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Most of the logic devices used in highly integrated circuits do not use bipolar transistors. Instead, they use field effect transistors. FETs perform a similar function to the bipolar transistors discussed earlier, but they are voltage controlled.
While the current flowing in the base controls bipolar transistors, the voltage between the gate and source controls field effect transistors. The gate voltage of a field effect transistor controls the current flowing in the drain-source circuit. The symbol for the FET shows the gate to be insulated from the source-drain circuit, as shown in figure above.
This type of FET is referred to as a MOSFET (metal oxide semiconductor FET), since the insulating material is silicon dioxide (SiO2), commonly known as glass (for early devices, the gate was made of metal). Like bipolar NPN and PNP transistors with opposite polarity, FETs come in N- and P- channel varieties. The N- and P- channels refer to the polarity of the source drain element of the device. A cross-section view of a FET is shown in figure bellow.
