Small signal analysis of an infrared imaging device based on equivalent circuit model

Abstract

This paper presents an analytical model of an infrared thermal imaging device. This device is composed of a Quantum Well Infrared Photodetector (QWIP), a Heterojunction Bipolar transistor (HBT) and a Light Emitting Diode (LED). It is called as QWIP-HBT-LED Optoelectronic Integrated Device. The device is modeled based on its equivalent circuit by considering a nonlinear gain HBT, early effect. Analytical expressions describing the current time response, rise time, and output derivative as a measure of device speed have been derived. The numerical results show that the transient performance of this device version is enhanced by the injected current from QWIP to the base of HBT, also the output current is increased with the increase of the gain and early coefficient of HBT, on the other hand, it degrades when the base recombination factor of HBT or the load resistance is increased. Also, the rise time increases when the current gain or the early coefficient is increased. This type of models can be exploited as a pixel in thermal image processing applications.