TABLE OF CONTENTS Strictly speaking, the drain current models developed in the previous sections are valid for enhancement devices only. However, in SPICE these models have been used for depletion type devices also simply by changing the sign of the threshold voltage as was pointed out in section 5.2.2. If the depletion device is used only as a load element (source and gate tied together) in a circuit, then this zero order model is quite satisfactory. However, for device to be used in a more general configuration requires a separate model. Although a general model, similar to the charge sheet model for the enhancement devices, has been developed [35] but it will not be discussed here due to its complexity. Moreover, such models are not very suitable for circuit simulators. Here we will discuss only piece-wise models that are normally used for circuit simulations [36] [45].
Recall that depletion devices have a deep channel implant which is of opposite type to that of the substrate, thereby forming a pn junction underneath the gate. Unlike the enhancement devices, the depletion devices conduct even at zero V gs and have many modes of operation depending upon the applied gate and drain voltages, channel doping concentration and implant depth [38] [45]. These different modes of operation are named according to the condition at the silicon surface. Thus, if the entire surface is accumulated, depleted or inverted, the device is said to be operating in accumulation, depletion or inversion mode, respectively,...
