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<sup>+</sup>/N<sup>-</sup>/P<sup>+</sup>, N<sup>+</sup>/P<sup>-</sup>/P<sup>+</sup>, 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<sup>+</sup>/N<sup>-</sup>/P<sup>+</sup> and N<sup>+</sup>/P<sup>-</sup>/P<sup>+</sup> HgCdTe architectures will be presented.
The High Operating Temperature Auger suppressed infrared detector concept is being pursued using the high density
vertically integrated photodiode (HDVIP®) architecture and an n<sup>+</sup>-p device structure. Dark current densities as low as 2.5
mA/cm<sup>2</sup> normalized to a 5 μm cutoff at 250K have been demonstrated on these diodes. These dark currents imply
minority carrier lifetimes in excess of 300μsec. 1/f noise in these devices arises from the tunneling of charge into the
passivation interface, giving rise to a modulation in the surface positive charge and hence to the width of the depletion
region in the p-side of the device and a modulation in the total dark current. The measured 1/f noise is in agreement with
the predictions of this model, with very low noise being observed when the lifetimes are high.
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
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.
Electron injection avalanche photodiodes in SWIR to LWIR HgCdTe show gain and excess noise properties indicative of a single ionizing carrier gain process. The result is an electron avalanche photodiode (EAPD) with "ideal" APD characteristics including near noiseless gain. This paper reports results obtained on long-wave, mid-wave, and short wave cutoff infrared HgCdTe EAPDs that utilize a cylindrical "p-around-n", front side illuminated, n+/n-/p geometry that favors electron injection into the gain region. These devices are characterized by a uniform, exponential, gain voltage characteristic that is consistent with a hole-to-electron ionization coefficient ratio, k, of zero. Gains of greater than 1000 have been measured in MWIR EAPDS without any sign of avalanche breakdown. Excess noise measurements on MWIR and SWIR EAPDs show a gain independent excess noise factor at high gains that has a limiting value less than 2. At 77 K, 4.3 μm cutoff devices show excess noise factors of close to unity out to gains of 1000. The excess noise factor at room temperature on SWIR EAPDs, while still consistent with the k = 0 operation, approaches a gain independent limiting value of just under 2. The k = 0 operation is explained by the band structure of the HgCdTe. Monte Carlo modeling based on the band structure and scattering models for HgCdTe predict the measured gain and excess noise behavior. A noise equivalent input of 7.5 photons at a 10 ns pulsed signal gain of 964, measured on an MWIR APD at 77 K, provides an indication of the capability of the HgCdTe EAPD.
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 [λ<sub>c</sub>(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, α<sub>PHOTO</sub> = 1.8 x 10<sup>-5</sup>. The value of α<sub>PHOTO</sub> is the same at both zero bias and 100 mV reverse bias. At 160 K, α<sub>PHOTO</sub> is slightly lower but still in the low 10<sup>-5</sup> range. Excess low-frequency noise measured at 140 K and 100 mV reverse bias in the dark has α<sub>DARK</sub> = 1.4 x 10<sup>-5</sup>. At 160 K and 100 mV reverse bias, α<sub>DARK</sub> is in the mid 10<sup>-5</sup> 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. α<sub>DARK</sub> = 1.85 x 10<sup>-3</sup> at -8.2 V. This ratio is two orders of magnitude greater than α<sub>PHOTO</sub>. 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, α<sub>DARK</sub> approximately α<sub>PHOTO</sub> in the low to mid 10<sup>-5</sup> 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.
This paper reports results obtained on mid-wave IR x equals 0.3 Hg<SUB>1-x</SUB>Cd<SUB>x</SUB>Te avalanche photodiodes (APDs) that utilize a cylindrical 'p-around-n' front side illuminated n+/n-/p geometry. This 'p-around-n' geometry favors electron avalanche gain. These devices are characterized by a uniform, exponential, gain voltage characteristic that is consistent with a hole to electron ionization ratio, k equals (alpha) <SUB>h</SUB>/(alpha) <SUB>e</SUB>, of zero. At 6 bias and 77 K, gains are typically near 50, and gains of over 100 have been measured at higher biases. Response times have been modeled and measured on these devices. The modeling indicates that the geometry and dimensions of the diode control the diffusion limited device bandwidth. Rise times of less than 0.35 nsec should be possible according to this analysis. TO dat 10 percent to 90 percent rise times as low as 1 nsec have been measured. The gain is approximately noiseless up to gains of over fifty which is consistent with insignificant hole ionization. The noiseless gain behavior reported here is inconsistent with the original theory of McIntyre that predicts an excess noise factor of 2 for the k equals 0 case. The explanation for these results will require application of the modified 'history dependent' theory for excess noise later proposed by McIntyre.