R. K. Bhan

Virtual Ground Technique for Crosstalk Suppression in Networked Resistive Sensors

Two-dimensional resistive sensor arrays that utilize shared row and column connections to simplify the interconnect complexity suffer from the crosstalk problem among its elements introduced due to the interconnection overloading. In this letter, we present a method of overcoming the problem of crosstalk by putting all of the row nodes at virtually equal potential using virtual ground of high-gain operational amplifiers (opamps) in negative feedback. The circuit, though it requires a large number of opamps, solves the crosstalk problem to a large extent and provides faster scanning. We verified the circuit functionality with PSPICE simulations. We have also derived the expressions of crosstalk rejection and sensitivity to show that, by using high-gain, low-noise opamps, we may get excellent performance.

Analysis of Crosstalk in Networked Arrays of Resistive Sensors

In this paper, we present the analysis of the crosstalk limitation in 2-D resistive sensor arrays that utilize shared row and column connections for simplified interconnections. The mathematical expression for crosstalk among all of the elements introduced due to the interconnection overloading has been analytically derived and verified by the circuit simulations. Based on this analysis, we examined and proved that the solution of crosstalk and snapshot capability cannot be achieved simultaneously in networked resistive sensor arrays.

An analysis of the dynamic resistance variation as a function of reverse bias voltage in a HgCdTe diode

This paper reports an analytical approach to interpret the variation of dynamic resistance as a function of the reverse bias voltage in HgCdTe diodes. The method consists of estimating the trap density contributing to the trap assisted tunnelling in the given diode from the observed position of maxima in a dynamic resistance versus reverse bias voltage curve. Good agreement between theory and experiment is reported by using the estimated trap density in this way.

Carrier density approximation for non-parabolic and highly degenerate HgCdTe semiconductors

We propose simple, analytical, approximations of carrier density for HgCdTe semiconductors that have non-parabolic energy bands and are highly degenerate. The proposed expressions are in the form similar to the classical Boltzmanns approximation without any adjustable parameter. These relations can be applied to HgCdTe for the case where electron densities are very high and the material is highly degenerate, e.g. highly accumulated surface due to passivant induced negative fixed charge density in n-HgCdTe photoconductor device. The expressions are valid in the LWIR and MWIR bands, and can be tuned further to apply to other bands.

Effect of Excessive Bias Heating on a Titanium Microbolometer Infrared Detector

In a metal film infrared microbolometer, the responsivity is improved by high bias current to compensate for its low-temperature coefficient of resistance (TCR). However, what are the upper limits of this current without damaging the microbolometer element is not well understood. To study the effects of large bias current, we performed the destructive I-V measurements on an element of a 16 times 16 Ti-microbolometer array developed at our laboratory and report here the experimental observations of its electrical and physical damages. In this study, we performed the I-V measurements repeatedly on a microbolometer element and increased the final bias current in steps of 50 muA in each repetition. The effect of the heating due to I2R power dissipation has been analyzed at each step by monitoring I-V characteristics, specific detectivity and physical health. We report a significant decrease in the detectivity when bias stress is increased beyond 450 muA, which corresponds to the element temperature of 370degC. Further, we found that the resistance started decreasing, when the power dissipated and the element had increased to about 2.5 mW, resulting in a peaked I-V characteristics. This corresponds to the bias stresses more than 650 muA. Using a new I-V model, we extracted the temperature to be about 750degC at these peaks. A further increase in bias stress has resulted in the complete physical damage of the element.

Effect of built-in electric field on crosstalk in focal plane arrays using HgCdTe epilayers

A simple two-dimensional model, for calculating the effect of the built-in electric field arising in HgCdTe epilayers due to the composition gradient that occurs in LPE growth, on the crosstalk is developed for the case of a back illuminated focal plane array. The calculations predict that the crosstalk does not decrease monotonically with increasing electric field (i.e., weak peaks occur), unlike in the simple one-dimensional model of Kamins and Fong. The zero-field crosstalk varies between 1% and 10%, depending upon the material parameters. For electric fields in the range of 1–10 V cm−1, the crosstalk is reduced to negligible values (<0.01%). The model is approximate to the extent that the effect of the electric field in reducing the crosstalk is somewhat overestimated. The crosstalk depends strongly on the diffusion length and on the absorption coefficient, but very weakly on the diffusion velocity of carriers. The effects of diode size and dead space are similar to those of other models. The effect of the CdTe–HgCdTe interface recombination velocity has been taken into account, whereas that of the top surface recombination velocity has been neglected. The crosstalk to the second nearest neighbour diode is an order of magnitude smaller than that to the first neighbour diode. It goes to zero at a much smaller electric field. The crosstalk is shown to be slightly higher in medium wavelength IR arrays (MWIR) than in long-wavelength IR (LWIR) arrays.

