**4. Operation and performance of SiGe photodetectors**

A photodetector may be basically defined as a device that converts an optical signal (photons) into an electrical one (electrons). There exist three primary classes of semiconductor based photodetectors: avalanche photodiodes (APDs), metal-semiconductor-metal (MSM) detectors, and p-i-n (*pin*) detectors. [An additional type of detector device, known as a metal-insulatorsemiconductor (MIS) photodetector featuring an insulator layer inserted between metal and semiconductor layers [30], has been developed as well but is not that common.] Detector devices of each of these three main classifications based on SiGe technology have been demonstrated. For the visible-NIR detection applications in view, SiGe based MSM and *pin* devices are the best suited, with each having associated advantages and disadvantages that will be discussed in some detail throughout the remainder of this section.

#### **4.1. Avalanche Photodiodes (APDs)**

Minimizing dark current in SiGe detectors, especially for those with smaller pixels, is a driving requirement. Figure 9 shows the SNR vs. dark current density for 7.5 and 12 μm pixels as a function of dark current density. The level lines are the readout and background SNRs and the slanted lines are the dark current SNRs. The background SNR is the best attainable SNR. The intersection of the lines thus signifies the dark current level where the dark current SNR is equal to the readout or background SNR. In Figure 9(a), the yellow lines signify the back‐ ground or BLIP SNRs for clear skies with no moonlight for the two pixel sizes, with the upper yellow line showing the SNR for 12 μm pixels and 0.89 moonlight conditions. In Figure 9(b), the readout noise SNR (red squares) has been added for both large and small pixels (based on

> **Figure 9.** (a) SNR vs. dark current density for 7.5 and 12 µm pixels for dark sky, 0.89 moonlight conditions, *f*/1.25, 33 ms integration time, and 400-1750 nm bandpass; (b) shows the addition of read noise based SNR (10 electrons).

**Figure 9.** (a) SNR vs. dark current density for 7.5 and 12 μm pixels for dark sky, 0.89 moonlight conditions, *f*/1.25, 33 ms integration time, and 400-1750 nm bandpass; (b) shows the addition of read noise based SNR (10 electrons).

1.0

10.0

100.0

SNR dk 7.5u SNR blip 7.5u SNR dk 12u SNR blip 12u SNR dkm 12u SNR blipm 12u **SNR's for two pixel sizes, no moon vs. Jdark for wide band f/1.25, 33 ms, 0.4-1.75um**

> SNR dk 7.5u SNR blip 7.5u SNR(10e) 7.5u SNR dk 12u SNR blip 12u SNR(10e) 12u

1.0E-11 1.0E-10 1.0E-09 1.0E-08 1.0E-07 **A/cm2**

(a) (b)

Dark current appears to be the performance limiting factor for small pixel NIR FPAs operating at RT. The performance can be improved by compensating for the dark currents using lookup tables, though nonuniformity due to uncompensated variance in dark current over the FPA must also be characterized. While utilizing lookup tables should smooth out most of the nonuniformity, there will be residual nonuniformity as a result of the FPA pixels' dark current to temperature difference ratios at RT in combination with the temperature increment used in the lookup tables. The dark current residual nonuniformity must be kept below the average dark

Dark current appears to be the performance limiting factor for small pixel NIR FPAs operating at RT. The performance can be improved by compensating for the dark currents using lookup tables, though nonuniformity due to uncompensated variance in dark current over the FPA must also be characterized. While utilizing lookup tables should smooth out most of the nonuniformity, there will be residual nonuniformity as a result of the FPA pixels' dark current to temperature difference ratios at RT in combination with the temperature increment used in the lookup tables. The dark current residual nonuniformity must be kept below the average

Miniature SiGe detector based FPAs that can be incorporated into handheld cameras or inserted into smartphones require *f*/2 to *f*/3 optics and pixels approximately 5-7 µm in size. These detectors will consequently have reduced light collection with relatively high dark currents at RT. Such higher *f*-numbers, which are about twice those characteristic of an ideal NIR camera, effectively reduce the signal and SNR by about a factor of four. While small optics of *f*/1 to *f*/1.5 are possible, these may require an increase in optics diameter. Another method to improve the SNR in dark current limited NIR sensors is to incorporate a microlens array (e.g., having 20 µm lens centers) to focus light from the scene onto 5-7 *µ*m detector pixels. This maintains the signal strength while reducing detector dark current, enabling small pixel sizes within a larger cell that

