Chang-Feng Wan

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.

Recent Development of Ultra Small Pixel Uncooled Focal Plane Arrays at DRS

DRS is a major supplier of the 25μm pixel pitch 640x480 and 320x240 infrared uncooled focal plane arrays (UFPAs) and camera products for commercial and military markets. The state-of-the-art 25μm pixel focal plane arrays currently in production provide excellent performance for soldier thermal weapon sights (TWS), vehicle driver vision enhancers (DVE), and aerial surveillance and industrial thermograph applications. To further improve sensor resolution and reduce the sensor system size, weight and cost, it is highly desired to reduce the UFPA pixel size. However, the 17μm pixel FPA presents significant design and fabrication challenges as compared with 25μm pixel FPAs. The design objectives, engineering trade-offs, and performance goals will be discussed. This paper presents an overview of the 17μm microblometer uncooled focal plane arrays and sensor electronics production and development activities at DRS. The 17 μm pixel performance data from several initial fabrication lots will be summarized. Relevant 25μm pixel performance data are provided for comparison. Thermal images and video from the 17μm pixel 640x480 UFPA will also be presented.

High Operating Temperature MWIR detectors

The utilization of the non-equilibrium photodiode concept for high operating temperature (HOT) FPAs is discussed, both generically, and with regard to the specific example of MWIR HgCdTe. The issues of dark current, surface passivation, and 1/f noise are considered for three different architectures, namely N+/N-/P+, N+/P-/P+, and nBn. These architectures are examined with regard to possible FPA performance limitations, and potential difficulty in reduction to practice. Performance data obtained at DRS for the N+/N-/P+ and N+/P-/P+ HgCdTe architectures will be presented.

Universal 1/f noise model for reverse biased diodes

A 1/f noise model is developed for reverse biased diodes based on McWhorter’s concept of charge tunneling into semiconductor states at passivation layer interfaces [A. L. McWhorter, in Semiconductor Surface Physics, edited by R. H. Kingston (University of Pennsylvania Press, Philadelphia, 1957), pp. 207–228]. The charge modulates the width of semiconductor surface charge layers on either side of the junction, resulting in fluctuations in dark current from these volumes due to the net difference in depletion and diffusion current generation rates per unit volume in the semiconductor. The 1/f spectrum associated with the fluctuating surface charge translates into a 1/f spectrum in thermally generated diode dark current. The model is applied to midwavelength infrared HgCdTe N+/P diode architectures.

Study of diffusion length in two-dimensional HgCdTe avalanche photodiodes by optical beam induced current

Optical beam induced current (OBIC) imaging has been used to measure the diffusion length of HgCdTe avalanche photodiodes (APDs) having a cylindrical structure. We show that the effective diffusion length extracted from current profiles is dependent on the electrode size and shape and the distance between electrode and excitation spot. To obtain the bulk diffusion length, a two-dimensional diffusion model was developed. The simulations provide good fits to experiment and indicate a bulk diffusion length of 70μm at room temperature.

Performance and modeling of the MWIR HgCdTe electron avalanche photodiode

The operation of the mid-wave infrared (MWIR) HgCdTe cylindrical electron injection avalanche photodiode (e-APD) is described. The measured gain and excess noise factor are related to the to the collection region fill factor. A 2D diffusion model calculates the time dependent response and steady state pixel point spread function for cylindrical diodes, and predicts bandwidths near 1 GHz for small geometries. A 2 μm diameter spot scan system was developed for point spread function and crosstalk measurements at 80 K. An electron diffusion length of 13.4 μm was extracted from spot scan data. Bandwidth data are shown that indicate bandwidths in excess of 300 MHz for small unit cells geometries. Dark current data, at high gain levels, indicate an effective gain normalized dark density count as low as 1000 counts per μs per cm2 at an APD gain of 444. A junction doping profile was determined from capacitance-voltage data. Spectral response data shows a gain independent characteristic.

HgCdTe HDVIP detectors and FPAs for strategic applications

Detector characteristics of Cu- and Au-doped High Density Vertically Integrated Photodiode (HDVIP) detectors as well as Cu-doped HDVIP Focal Plane Arrays (FPAs) are presented in this paper. Individual photodiodes in test bars were examined by measuring I-V curves and the associated resistance-area (RA) product as a function of temperature. The Au-doped MWIR [λc(78 K) = 5 μm] HDVIP detectors RoA performance was within a factor of two or three of theoretical. Noise as a function of frequency has been measured on Au-doped MWIR HgCdTe HDVIP diodes at several temperatures under dark and illuminated conditions. Low-frequency noise performance of the Au-doped MWIR diode in the various environments is characterized by the ratio α of the noise current spectral density at 1 Hz to the value of the diode current. For photocurrent at 140 K, αPHOTO = 1.8 x 10-5. The value of αPHOTO is the same at both zero bias and 100 mV reverse bias. At 160 K, αPHOTO is slightly lower but still in the low 10-5 range. Excess low-frequency noise measured at 140 K and 100 mV reverse bias in the dark has αDARK = 1.4 x 10-5. At 160 K and 100 mV reverse bias, αDARK is in the mid 10-5 range. At 140 K,the dark current at 8.2 V reverse bias was equal to the photocurrent at 100 mV reverse bias and close to the photocurrent at zero bias. αDARK = 1.85 x 10-3 at -8.2 V. This ratio is two orders of magnitude greater than αPHOTO. At 8.2 V reverse bias, the current was amplified by avalanche processes. Similar results were obtained on the Au-doped diode at 160 K. Diffusion current dominates dark current at 100 mV reverse bias at T = 185 K and T = 220 K. The ratio, αDARK approximately αPHOTO in the low to mid 10-5 range, i.e. dark diffusion current generates excess low frequency noise in the same manner as photocurrent. In addition, 256 x 256 Cu-doped detector arrays were fabricated. Initial measurements had seven out of ten FPAs having operabilities greater than 99.45% with the best 256 x 256 array having only two inoperable pixels.