The effects of fixed bulk charge on the thermal noise in metal-oxide-semiconductor transistors

The theory of the thermal noise due to channel conductance fluctuation is extended for insulated gate (MOS) field effect transistors with the gate voltage induced channel structure by including the bulk charge from the ionized impurities in the semiconductor substrate. In the saturation range of the drain characteristics, the theory shows that
R_{gns} g_{ms} \geq 2/3
, where the equality condition corresponds to the previously obtained result for an intrinsic or chemically pure semiconductor substrate. Satisfactory correlations between theory and experimental measurements are obtained for both P-channel and N-channel silicon devices with either a thin oxide (2000A) or a thick oxide (6200 and 8400A) gate.