Miniature SiGe detector based FPAs that can be incorporated into handheld cameras or inserted into smartphones require *f*/2 to *f*/3 optics and pixels approximately 5-7 μm in size. These detectors will consequently have reduced light collection with relatively high dark currents at RT. Such higher *f*-numbers, which are about twice those characteristic of an ideal NIR camera, effectively reduce the signal and SNR by about a factor of four. While small optics of *f*/1 to *f*/1.5 are possible, these may require an increase in optics diameter. Another method

current noise level to preserve the performance, as illustrated in Figure 9.

dark current noise level to preserve the performance, as illustrated in Figure 9.

**3.7 SiGe Imager Performance Based on Modeling Results** 

**3.7. SiGe imager performance based on modeling results**

allows for extra on-chip signal processing electronics.

14

Predictions based on the modeling that has been detailed in this section are summarized as follows: Imaging under rural night sky conditions becomes challenging for small pixel, small optics designs, and dark currents can significantly impact performance in an uncooled NIR camera. A small NIR camera will respond well to minimal amounts of illumination from a direct

10 noise electrons per integration).

1.0

10.0

100.0

**SNR's for no moon and .89 moon for two pixel sizes vs Jdark for wide band f/1.25, 33 ms, 0.4-1.75um** 

328 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

1.0E-11 1.0E-10 1.0E-09 1.0E-08 1.0E-07 **A/cm2**

> APDs, which are commonly employed in high bitrate optical communication systems, achieve high built-in gain through avalanche multiplication, and require high bias voltages (~20 V/μm) to achieve desired ionization rates and provide detection of low power signals with high sensitivity [12]. Under sufficiently high external bias, the electrical field in an APD's depletion region causes photogenerated electrons from the absorption layer to undergo a series of impact ionization processes. This enables a single absorbed incoming photon to generate a large number of electron/hole pairs (EHPs), which effectively amplifies the photocurrent and improves the sensitivity, providing a QE potentially greater than unity. SiGe APDs typically have separate absorption-charge-multiplication (SACM) structures (see Figure 10), in which light is absorbed in an intrinsic Ge film and electrons are multiplied in an intrinsic Si film; such structures allow optimization of both QE and multiplication gain [10].

**Figure 10.** Schematic cross-section of a SiGe based APD device with a separate absorption-charge-multiplication (SACM) structure and its internal electric field distribution [10].

The most important performance metrics for APDs are ionization ratio (which should be minimized), internal electric field distribution, excess noise factor, gain-bandwidth product, and sensitivity [4]. The device structure of a basic APD is similar to that of a *pin* photodetector. Compared with their *pin* counterparts, APDs offer 5-10 dB better sensitivity and higher SNR due to their internal multiplication gain, as well as high bandwidth-efficiency products [4,31]. However, the comparatively low operation bandwidths and the requirement of very high bias voltages limit the integration potential of SiGe based APDs into practical CMOS-based devices.

#### **4.2. Metal-Semiconductor-Metal (MSM) photodetectors**

MSM photodetectors comprise two back-to-back Schottky contacts and feature a closely spaced interdigitated metal electrode configuration on top of an active light absorption semiconductor layer [32]. The material, physical, and electrical properties of MSM devices are depicted in Figure 11(a), (b), and (c), respectively. MSM detectors are photoconductive devices not functional under zero bias, and require sufficient external bias for the semiconductor layer to become fully depleted. The Schottky junctions present in MSM detectors exhibit rectified current-voltage (I-V) characteristics as do *pn* junctions, but occur at the metal-semiconductor rather than semiconductor-semiconductor interfaces. Also, while *pn* junctions allow both electrons and holes to flow under forward bias, Schottky junctions allow only majority carriers to flow.

Advantages of MSM detectors include low capacitance and consequent low RC delay, which enables high-speed operation. Detection bandwidths for SiGe based MSM devices are comparatively high, making them suitable for fast optical fiber communications. In addition,

since MSM detectors are inherently planar and require only a single photolithography step, they are relatively easy to fabricate, boosting their potential for practical integration. However, the external QE and effective responsivity in MSM devices are generally lower than those in *pin* detectors due to shadowing of the metal electrodes, which typically occupy 25-50% of the surface area [18].

