Theory of the MOS transistor in weak inversion-new method to determine the number of surface states

The drain current IDversus gate voltage VGof an MOST operating in weak inversion, and the influence of surface potential fluctuations on this characteristic have been studied before [1], [2]. The purpose of this paper is to derive an expression of the drain current IDversus the drain voltage VDfor devices with a channel length not smaller than 20 µm. It is demonstrated that the surface potential fluctuations do not affect the slope of the ID-VDcurve, whereas the density Nssof surface states strongly influences the slope for small drain voltages. This yields a simple and useful technique to determine Nsson MOS transistors.