Extra Class question of the day: field-effect transistors

August 24, 2012 Dan KB6NU

A field-effect transistor (FET) is a device that uses an electric field to control current flow through the device. Like the bipolar transistor, a FET normally has three terminals. The names of the three terminals of a field-effect transistor are gate, drain, source. (E6A17)

FETs are normally made with a technology called Complementary Metal-Oxide Semiconductor, or CMOS. The initials CMOS stand for Complementary Metal-Oxide Semiconductor. (E6A13) FETs made with CMOS technology are sometimes call MOSFETs.

In Figure E6-2 (below), schematic symbol 1 is the symbol for a P-channel junction FET. (E6A11) In Figure E6-2 (below), schematic symbol 4 is the symbol for an N-channel dual-gate MOSFET. (E6A10)

One characteristic of the MOSFET is that they have a high input impedance. This makes them more attractive for use in many test equipment applications than bipolar transistors. How does DC input impedance at the gate of a field-effect transistor compare with the DC input impedance of a bipolar transistor? An FET has high input impedance; a bipolar transistor has low input impedance. (E6A14)

One disadvantage of using MOSFETs is that they are very sensitive to electrostatic discharge (ESD). Sometimes, they are damaged by static discharges so low that you never even see the spark or feel the shock. To reduce the chance of the gate insulation being punctured by static discharges or excessive voltages many MOSFET devices have internally connected Zener diodes on the gates. (E6A12)

Most FETs are enhancement-mode devices. When using an enhancement-mode FET, you must apply a voltage to the gate to get current to flow from source to drain. Some FETs are, however, depletion mode devices. A depletion-mode FET is an FET that exhibits a current flow between source and drain when no gate voltage is applied. (E6A09)

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