**Figure 11.** (a) Cross-section of MSM photodetector fabricated on Ge layer grown on Si substrate [33]. (b) Scanning electron microscope (SEM) image of an evanescent waveguide-coupled Ge-on-SOI MSM photodetector [34]. (c) **Figure 11.** (a) Cross-section of MSM photodetector fabricated on Ge layer grown on Si substrate [33]. (b) Scanning elec‐ tron microscope (SEM) image of an evanescent waveguide-coupled Ge-on-SOI MSM photodetector [34]. (c) Schematic diagram of MSM structure and corresponding energy band diagram at thermal equilibrium [12].

Schematic diagram of MSM structure and corresponding energy band diagram at thermal equilibrium [12]. In addition, high dark current associated with SiGe based MSM devices, primarily as a result of hole injection over the Schottky barrier [35], is a significant problem that raises the noise floor and increases standby power consumption [10]. This dark current may include current associated with thermally generated electron-hole pairs and carrier injection over the Schottky barriers, since SiGe MSM detectors typically have poor Schottky contacts with Ge [12]. While techniques to suppress dark current in MSM devices, such as dopant segregation and utilizing an intermediate layer of amorphous Ge and SiC, have suppressed dark current in detection devices significantly [36], MSM detectors generally still exhibit higher levels of dark current than comparable SiGe based *pin* devices [37] often resulting in an inferior level of performance [4]. In addition, high dark current associated with SiGe based MSM devices, primarily as a result of hole injection over the Schottky barrier [35], is a significant problem that raises the noise floor and increases standby power consumption [10]. This dark current may include current associated with thermally generated electron-hole pairs and carrier injection over the Schottky barriers, since SiGe MSM detectors typically have poor Schottky contacts with Ge [12]. While techniques to suppress dark current in MSM devices, such as dopant segregation and utilizing an intermediate layer of amorphous Ge and SiC, have sup‐ pressed dark current in detection devices significantly [36], MSM detectors generally still exhibit higher levels of dark current than comparable SiGe based *pin* devices [37] often resulting in an inferior level of performance [4].

#### **4.3** *pin* **Photodetectors 4.3.** *Pin* **photodetectors**

**Figure 10.** Schematic cross-section of a SiGe based APD device with a separate absorption-charge-multiplication

The most important performance metrics for APDs are ionization ratio (which should be minimized), internal electric field distribution, excess noise factor, gain-bandwidth product, and sensitivity [4]. The device structure of a basic APD is similar to that of a *pin* photodetector. Compared with their *pin* counterparts, APDs offer 5-10 dB better sensitivity and higher SNR due to their internal multiplication gain, as well as high bandwidth-efficiency products [4,31]. However, the comparatively low operation bandwidths and the requirement of very high bias voltages limit the integration potential of SiGe based APDs into practical CMOS-based devices.

MSM photodetectors comprise two back-to-back Schottky contacts and feature a closely spaced interdigitated metal electrode configuration on top of an active light absorption semiconductor layer [32]. The material, physical, and electrical properties of MSM devices are depicted in Figure 11(a), (b), and (c), respectively. MSM detectors are photoconductive devices not functional under zero bias, and require sufficient external bias for the semiconductor layer to become fully depleted. The Schottky junctions present in MSM detectors exhibit rectified current-voltage (I-V) characteristics as do *pn* junctions, but occur at the metal-semiconductor rather than semiconductor-semiconductor interfaces. Also, while *pn* junctions allow both electrons and holes to flow under forward bias, Schottky junctions allow only majority carriers

Advantages of MSM detectors include low capacitance and consequent low RC delay, which enables high-speed operation. Detection bandwidths for SiGe based MSM devices are comparatively high, making them suitable for fast optical fiber communications. In addition,

(SACM) structure and its internal electric field distribution [10].

330 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**4.2. Metal-Semiconductor-Metal (MSM) photodetectors**

to flow.

