Temperature dependence of n-type MOS transistors

Ionization of surface states with increasing temperature is shown to be responsible for the positive temperature coefficient of the drain current often observed in
n
-type silicon MOS transistors. Competition of this effect with a decrease in mobility for increasing temperature is demonstrated to yield transistors with negative, positive, or zero temperature coefficients. The reflection of surface state ionization with increasing temperature as a linear decrease in gate voltage (for constant drain current) is theoretically explained. Since this linear decrease in gate voltage is a direct function of surface state density, a new method for determining the surface state density near the conduction band is developed. For many transistors gate voltage decreased about 40 millivolts per degree centigrade increase in temperature. This corresponds to a surface state density on the order of 10
¹³
per square centimeter per electron volt.