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

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 C
s
, the semiconductor space-charge capacitance, with reasonable accuracy the ratio of C
s
to C
I
, 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 C
s
when the ratio C
s
/C
r
is 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 C
s
/C
I
≥ 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.