As their name may suggest, *pin* photodetectors consist of an intrinsic (*i*) region sandwiched between heavily doped *p*<sup>+</sup> and *n*<sup>+</sup> semiconductor layers. A typical *pin* photodetector design cross-section is depicted in Figure 12(a). The *p*<sup>+</sup> /*n*+ regions may be formed by implantation, *in situ* doping, or consist of a highly doped monocrystalline Si substrate [15,38]. The depletion layer in which all absorption occurs is almost entirely defined by the thicker highly resistive intrinsic region. This is in contrast to a common *pn* photodiode, in which the width of the depletion region (usually thinner than that in *pin* devices) is governed by the applied external electric field. As their name may suggest, *pin* photodetectors consist of an intrinsic (*i*) region sandwiched between heavily doped *p*<sup>+</sup> and *n*<sup>+</sup> semiconductor layers. A typical *pin* photodetector design cross-section is depicted in Figure 12(a). The *p*<sup>+</sup> /*n*<sup>+</sup> regions may be formed by implantation, *in situ* doping, or consist of a highly doped monocrystalline Si substrate [15,38]. The depletion layer in which all absorption occurs is almost entirely defined by the thicker highly resistive intrinsic region. This is in contrast to a common *pn* photodiode, in which the width of the depletion region (usually thinner than that in *pin* devices) is governed by the applied external electric field.

SiGe based *pin* photodetector structures can exhibit significant built-in electric fields of several kV cm-1 inside the *i*-Ge/SiGe layers, which overcome recombination processes at lattice defects, improving the device quality and enabling smaller devices [10]. The uniform electric field *F* in the intrinsic region in a *pin* photodetector is given approximately by [41]: SiGe based *pin* photodetector structures can exhibit significant built-in electric fields of several kV cm-1 inside the *<sup>i</sup>*-Ge/SiGe layers, which overcome recombination processes at lattice defects,improving the device quality and enabling smaller devices [10]. The uniform electric field *<sup>F</sup>* in the intrinsic region in a *pin* photodetector is given approximately by [41]:

*F Vbi V wD* / (10)

17

$$F = \left(V\_{bi} - V\right) / \text{tr}\_D \tag{10}$$

**Figure 12.** (a) Cross-sectional schematic view of SiGe based *pin* photodetector structure [39]. (b) Band diagram of *pin* Ge/Si heterojunction [40]. **Figure 12.** (a) Cross-sectional schematic view of SiGe based *pin* photodetector structure [39]. (b) Band diagram of *pin* Ge/Si heterojunction [40].

where *V* is the applied voltage, *Vbi* is the built-in voltage, and *wD* is the thickness of the depletion or intrinsic region. Upon absorption of a photon in the intrinsic region of energy *hν* > *Eg*, EHPs are created and immediately separated by the external electric field resulting from reverse biasing the device, leading to generation of photocurrent. Absorption outside the intrinsic region will also result in photocurrent if the minority carriers manage to diffuse to the intrinsic region. Since there are few charge carriers in the intrinsic region, the space charge region reaches completely from the *p*-type to the *n*-type region. Figure 12(b) shows the energy band diagram of a *p*-Ge/*i*-Ge/*n*-Si heterojunction. where *V* is the applied voltage, *V*bi is the built-in voltage, and *w*D is the thickness of the depletion or intrinsic region. Upon absorption of a photon in the intrinsic region of energy *hν* > *E*g, EHPs are created and immediately separated by the external electric field resulting from reverse biasing the device, leading to generation of photocurrent. Absorption outside the intrinsic region will also result in photocurrent if the minority carriers manage to diffuse to the intrinsic region. Since there are few charge carriers in the intrinsic region, the space charge region reaches completely from the *p*-type to the *n*-type region. Figure 12(b) shows the energy band diagram of a *p*-Si/*i*-Ge/*n*-Ge heterojunction.

The intrinsic/depletion region thickness, which is normally made substantially larger than that of the *p*<sup>+</sup> and *n*<sup>+</sup> regions, can be tailored to optimize detector performance [4]. Having a relatively thick depletion region with a strong internal electric field causes most of the generated EHPs to be transferred to the *p*<sup>+</sup> /*n*+ regions and collected as a result of carrier drift rather than diffusion. (Since the valence band offset is much larger than the conduction band offset, the higher barrier in the valence band limits the movement of holes, so the conductivity in *pin* heterojunctions is due to electrons in the conduction band [40].) This in turn results in less carrier recombination at dislocations or point defects, leading to higher collection efficiency [10]. Because of this as well as other factors, a thicker depletion region in a *pin* photodetector is associated with higher responsivity and QE. On the other hand, having a thinner intrinsic region in a *pin* detector reduces the transit time, thereby enhancing the response bandwidth [4]. However, a thinner intrinsic layer also effects a larger capacitance, which produces greater RC delay that can have a limiting effect on the detector speed of operation [12]. There are two main classifications of SiGe based *pin* photodetectors structures: normal incidence The intrinsic/depletion region thickness, which is normally made substantially larger than that of the *p*<sup>+</sup> and *n*<sup>+</sup> regions, can be tailored to optimize detector performance [4]. Having a relatively thick depletion region with a strong internal electric field causes most of the generated EHPs to be transferred to the *p*<sup>+</sup> /*n*<sup>+</sup> regions and collected as a result of carrier drift rather than diffusion. (Since the valence band offset is much larger than the conduction band offset, the higher barrier in the valence band limits the movement of holes, so the conductivity in *pin* heterojunctions is due to electrons in the conduction band [40].) This in turn results in less carrier recombination at dislocations or point defects, leading to higher collection efficiency [10]. Because of this as well as other factors, a thicker depletion region in a *pin* photodetector is associated with higher responsivity and QE. On the other hand, having a thinner intrinsic region in a *pin* detector reduces the transit time, thereby enhancing the response bandwidth [4]. However, a thinner intrinsic layer also effects a larger capacitance, which produces greater RC delay that can have a limiting effect on the detector speed of operation [12].

