An MOS Based Logarithmic Photoreceptor Element Biased in a Subthreshold Region

Abstract

We studied three types of logarithmic photo-receptor elements, consisting of a diode connected MOSFET in series with a photodiode. The performance limits were established and the effects of the device configuration and the operation mode on the logarithmic nature of the response were experimentally verified. It was found that the photoreceptor element biased to a punchthrough mode had a wider dynamic range and output voltage swing than that obtained under subthreshold current mode operation. We measured the logarithmic response of the photoreceptor with a channel length of 3.3 μm and a dark current density of approximately 100 nA/cm². Measurement was conducted over a range of 10⁴ of incident light intensity with a sensitivity of 650 mV/ decade.