Four-terminal operation of m.o.s. transistors

The charge induced in the channel of a silicon m.o.s.t. due to the capacitance of the depletion layer in the bulk material has a significant effect if the resistivity is less than about 10¿cm. With the substrate short-circuited to the source, the pinchoff voltage is reduced. The substrate provides a second control terminal, and 4-terminal operation is particularly useful if the fractional changes in voltage are limited so that the capacitance between the substrate and the channel is essentially constant. The drain current can be made to vary according to the sum of the signal voltages applied to the gate and the substrate. It is also possible to square this sum or to multiply it by a third signal voltage applied to both the drain and source in antiphase. With sinusoidal generators there is selective mixing.