Jeffrey D. Beck

Recent advances in avalanche photodiodes

Until the early 2000s, the avalanche photodiode (APD) was widely deployed in high-performance optical receivers that operated up to 10 Gb/s. In subsequent years, the use of APDs for high-capacity systems declined as a result of their limited gain bandwidth, the transition to coherent detection, and the development of high-efficiency modulation techniques. Recently, the rapid growth of optical-fiber communications systems that utilize baud rates up to 25 Gb/s represented by a 100-Gb/s Ethernet has led to a resurgence of research on APDs and the development of low-noise APDs with enhanced gain bandwidth. This paper presents a brief review of APD fundamentals and describes some of the recent advances.

Monte Carlo simulations of Hg0.7Cd0.3Te avalanche photodiodes and resonance phenomenon in the multiplication noise

Monte Carlo simulations of Hg0.7Cd0.3Te avalanche photodiodes are presented. The simulated very low excess noise and exponential gain curve are consistent with those that have been experimentally observed and are consistent with the speculated large ratio of electron and hole impact ionization rates. The simulations suggest that there is a large difference between the scattering rates of electrons and holes, a direct consequence of the band structure. A resonance behavior in the excess noise factor at gain values near 2, 4, 8, and 16 is also revealed in the simulations. This effect is explained by comparing to the gain and noise of a photomultiplier tube.

A Highly Sensitive Multi-element HgCdTe e-APD Detector for IPDA Lidar Applications

An HgCdTe electron avalanche photodiode (e-APD) detector has been developed for lidar receivers, one application of which is integrated path differential absorption lidar measurements of such atmospheric trace gases as CO2 and CH4. The HgCdTe APD has a wide, visible to mid-wave-infrared, spectral response, high dynamic range, substantially improved sensitivity, and an expected improvement in operational lifetime. A demonstration sensor-chip assembly consisting of a 4.3 μm cutoff HgCdTe 4 × 4 APD detector array with 80 μm pitch pixels and a custom complementary metal–oxide–semiconductor readout integrated circuit was developed. For one typical array the APD gain was 654 at 12 V with corresponding gain normalized dark currents ranging from 1.2 fA to 3.2 fA. The 4 × 4 detector system was characterized at 77 K with a 1.55 μm wavelength, 1 μs wide, laser pulse. The measured unit gain detector photon conversion efficiency was 91.1%. At 11 V bias the mean measured APD gain at 77 K was 307.8 with σ/mean uniformity of 1.23%. The average, noise-bandwidth normalized, system noise-equivalent power (NEP) was 1.04 fW/Hz1/2 with a σ/mean of 3.8%. The measured, electronics-limited, bandwidth of 6.8 MHz was more than adequate for 1 μs pulse detection. The system had an NEP (3 MHz) of 0.4 fW/Hz1/2 at 12 V APD bias and a linear dynamic range close to 1000. A gain-independent quantum-limited SNR of 80% of full theoretical was indicative of a gain-independent excess noise factor very close to 1.0 and the expected APD mode quantum efficiency.

HgCdTe e-APD detector arrays with single photon sensitivity for space lidar applications

A multi-element HgCdTe electron initiated avalanche photodiode (e-APD) array has been developed for space lidar. The detector array was fabricated with 4.3μm cutoff HgCdTe with a spectral response from 0.4 to 4.3 μm. We have demonstrated a 4x4 e-APD array with 80 μm square elements followed by a custom cryogenic CMOS read-out integrated circuit (ROIC). The device operates at 77K inside a small closed-cycle cooler-Dewar with the support electronics integrated in a field programmable gate array. Measurements showed a unity gain quantum efficiency of about 90% at 1.5-1.6 μm wavelength. The bulk dark current of the HgCdTe e-APD at 77K was less than 50,000 input referred electrons/s at 12 V APD bias where the APD gain was 620 and the measured noise equivalent power (NEP) was 0.4 fW/Hz1/2. The electrical bandwidth of the device was about 6 MHz, mostly limited by the ROIC, but sufficient for the lidar application. Although the devices were designed for low bandwidth pulse detections, the high gain and low dark current enabled them to be used for single photon detections. Because the APD was biased below the break-down voltage, the output is linear to the input signal and there were no nonlinear effect such as dead-time and afterpulsing, and no need for gated operation. A new series of HgCdTe e-APDs have also been developed with a much wider bandwidth ROIC and higher APD gain, which is expected to give a much better performance in single photon detections.

HgCdTe Avalanche Photodiode Detectors for Airborne and Spaceborne Lidar at Infrared Wavelengths

We report results from characterizing the HgCdTe avalanche photodiode (APD) arrays developed for lidar at infrared wavelengths by using the high density vertically integrated photodiodes (HDVIP®) technique. The results show >90% quantum efficiencies between 0.8 μm and the cut-off wavelength, >600 APD gain, near unity excess noise factor, 6-10 MHz electrical bandwidth and <0.5 fW/Hz1/2 noise equivalent power (NEP). The detectors provide linear analog output with a dynamic range of 2-3 orders of magnitude at a fixed APD gain without averaging, and over 5 orders of magnitude by adjusting the APD gain settings. They have been used successfully in airborne CO2 and CH4 integrated path differential absorption (IPDA) lidar as precursors for use in space lidar.

