Subthreshold drain leakage currents in MOS field-effect transistors

There are two contributions to the drain-source leakage current in MOS field-effect transistors for gate voltages below the extrapolated threshold voltage (Vtx) : 1) reverse-bias drain junction leakage current, and 2) a surface channel current that flows when the surface is weakly inverted. Nearly six orders of magnitude of drain-source current from the background limit imposed by the drain junction leakage to the lower limits of detection of most curve tracers (0.05 µA) are controlled by gate-source voltages below the extrapolated threshold voltage. It is shown that this current flows only for gate voltages above the intrinsic voltage Vi, the gate voltage at which the silicon surface becomes intrinsic. For gate voltages between Viand Vtxthe surface is weakly inverted with the resulting channel conductivity being responsible for the drain-source current "tails" observed for gate voltages below Vtx. The importance of the intrinsic voltage in designing low-leakage CMOS and standard PMOS circuitry is discussed.