CIE Student Area

FETs Explained

Q. I have read the explanations of the operation of different kinds of FETS, but I am having trouble understanding them. Could you please explain them?

A. Recall that an electric field extends into space around a charged object and can attract or repel charged objects that are within the field.

The voltage applied to the gate of the FET gives it an electric charge, and the resulting fleld extends into the channel. The holes or electrons (depending on whether it is P- or N-type material) move in response the field on the gate. This is why it is called a “field-effect” transistor.

The greater the voltage on the gate, the stronger the field, and the more holes or electrons it can move. If the polarity of the field is the same as that of the charge carriers in the channel, the gate will repel them, and restrict their flow through the gate. The strength of the voltage on the gate thus determines how restricted the flow of current through the channel will be.

Where the polarity of the gate voltage is the same as that of the charge carriers in the channel, the current is controlled by repulsion, since like charges repel. This is known as the depletion mode, since the field on the gate depletes the channel of charge carriers.

The opposite effect is used in some IGFETS to attract charge carriers. This is known as the enhancement mode. In this type of operation, the number of charge carriers in the channel is increased (enhanced) by using an opposite polarity for the gate voltage. Since opposite charges attract, current flow in the channel is enhanced by the voltage on the gate.