(NI) and lateral. As might be expected, in the former type of device light is incident vertically or normal to its top or bottom surface, while in the latter type the photons are incident horizontally or laterally. In addition, a substantial portion of SiGe *pin* detectors now have waveguide-coupled (WC) designs, which circumvent alignment issues by featuring either evanescent coupling or butt-coupling between integrated SiGe detectors and Si optical waveguides. Practically all lateral (and some NI) *pin* photodetectors are WC. The advent of practical WC SiGe *pin* There are two main classifications of SiGe based *pin* photodetectors structures: normal incidence (NI) and lateral. As might be expected, in the former type of device light is incident vertically or normal to its top or bottom surface, while in the latter type the photons are incident horizontally or laterally. In addition, a substantial portion of SiGe *pin* detectors now have

18

photodetectors was relatively recent— virtually all such detector devices have been reported

waveguide-coupled (WC) designs, which circumvent alignment issues by featuring either evanescent coupling or butt-coupling between integrated SiGe detectors and Si optical waveguides. Practically all lateral (and some NI) *pin* photodetectors are WC. The advent of practical WC SiGe *pin* photodetectors was relatively recent— virtually all such detector devices have been reported within the past decade. While lengths of early WC *pin* photodetectors were typically on the order of 100 μm to ensure full absorption of light around 1550 nm [4], more recent devices are have been designed smaller (~10X) to reduce the RC delay and maximize potential bandwidths [42].

#### **4.4. Reported performance of SiGe** *pin* **photodetectors**

= - ( ) *bi <sup>D</sup> F V V /w* (10)

18

**Figure 12.** (a) Cross-sectional schematic view of SiGe based *pin* photodetector

**Figure 12.** (a) Cross-sectional schematic view of SiGe based *pin* photodetector structure [39]. (b) Band diagram of *pin*

where *V* is the applied voltage, *Vbi* is the built-in voltage, and *wD* is the thickness of the depletion or intrinsic region. Upon absorption of a photon in the intrinsic region of energy *hν* > *Eg*, EHPs are created and immediately separated by the external electric field resulting from reverse biasing the device, leading to generation of photocurrent. Absorption outside the intrinsic region will also result in photocurrent if the minority carriers manage to diffuse to the intrinsic region. Since there are few charge carriers in the intrinsic region, the space charge region reaches completely from the *p*-type to the *n*-type region. Figure 12(b) shows the energy band diagram of

where *V* is the applied voltage, *V*bi is the built-in voltage, and *w*D is the thickness of the depletion or intrinsic region. Upon absorption of a photon in the intrinsic region of energy *hν* > *E*g, EHPs are created and immediately separated by the external electric field resulting from reverse biasing the device, leading to generation of photocurrent. Absorption outside the intrinsic region will also result in photocurrent if the minority carriers manage to diffuse to the intrinsic region. Since there are few charge carriers in the intrinsic region, the space charge region reaches completely from the *p*-type to the *n*-type region. Figure 12(b) shows the energy band

The intrinsic/depletion region thickness, which is normally made substantially larger than that of

The intrinsic/depletion region thickness, which is normally made substantially larger than that

thick depletion region with a strong internal electric field causes most of the generated EHPs to

thick depletion region with a strong internal electric field causes most of the generated EHPs

