Theory of the M. O. S. transistor in weak inversion - new method to determine the number of surface states

The drain current IDversus gate voltage VGof a MOST operating in weak inversion, and the influence of surface potential fluctuations on this characteristic have been studied before [1] The purpose of this paper is to derive an expression of the drain current IDversus the drain voltage VDfor long channel devices. It is demonstrated that the surface potential fluctuations don´t 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 M.O.S. transistors.