Depletion-Type FETs

Schematic of a p-channel enhancement-type insulated gate MOSFET. The circuit symbol is shown to the right. The arrow is reversed for hannel. 

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A nonzero Ron is the only resistance preventing perfect efficiency if L and C in Fig. 7.62 are considered ideal. In reality, power efficiency near 98% is possible with very low on-resistance depletion type FETs. Cbiock is necessary because the current through Q1 has a DC component. 

FETs can be categorized as p-channel or n-charmel depending on which carriers are used and as enhancement-type, which are nonconductive in their normal state, and depletion type, which are conductive in their normal state. We will consider the enhancement-type first. 

Although the two choices each of type (JFET, MOSFET) of channel (n-channel, p-channel) and doping (enhancement, depletion) could yield eight choices, in practice only five main FET types exist ... 

Figure 25.20 (a) A schematic picture and fabric circuit diagram showing a classical circuit diagram design of an inverter, based on a p-type depletion FET, and three resistors, where R2 + R3 = Rl. (b) Optical micrograph of a fabric/mesh constructed from the design of a. 

The most widespread types are depleted ones. Control of such MIS-FETs can be brought about by varying Uqs from negative to positive values. 

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