(Since the valence band offset is much larger than the conduction band offset, the higher barrier in the valence band limits the movement of holes, so the conductivity in *pin* heterojunctions is due to electrons in the conduction band [40].) This in turn results in less carrier recombination at dislocations or point defects, leading to higher collection efficiency [10]. Because of this as well as other factors, a thicker depletion region in a *pin* photodetector is associated with higher responsivity and QE. On the other hand, having a thinner intrinsic region in a *pin* detector reduces the transit time, thereby enhancing the response bandwidth [4]. However, a thinner intrinsic layer also effects a larger capacitance, which produces greater RC delay that can have a

(Since the valence band offset is much larger than the conduction band offset, the higher barrier in the valence band limits the movement of holes, so the conductivity in *pin* heterojunctions is due to electrons in the conduction band [40].) This in turn results in less carrier recombination at dislocations or point defects, leading to higher collection efficiency [10]. Because of this as well as other factors, a thicker depletion region in a *pin* photodetector is associated with higher responsivity and QE. On the other hand, having a thinner intrinsic region in a *pin* detector reduces the transit time, thereby enhancing the response bandwidth [4]. However, a thinner intrinsic layer also effects a larger capacitance, which produces greater RC delay that can have

There are two main classifications of SiGe based *pin* photodetectors structures: normal incidence (NI) and lateral. As might be expected, in the former type of device light is incident vertically or normal to its top or bottom surface, while in the latter type the photons are incident horizontally or laterally. In addition, a substantial portion of SiGe *pin* detectors now have waveguide-coupled (WC) designs, which circumvent alignment issues by featuring either evanescent coupling or butt-coupling between integrated SiGe detectors and Si optical waveguides. Practically all lateral (and some NI) *pin* photodetectors are WC. The advent of practical WC SiGe *pin* photodetectors was relatively recent— virtually all such detector devices have been reported

There are two main classifications of SiGe based *pin* photodetectors structures: normal incidence (NI) and lateral. As might be expected, in the former type of device light is incident vertically or normal to its top or bottom surface, while in the latter type the photons are incident horizontally or laterally. In addition, a substantial portion of SiGe *pin* detectors now have

regions, can be tailored to optimize detector performance [4]. Having a relatively

regions, can be tailored to optimize detector performance [4]. Having a relatively

regions and collected as a result of carrier drift rather than diffusion.

regions and collected as a result of carrier drift rather than diffusion.

structure [39]. (b) Band diagram of *pin* Ge/Si heterojunction [40].

(b) (a)

332 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

a *p*-Ge/*i*-Ge/*n*-Si heterojunction.

/*n*+

/*n*<sup>+</sup>

diagram of a *p*-Si/*i*-Ge/*n*-Ge heterojunction.

limiting effect on the detector speed of operation [12].

a limiting effect on the detector speed of operation [12].

the *p*<sup>+</sup>

of the *p*<sup>+</sup>

and *n*<sup>+</sup>

and *n*<sup>+</sup>

Ge/Si heterojunction [40].

be transferred to the *p*<sup>+</sup>

to be transferred to the *p*<sup>+</sup>

Due to their comparative ease of fabrication, performance advantages, and prevalence, the focus throughout the remainder of this chapter centers primarily on SiGe based *pin* photode‐ tectors and associated technology. Performance specifications reported for NI and WC *pin* SiGe photodetectors are given in Table 1 and Table 2, respectively. In Table 3 and Table 4, typical ranges of performance results for MSM detectors and typical APD specifications, respectively, are presented for comparison.


**Table 1.** Reported performance specifications of NI *pin* SiGe photodetectors.

Compared to NI detectors, WC SiGe *pin* photodetectors generally have similar bandwidths (up to 47 GHz), higher responsivity and QE, and dark currents and dark current densities that are comparable in magnitude. Among *pin* WC detectors, lateral structures have demonstrated higher QE values compared to NI devices due to less light consumption in the highly doped region, while achieving similar response speeds [42].


**Table 2.** Performance specifications of WC *pin* SiGe photodetectors.

In comparison to SiGe based MSM devices, SiGe *pin* detectors offer high bandwidths, low noise, and high responsivities. Responsivities reported for some *pin* devices are as high as ~1 A/W (and even greater for certain WC detectors), and typically are substantially better than those of MSM devices [50,52]. SiGe *pin* detectors generally also offer higher responsivities and lower dark currents than SiGe APDs. SiGe *pin* detector devices have demonstrated respon‐ sivities at 1310 and 1550 nm that are similar to those of commercially available InGaAs photodetectors [53].


