A Guarded Insulated Gate Field Effect Electrometer
An investigation has been made of a new type of insulated gate field effect transistor (IGFET), in which the gate lead is brought through the header in a guarded configuration. Measurements of gate leakage current with the drain connected to the source on this guarded IGFET show that its gate leakage current is less than 1/10 that of the conventional device, demonstrating that most of the leakage in an IGFET is across the header. When the guarded IGFET is operated in the active region, it is possible to set the operating conditions such that the gate leakage current cannot be measured on the best vibrating reed electrometers. Models of the guarded IGFET which explain the effect of bias on gate leakage current are presented. Others have shown that the IGFET, under proper bias, exhibits a zero temperature coefficient. It is shown how to simultaneously satisfy the bias conditions for minimum gate leakage current and zero temperature coefficient, as well as eliminate drift with time. Biased in this manner, a single guarded IGFET electrometer, which can be battery operated, was built. Its current sensitivity is better than 10-17 amperes.