Gated IR Imaging with 128 × 128 HgCdTe Electron Avalanche Photodiode FPA

The next generation of IR sensor systems will include active imaging capabilities. One example of such a system is a gated-active/passive system. The gated-active/passive system promises long-range target detection and identification. A detector that is capable of both active and passive modes of operation opens up the possibility of a self-aligned system that uses a single focal plane. The detector would need to be sensitive in the 3-5 μm band for passive mode operation. In the active mode, the detector would need to be sensitive in eye-safe range, e.g. 1.55 μm, and have internal gain to achieve the required system sensitivity. The MWIR HgCdTe electron injection avalanche photodiode (e-APD) not only provides state-of-the-art 3-5 μm spectral sensitivity, but also high avalanche photodiode gain without minimal excess noise. Gains of greater than 1000 have been measured in MWIR e-APDs with a gain independent excess noise factor of 1.3. This paper reports the application of the mid-wave HgCdTe e-APD for near-IR gated-active/passive imaging. Specifically a 128x128 FPA composed of 40 μm pitch, 4.2 μm to 5 μm cutoff, APD detectors with a custom readout integrated circuit was designed, fabricated, and tested. Median gains as high as 946 at 11 V bias with noise equivalent inputs as low as 0.4 photon were measured at 80 K. A gated imaging demonstration system was designed and built using commercially available parts. High resolution gated imagery out to 9 km was obtained with this system that demonstrated predicted MTF, precision gating, and sub 10 photon sensitivity.

Update on linear mode photon counting with the HgCdTe linear mode avalanche photodiode

Abstract. The behavior of the gain-voltage characteristic of the mid-wavelength infrared cutoff HgCdTe linear mode avalanche photodiode (e-APD) is discussed both experimentally and theoretically as a function of the width of the multiplication region. Data are shown that demonstrate a strong dependence of the gain at a given bias voltage on the width of the n− gain region. Geometrical and fundamental theoretical models are examined to explain this behavior. The geometrical model takes into account the gain-dependent optical fill factor of the cylindrical APD. The theoretical model is based on the ballistic ionization model being developed for the HgCdTe APD. It is concluded that the fundamental theoretical explanation is the dominant effect. A model is developed that combines both the geometrical and fundamental effects. The model also takes into account the effect of the varying multiplication width in the low bias region of the gain-voltage curve. It is concluded that the lower than expected gain seen in the first 2×8 HgCdTe linear mode photon counting APD arrays, and higher excess noise factor, was very likely due to the larger than typical multiplication region length in the photon counting APD pixel design. The implications of these effects on device photon counting performance are discussed.

Linear mode photon counting with the noiseless gain HgCdTe e-avalanche photodiode

Abstract. A linear mode photon counting focal plane array using HgCdTe mid-wave infrared (MWIR) cutoff electron initiated avalanche photodiodes (e-APDs) has been designed, fabricated, and characterized. The broad spectral range (0.4 to 4.3 μm) is unique among photon counters, making this a “first of its kind” system spanning the visible to the MWIR. The low excess noise [F(M)≈1] of the e-APDs allows for robust photon detection while operating at a stable linear avalanche gain in the range of 500–1000. The readout integrated circuit (ROIC) design included a very high gain-bandwidth product resistive transimpedance amplifier (3×1013  Ω-Hz) and a 4 ns output digital pulse width comparator. The ROIC had 16 high-bandwidth analogs and 16 low-voltage differential signaling digital outputs. The 2×8 array was integrated into an LN2 Dewar with a custom leadless chip carrier and daughter board design that preserved high-bandwidth analog and digital signal integrity. The 2×8 e-APD arrays were fabricated on 4.3 μm cutoff HgCdTe and operated at 84 K. The measured dark currents were approximately 1 pA at 13 V bias where the measured avalanche photodiode gain was 500. This translates to a predicted dark current induced dark count rate of less than 20 KHz. Single photon detection was achieved with a photon pulse signal-to-noise ratio of 13.7 above the amplifier noise floor. A photon detection efficiency of 50% was measured at a photon background limited false event rate of about 1 MHz. The measured jitter was in the range of 550–800 ps. The demonstrated minimum time between distinguishable events was less than 10 ns.

Comparison of flash lidar detector options

Abstract. Three lidar receiver technologies using the total laser energy required to perform a set of imaging tasks are compared. The tasks are combinations of two collection types (3-D mapping from near and far), two scene types (foliated and unobscured), and three types of data products (geometry only, geometry plus 3-bit intensity, and geometry plus 6-bit intensity). The receiver technologies are based on Geiger mode avalanche photodiodes (GMAPD), linear mode avalanche photodiodes (LMAPD), and optical time-of-flight lidar, which combine rapid polarization rotation of the image and dual low-bandwidth cameras to generate a 3-D image. We choose scenarios to highlight the strengths and weaknesses of various lidars. We consider HgCdTe and InGaAs variations of LMAPD cameras. The InGaAs GMAPD and the HgCdTe LMAPD cameras required the least energy to 3-D map both scenarios for bare earth, with the GMAPD taking slightly less energy. We comment on the strengths and weaknesses of each receiver technology. Six bits of intensity gray levels requires substantial energy using all camera modalities.

A comparison flash lidar detector options

This paper will discussion multiple flash lidar camera options and will compare sensitivity by calculating the required energy to map a certain area under specific conditions. We define two basic scenarios, and in each scenario look at bare earth 3D imaging, 3D imaging with 64 grey levels, or 6 bits of grey scale, 3D imaging with 3 return pulses from different ranges per detector element, and 3D imaging with both grey scale and multiple returns in each detector. We will compare Gieger Mode Avalanche Photo-Diodes, GMAPDs, Linear Mode Avalanche PhotoDiodes, LMAPDs, and low bandwidth cameras traditionally used for 2D imaging, but capable of being used for 3D imaging in conjunction with a rapid polarization rotation stage.