures absorp‐ tion is limited to the base and the base-collector depletion region. The basic HPT is a two terminal device. A number of modifications to the basic HPT structure have been investigat‐ ed to improve performance. A base bias can be provided, either optically or by an electrical 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 of HPTs to the tens of GHz range [10,11].

The heterostructures were grown using molecular beam epitaxy at the US Army Research Laboratory, Adelphi, MD. Cracking defects in the thin films were revealed during device fabrication, leading to an investigation into an alternative device structure with indium

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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

**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‐

phosphide (InP) layers to improve the growth quality [22,23].

**2. Device Structure and Simulation**

device, as shown, is configured for top illumination.

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

HPTs have been demonstrated for optoelectronic mixing applications, where the LO signal was provided electrically [10,13] or optically [14].

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‐ ceiver performance.

#### **1.3. Symmetric gain optoelectronic mixers**

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.

The heterostructures were grown using molecular beam epitaxy at the US Army Research Laboratory, Adelphi, MD. Cracking defects in the thin films were revealed during device fabrication, leading to an investigation into an alternative device structure with indium phosphide (InP) layers to improve the growth quality [22,23].