**Table 3.** General performance specification ranges of MSM type SiGe photodetectors.

Bandwidths of Ge/SiGe *pin* photodetectors have improved from several gigahertz to close to 50 GHz in recent years, and are presently comparable to those of MSM devices. Currently the highest *pin* detector bandwidth is 49 GHz at-2 V reverse bias as reported by Klinger *et al*., and three reported WC detectors can operate above 45 GHz, fast enough to accommodate future 40 Gb/s telecommunications applications [50]. Techniques considered to enhance bandwidths further include limiting the thickness of the Ge/SiGe intrinsic layers in *pin* photodiodes to reduce carrier transition times, and altering the device structure to limit undesirable parasitic effects.


**Table 4.** General performance specifications of SiGe APDs.

**Resp. (A/W) @ 1.55 µm**

photodetectors [53].

effects.

**DC Dens. (mA/cm2**

**) DC (µA)**

334 Advances in Optical Fiber Technology: Fundamental Optical Phenomena and Applications

**Table 2.** Performance specifications of WC *pin* SiGe photodetectors.

**BW VBias (V)**

0.87 1.3×103 0.9 -1 7.5 -1 2007 D. Ahn [54] 0.89 51 0.17 -2 31.3 -2 2007 T. Yin [55] 1.0 0.7 0.0002 -1 4.5 -3 2008 M. Beals [56] 0.65 — 0.06 -1 18 -1 2008 J. Wang [42] 1.0 60 ~1 -1 42 -4 2009 L. Vivien [57] 1.1 1.6×104 1.3 -1 32 -1 2009 D. Feng [52] 0.8 — 0.072 -1 47 -3 2009 D. Suh [58] 1.1 28 1.3 -1 36 -3 2010 D. Feng [59] 0.78 40 0.003 -1 45 -1 2011 C. DeRose [60] 0.8 71 0.025 -1 45 0 2013 L. Virot [61]

In comparison to SiGe based MSM devices, SiGe *pin* detectors offer high bandwidths, low noise, and high responsivities. Responsivities reported for some *pin* devices are as high as ~1 A/W (and even greater for certain WC detectors), and typically are substantially better than those of MSM devices [50,52]. SiGe *pin* detectors generally also offer higher responsivities and lower dark currents than SiGe APDs. SiGe *pin* detector devices have demonstrated respon‐ sivities at 1310 and 1550 nm that are similar to those of commercially available InGaAs

**Parameter Best Typical Worst** Responsivity @ 1.55 μm 0.8-1.2 A/W 0.53-0.75 A/W 0.10-0.14 A/W Dark Current Density 85-100 mA/cm2 0.6-2.0 A/cm2 650-1000 A/cm2 Dark Current 0.011-0.020 μA 4-10 μA 90-4000 μA Bandwidth 36.5-40.0 GHz 10-25 GHz 1.0-4.3 GHz

Bandwidths of Ge/SiGe *pin* photodetectors have improved from several gigahertz to close to 50 GHz in recent years, and are presently comparable to those of MSM devices. Currently the highest *pin* detector bandwidth is 49 GHz at-2 V reverse bias as reported by Klinger *et al*., and three reported WC detectors can operate above 45 GHz, fast enough to accommodate future 40 Gb/s telecommunications applications [50]. Techniques considered to enhance bandwidths further include limiting the thickness of the Ge/SiGe intrinsic layers in *pin* photodiodes to reduce carrier transition times, and altering the device structure to limit undesirable parasitic

**Table 3.** General performance specification ranges of MSM type SiGe photodetectors.

**BW (GHz) VBias (V) Pub. Year 1st Author Ref.**

Reported dark currents and dark current densities in SiGe *pin* detectors are both on average approximately two orders of magnitude lower compared to those of MSM devices, with dark current densities of certain *pin* devices as low as in the μA/cm2 range [42]. The dark current and dark current density results presented were almost entirely measured for devices biased at 1 V. To minimize dark current and operating power further, there has recently been increasing research interest in the development of lower bias or even zerobias *pin* photodiodes. Zero-bias SiGe *pin* photodetectors have demonstrated responsivities at 0 V bias nearly equivalent to the saturated value at 2 V bias [15], as well as 3 dB bandwidths as high as 25 GHz [38].
