IAN LANG ELECTRONICS
The p Channel Enhancement MOSFET
The MOSFET is available as a discrete device and when first invented they were known as the insulated gate field effect transistor, because the gate was (and still is) insulated from the drain and source.
(Those of you more familiar with the bi-polar junction transistor will know about collector, base and emitter, drain, gate and source are the equivalent terms in a FET.
The symbol for a p channel enhancement device as a discrete component is shown below:
The symbol suggests that the gate is insulated from the drain and the source and this is the way of working of the device. It works in much the same way as a PNP bipolar junction transistor does in that the more negative the gate, the more current can flow, however the mechanics of this are somewhat different. The drawing below is representative of a p channel enhancement mode device such as may be found on an integrated circuit
In effect two p-n junctions are formed in the transistor , the gate is insulated by a layer of silicon dioxide and when the gate is made negative with respect to the n type substrate minority carriers are attracted to the channel between the two p-type sections whilst majority carriers are expelled. When the gate is sufficiently negative the carriers (in this case holes) cause a conducting channel between the source and drain whereby current is able to flow. Because the transistor can now be considered a resistive device and since Ohm's law tells us V=IR as more current flows, so more voltage.
As the gate varies between more and less positive, the depth of the p channel alters accordingly, in turn altering the current between source and drain. In this way it can be used as an amplifier or a switch. Unlike the BJT, the FET uses only one carrier, in this case the holes (but if it were an n type device it would be electrons) and so it is a unipolar device. It can also be considered to be voltage, rather than current controlled.