Analysis of the effect of surface passivant charges on HgCdTe photoconductive detectors

The effect of fixed surface charge density on the performance of HgCdTe (MCT) photoconductive detectors is presented in this study. The figure of merit such as the specific detectivity of photoconductive detectors operating at 77 K is calculated with 300 K background and field of view (FOV). The equations leading to the above calculations are developed for the case of a one-dimensional model. Our calculations show that: (i) shows maxima with varying ; (ii) improvement in due to accumulation is observed for values of up to for materials with low as well as high bulk minority carrier lifetimes ; and (iii) is limited by shunt resistance due to passivants having for low- materials. Furthermore, is found to degrade by an order of magnitude from its peak value with increase in from to in a material having ns, whereas no such degradation in is seen for the same range of in a material having . In addition, our calculations predict that the accumulation layer improves by a factor of 1.5 for surfaces having an initial surface recombination velocity as high as .

Crosstalk suppression in networked resistive sensor arrays using virtual ground technique

In 2D resistive sensor arrays, the interconnections are reduced considerably by sharing rows and columns among various sensor elements in such a way that one end of each sensor is connected to a row node and other end connected to a column node. This scheme results in total N + M interconnections for N × M array of sensors. Thus, it simplifies the interconnect complexity but suffers from the crosstalk problem among its elements. We experimentally demonstrate that this problem can be overcome by putting all the row nodes at virtually equal potential using virtual ground of high gain operational amplifiers in negative feedback. Although it requires large number of opamps, it solves the crosstalk problem to a large extent. Additionally, we get the response of all the sensors lying in a column simultaneously, resulting in a faster scanning capability. By performing lock-in-amplifier based measurements on a light dependent resistor at a randomly selected location in a 4 × 4 array of otherwise fixed valued resistors, we have shown that the technique can provide 86 dB crosstalk suppression even with a simple opamp. Finally, we demonstrate the circuit implementation of this technique for a 16 × 16 imaging array of light dependent resistors.

Conduction, dielectric and interface properties of Al2O3 films on GaAs deposited by the e-beam evaporation technique

We report the results of resistivity, conduction mechanism, dielectric and interface properties of 750 and 1500 A thick Al2O3 films deposited by the e-beam evaporation technique for surface passivation of GaAs-based devices. The typical near-zero bias leakage currents were varying from 85 fA to 6 pA corresponding to a resistivity variation of 3.6 × 1011 to 8.8 × 1011 Ω cm for 750 A films. Similarly for 1500 A films, the corresponding currents were varying from 85 fA to 3 pA indicating a resistivity variation of 1.5 × 1011 to 6.6 × 1012 Ω cm. The 750 A films showed typically almost constant current densities of under 5 × 10−8 A cm−2 compared to varying (1.2–7.5)×10−8 A cm−2 for 1500 A films. These observations were further corroborated by conduction mechanism results. Although the 750 A films showed trends of Frenkel–Poole (FP) conduction for medium electric field strengths with lesser variation, the 1500 A films also showed FP conduction but with more variation and scatter in data. Additionally, we report the results for the estimates of dynamic dielectric constant estimated from the FP conduction dominated region. The average value for 750 A films was 2.17 compared to 4.43 for 1500 A films. The static dielectric constant, interface properties were measured by the capacitance–voltage (CV) technique. The results indicate that a stable interface is possible by using the e-beam evaporation technique contrary to reports available in the literature. The range of values (including the data of both the films) obtained for dielectric constant, fixed charge density and fast interface state density were 3.6–4.6, (0.7–1.4) × 1012 cm−2 and (0.5–1) × 1013 cm−2 eV−1 respectively for these films.

Effect of 300 K ambient background on C–V characteristics of HgCdTe MIS structures

Abstract We propose a simple model that describes the effect of 300 K ambient background on low-frequency (LF) capacitance–voltage characteristics of metal–insulator–semiconductor (MIS) structures based on HgCdTe materials. The materials used for both photoconductive (PC) and photovoltaic (PV) infrared detectors are investigated. We modify the solution of the one-dimensional Poisson’s equation and calculate LF capacitance including the effect of incident photon flux corresponding to the 300 K background. We identify the situations where this effect cannot be neglected. Based on our calculations, we find that in n-type materials used for medium-wave infrared (MWIR) PC devices, the minimum capacitance ( C min ) can increase by as much as 4.4%, whereas this increase is about 9.6% for materials used for the long-wavelength infrared (LWIR) devices, for experimentally achievable minority carrier lifetimes and carrier concentrations. Similarly, in p-type materials used for LWIR PV devices, there is a very small increase of 1.5% in C min , whereas there is no change in the MWIR region. The predictions of our model agree with experimental results and imply that the effect of 300 K ambient background radiation is very small compared to that of traps in p-type materials used for LWIR PV devices.