A useful modification of the technique for measuring capacitance as a function of voltage

Abstract

Measurements of small signal capacitance as a function of applied bias voltage are widely used for the determination of information about metal-insulator-semiconductor (MIS) capacitors. The information that can be derived from the measurements includes interface-state density and flat-band charge density at the insulator-semiconductor (IS) interface, semiconductor doping, and charge stability under bias-temperature stress. A limitation on the use of this measurement method which has until now prevented its even more general application is the requirement that in order to determine Cs, the semiconductor space-charge capacitance, with reasonable accuracy the ratio of Csto CI, the insulating layer capacitance, must be ∼ 10. In the present work it is shown that a modification of the usual method can significantly relax this restriction and allow the accurate determination of Cswhen the ratio Cs/Cris as large as 100 or more, In fact, the inherent limit is no longer directly dependent on this ratio but on the noise level in the capacitance measurement. In some cases Cs/CI≥ 1 due to a thick insulating layer, A very large bias voltage is then required to span the capacitance range of interest; commercially available capacitance meters which typically have applied bias capabilities of ±600 V or less may be inadequate. A simple circuit modification has been employed to allow much larger bias voltages (up to ± 7 kV in the present Work) to be applied to the sample without alteration of or damage to the capacitance meter.