Modeling of MOSFET under Illumination for Optoelectronic Applications

Abstract

Monolithic integration of optics with Silicon electronics is essential to realize low cost and high performance interconnections. In this thesis, the focus of work was the development of a photodetector in standard CMOS process, to interact with light at a wavelength of 830 nm. An n-channel silicon MOSFET was chosen which would offer detection along with an inherent advantage of gain in a combined package. Thus the study of light-effect on MOSFET is important specifically at a wavelength of 830 nm. newlineThe thesis presents a detailed theoretical investigation of n-channel silicon MOSFET with multi-finger structure suitable at RF under varying optical illumination. The results of the study show that there is an increase in current with increase in illumination under AC and DC conditions. The change in parameters can be accounted for the alteration of the threshold voltage and hence the effective gate voltage of the device due to absorption of optical flux. Hence this device is referred as OG-MOSFET i.e. Optically Gated MOSFET. newlineA small signal model was developed to accommodate the secondary effects in the submicron dimension device. The variations in transconductances and capacitances with variations in voltage and light were evaluated, and signify the potential of the device as an optically controlled mixer. The device transconductance is found to have substantial increase with other admittance parameters remaining almost constant under illumination. Power gain analysis, figures of merit and noise performance indicate that the device will be useful as optically controlled amplifiers for high frequency operation. newlineIt has been shown for the first time that the conventional MOSFET structure in standard CMOS process is a promising candidate that can be used for detection at 830 nm wavelengths.