Study Of HgCdTe E-APD Active/Passive Dual Mode Imaging ROIC

HgCdTe APDs (avalanche photo-diodes), as a new type of detector, have been demonstrated to be apromising path to Focal Plane Arrays for low flux application and3D active/passive dual mode imaging overthe past decade, because of the advantates of high gain, high sensitivity and high speed.For the ratio k=β/α of the hole ionization coefficient β to the electron ionization coefficient α of HgCdTee-APDs (electron-initiated APDs) is much less than unity, only the electrons are multiplied due to theavalanche effect. HgCdTe e-APDs working in linear mode, have an exponential M that reaches high values(10-1000) for low values of reverse bias (5~15V) and with an excess noise factor close to unity.This paper discusses the realization and principles of active/passive dual mode imaging with HgCdTe APDs.1. Equivalent small signal mode of HgCdTe APDs. HgCdTe APDs work at reverse-bias, and the gainnormalized dark current at77K increases exponentially with increasing reverse-bias voltage. Conventionalfirst order linear equivalent small signal model is no longer suited to APD. A second order model withbroken line model for HgCdTe APDs is designed.2. Active/passive dual mode imaging. A capacitive feedback transimpedance amplifier (CTIA) with threefeedback capacitors is designed as preamplifier. Two of the capacitors can be switched to offer four differentintegrating capacitances including900fF,600fF,400fF and100fF. Different integrating capacitance is usedto handle different imaging mode. When designing circuit at transistor-level, power consumption, noise, andpixel area lead design toward a simple one-stage cascade operational transconductance amplifier (OTA)3. Providing APDs high reverse bias. ROIC not only provide APDs high reverse bias, also protectingROIC not to be damaged by avalanche breakdown, when some pixels of APDs break down because ofheterogeneity of HgCdTe material, leading ROIC be shorted to the common voltage of APDs. Conventional5V CMOS process is applied to implement the high voltage protection with the small area other thanLDMOS in high voltage BCD process in the limited100um×100um pitch area.4. Defending module against oversaturation. The common voltage of n-on-p APDs is below the zero such as-8.5V. The input voltage of CTIA will decline quickly because of saturation, resulting in the forward bias ofsubstrate PN junction. The protection module can detect and control integrating of CTIA, maintaining inputvoltage of CTIA over the zero.The test chips are fabricated with0.5μm CMOS process. Analysis and test of the chips were completed.The results show that the every function modules can work correctly.