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The use of MOCVD-grown wider-bandgap Hg 1−x Cd x Te as a capping layer for long-wavelength infrared (LWIR) Hg 1−x Cd x Te photoconductors has been studied using both theoretical and experimental results. A device model is derived which shows that in the presence of a suitable energy barrier between the Hg 1−x Cd x Te infrared absorbing layer and the overlaying passivation layer, the high surface recombination rate which is usually present at the semiconductor/passivant interface is prevented from having a significant effect on device performance. The energy barrier, which repels photogenerated minority carriers from the semiconductor surface, is introduced by employing an n-type Hg 1−x Cd x Te wafer which consists of a wider-bandgap capping layer that is grown in situ by MOCVD on an LWIR absorbing layer. The derived model allows the responsivity to be calculated by taking into account surface recombination at both the front and back interfaces, thickness of capping and absorbing layers, recombination at the heterointerface, and variations in equilibrium electron concentration. Calculations show that for an x = 0.22 Hg 1−x Cd x Te absorbing layer, the optimum capping layer consists of x ≥ 0.25 and a thickness of the order of 0.1 to 0.2 µm.Experimental results are presented for x = 0.22 n-type Hg 1−x Cd x Te conventional single-layer LWIR photoconductors, and for heterostructure photoconductors consisting of an LWIR absorbing layer of x = 0.22 capped by an n-type layer of x = 0.31. The model is used to extract the recombination velocities at the heterointerface and the semiconductor/substrate interface, which are determined to be 250 cm s −1 and 100 cm s −1 respectively. The experimental data clearly indicate that the use of a heterostructure barrier between the overlaying passivation layer and the underlying LWIR absorbing layer produces detectors that exhibit much higher performance and are insensitive to the condition of the semiconductor/passivant interface.

The performance of Hg,-,CdxTe infrared photoconductors is strongly dependent on the semiconductor surface conditions and, in particular, the degree to which the surface contributes to recombination of photogenerated excess carriers. Although published photoconductor fabrication processes based on bulk Hg,_,Cd,Te address this issue by fully passivating both major surfaces (i.e. front and back) with anodically grown native oxide, passivation of the sidewalls is neglected. In this paper it is shown both theoretically and experimentally that leaving the sidewalls unpassivated can result in approximately a factor of two reduction in responsivity for long-wavelength infrared (LWIR) detectors used in high-resolution thermal imaging systems. Detector arrays are typically fabricated on x = 0.23 Hg,_,Cd,Te representing a cut-off wavelength of 9.4 pm and use individual element sizes of approximately 50 x 50 &mZ.We describe in detail for the first time a device technology which enables the fabrication of Hg,-,Cd,Te photoconductor arrays such that the entire surface of the semiconductor is effectively passivated, including the sidewalls. Of particular interest is the fact that this improved device technology is compatible with present-day Hg,-,Cd,Te epitaxial growth processes. This is in contrast to current photoconductor technology which is primarily based on bulk Hg,_,Cd,Te. Experimental results are presented which compare device performance of LWIR detectors fabricated using the improved photoconductor technology with current published photoconductor technology. These results clearly indicate that detectors fabricated on liquid phase epitaxially (LPE) grown x = 0.23 Hg,_,CdxTe material using the improved photoconductor device technology achieve much higher responsivities and detectivities. Furthermore, it is shown that only a fully passivated device structure is capable of exploiting any future improvements in bulk minority carrier lifetime as it approaches the Auger recombination limit.

In this paper, a model is presented for predicting the phase modulation (PM) and amplitude modulation (AM) noise in bipolar junction transistor (BJT) amplifiers. The model correctly predicts the dependence of phase noise on the signal frequency (at a particular carrier offset frequency), explains the noise shaping of the phase noise about the signal frequency, and shows the functional dependence on the transistor parameters and the circuit parameters. Experimental studies on common emitter (CE) amplifiers have been used to validate the PM noise model at carrier frequencies between 10 and 100 MHz.

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