ransistor) One of two major categories of transistors; the other is the bipolar junction (BJT). FETs use a gate element that, when charged, creates an electromagnetic field that changes the conductivity of a silicon channel and turns the transistor on or off. FETs are fabricated as discrete components as well as by the billions on a single chip.
FET vs. BJT
FET-based silicon chips are easier to construct than their bipolar (BJT) counterparts. Although FETs switch slower, they use less power because once the gate terminal has been charged, no more current is needed to keep the transistor closed (see illustration below). While the current for one transistor may be negligible, it adds up when billions are switching simultaneously. The heat dissipated by BJT transistors also limits their number on a single chip.
MOSFET (Metal Oxide Semiconductor FET)
The most widely used FET is the MOSFET, which comes in n-channel NMOS and p-channel PMOS varieties. On a chip, NMOS and PMOS transistors are wired together in a complementary fashion to create CMOS logic, which is used in nearly every electronic device today. See CMOS
and n-type silicon
JFET (Junction FET)
The JFET is a MOSFET with a PN junction gate rather than poly-crystalline.
MESFET (Metal Semiconductor FET)
Similar to the JFET, the MESFET is used for microwave communications. The MESFET uses a Schottky metal gate and is made from gallium arsenide or indium phosphide, not silicon.
HEMT (High Electron Mobility Transistor)
Evolving from the MESFET, the HEMT and PHEMT (pseudomorphic HEMT) are used in high-frequency applications. HEMT varieties include MODFET (modulation doped FET), TEGFET (two-dimensional electron gas FET) and SDHT (selectively doped heterojunction transistor).
CHFET (Complementary Heterostructure FET)
Similar to CMOS, the CHFET is another high-frequency FET using gallium arsenide.
FET vs. Bipolar Junction (BJT)
Current Flow in a JFET
After the FET gate is initially charged, no more current is required to keep the transistor closed (turned on). In contrast, BJT gates require charging the entire time the transistor must be closed. See bipolar transistor
With no pulse on the JFET gate, current flows from source to drain. As electricity is applied to the gate, the depletion region grows, impeding current flow. Although illustrated at the electron level, this example is very simplified because there are trillions of electrons (e) flowing in a transistor.