Optical characteristics of CMOS-fabricated MOSFET´s

A CMOS fabricated MOSFET in a floating p-well on a semiconducting n-type substrate is optically sensitive and can be used for photoreception in CMOS circuitry. Experimental results for the logarithmic dependence of threshold voltage V/SUB th/ and channel current I/SUB D/ on light intensity compare well with the theory. However, the floating p-well also causes I/SUB D/ and V/SUB th/ to depend on the drain voltage due to impact ionization, and allows parasitic bipolar junction transistors to become dominant when p-well potential exceeds 0.65 V or when source area is very large. Solutions are proposed to minimize these negative effects.