An analysis of deep depletion thin-film MOS transistors

Consideration is given to the problems associated with the use of an insulated gate to obtain depletion in the channel of a field-effect transistor. It is shown that if an inversion layer forms at the insulator semiconductor interface before the channel is completely depleted complete pinch-off of drain current by the gate will not be observed. It is further shown that channel pinch-off at the drain will always occur and, hence, that drain current saturation will always be observed. A quantitative analysis based on the proposed model is performed and theoretical expressions for the device behavior are derived and plotted. The analysis of the device is divided into two ranges, a high-frequency range and a low-frequency range, where the dividing frequency is the frequency response of the surface inversion layer. This dividing frequency is generally orders of magnitude lower than the upper operating frequency limit of the transistor itself. Finally, the theoretical results are compared with experiment and shown to be in good agreement.