Characterization of depletion mode MOSFET's

Simple models have been used for the depletion mode MOSFET which consider the device to be identical to the well studied enhancement mode transistor with a negative threshold voltage. Closer examination shows this model to be inadequate to accurately predict the performance of the device in many cases. In particular this paper will show that the threshold of a depletion mode device (DMD) is a function not only of the drain and substrate bias along with physical dimensions of the device as for the enhancement mode device, but is also a function of the mode of operation, be it linear or saturated. This effect is shown to be caused by a change in conduction mechanism from surface to buried-channel conduction which is present in the DMD due to the threshold adjust implant. Both experimental and computer modeled results will be presented which show this effect to be present even for long, wide channels at room temperature. Methods for characterizing these devices to adequately predict their behavior will be presented.