A solid state color image sensor using ZnSe-Zn1-xCdxTe heterojunction thin-film photoconductor

Abstract

A 413H X 506V BBD* image sensor structure using a ZnSe-Znl-,Cd,Te heterojunction thin-film photoconductor for color imaging applications will be described. The device, with its photoconductive-layer-on-solid-scanner (PLOSS), can extend the aperture of the photoconductor to the scanning circuit area to obtain the high blue sensitivity of the heterojunction diode. Low light level sensing of a color image is thus possible. Blooming suppression is also available. The unit cell of the sensor, which consists of a thin-film photoconductor, a read-gate FET and a bucket-brigade transfer gate FET is shown schematically in Figure 1. The heterojunction film which is composed of ZnSe-Znl-,Cd,Te multilayer is vacuum-deposited on these FETs. A transparent electrode of IT0 film is deposited on the Znl-xCdxTe layer for reverse biasing of the heterojunction diode. The metal electrodes for ZnSe layer make contact with the N+ diffused source area of the read-gate FET to read a photogenerated charge from the reverse-biased heterojunction diode into the transfer stage of BBD-shift register through with the color filter located on the sensor within lop gap. The color difference signals (R-G, Cy-G) werc derived directly from the device at line sequential via a color filter and then encoded into an NTSC signal. The device has an acceptable color image quality under a low light ‘scene illumination level of 500 I x (F:2.0) which is 50% of the saturation illumination. In this state the SN is 43dB. Highlight exposure for a 10% spot of the imaging area was up to 120 times as intense as the saturation exposure within 5% blooming for the saturation signal.