Latest revision as of 17:01, 6 September 2024

Field-effect transistor (FET) is a transistor that makes use of the field effect in order to control the flow of electrons. The most common type of FET is the MOSFET (metal-oxide-semiconductor field-effect transistor).

Overview[edit]

Cross-sectional view of a field-effect transistor, showing source, gate and drain terminals

The field-effect transistor (FET) is a type of transistor which uses an electric field to control the flow of current. FETs are devices with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and source.

FETs are also known as unipolar transistors since they involve single-carrier-type operation. That is, FETs use electrons or holes as charge carriers in their operation, but not both. Many different types of field effect transistors exist. Field effect transistors generally display very high input impedance at low frequencies. The most widely used field-effect transistor is the MOSFET (metal-oxide-semiconductor field-effect transistor).

FETs can be majority-charge-carrier devices, in which the current is carried predominantly by majority carriers, or minority-charge-carrier devices, in which the current is mainly due to a flow of minority carriers.^[1] The device consists of an active channel through which charge carriers, electrons or holes, flow from the source to the drain. Source and drain terminal conductors are connected to the semiconductor through ohmic contacts. The conductivity of the channel is a function of the potential applied across the gate and source terminals.

The FET's three terminals are:^[2]

1. source (S), through which the carriers enter the channel. Conventionally, current entering the channel at S is designated by I_S.
2. drain (D), through which the carriers leave the channel. Conventionally, current entering the channel at D is designated by I_D. Drain-to-source voltage is V_DS.
3. gate (G), the terminal that modulates the channel conductivity. By applying voltage to G, one can control I_D.

History[edit]

Further information: History of the transistor

The concept of a field-effect transistor (FET) was first patented by Austro-Hungarian physicist Julius Edgar Lilienfeld in 1925 and by Oskar Heil in 1934, but they were unable to build a working practical semiconducting device based on the concept. The transistor effect was later observed and explained by John Bardeen and Walter Houser Brattain while working under William Shockley at Bell Labs in 1947, shortly after the 17-year patent expired. Shockley initially attempted to build a working FET, by trying to modulate the conductivity of a semiconductor, but was unsuccessful, mainly due to problems with the surface states, the dangling bond, and the germanium and copper compound materials. In the course of trying to understand the mysterious reasons behind their failure to build a working FET, this led to Bardeen and Brattain instead building a point-contact transistor in 1947, which was followed by Shockley's bipolar junction transistor in 1948.^[3][4]

The first FET device to be successfully built was the junction field-effect transistor (JFET).^[3] A JFET was first patented by Heinrich Welker in 1945.^[5] The static induction transistor (SIT), a type of JFET with a short channel, was invented by Japanese engineers Jun-ichi Nishizawa and Y. Watanabe in 1950. Following Shockley's theoretical treatment on the JFET in 1952, a working practical JFET was built by George F. Dacey and Ian M. Ross in 1953.^[6] However, the JFET still had issues affecting junction transistors in general.^[7] Junction transistors were relatively bulky devices that were difficult to manufacture on a mass-production basis, which limited them to a number of specialised applications. The insulated-gate field-effect transistor (IGFET) was theorized as a potential alternative to junction transistors, but researchers were unable to build working IGFETs, largely due to the troublesome surface state barrier that prevented the external electric field from penetrating into the material.^[7] By the mid-1950s, researchers had largely given up on the FET concept, and instead focused on bipolar junction transistor (BJT) technology.^[8]

William Shockley, John Bardeen and Walter Brattain envisioned the FET concept in 1945, but they were unable to build a working device. The next year, Bardeen explained his failure in terms of surface states. Bardeen applied the theory of surface states on semiconductors (previous work on surface states was done by Shockley in 1939 and Igor Tamm in 1932) and realized that the external field was blocked at the surface because of extra electrons which are drawn to the semiconductor surface. Electrons become trapped in those localized states forming an inversion layer. Bardeen's hypothesis marked the birth of surface physics. Bardeen then decided to make use of an inversion layer and use it instead of a very thin layer of semiconductor which Shockley envisioned in his FET designs. Based on his theory, in 1948 Bardeen patented an insulated-gate FET (IGFET) with an inversion layer. The inversion layer confines the flow of minority carriers, increases modulation and conductivity, although its electron transport depends on gate's insulator or quality of oxide if used as an insulator, deposited above the inversion layer. Bardeen's patent as well as the concept of inversion layer forms the basis of CMOS technology today. In 1976 Shockley has described Bardeen's surface state hypothesis "as one of the most significant research ideas in the semiconductor program".^[9]

After Bardeen's surface state theory the trio tried to overcome the effect of surface states. In late 1947, Robert Gibney and Brattain suggested the use of electrolyte placed between metal and semiconductor to overcome the effects of surface states. Their FET device worked, but amplification was poor. Bardeen went further and suggested to rather focus on the conductivity of the inversion layer. Further experiments led them to replace electrolyte with a solid oxide layer in the hope of getting better results. Their goal was to penetrate the oxide layer and get to the inversion layer. However, Bardeen suggested they switch from silicon to germanium and in the process their oxide got inadvertently washed off. They stumbled upon completely different transistor, the point-contact transistor. Lillian Hoddeson argues that "had Brattain and Bardeen been working with silicon instead of germanium they would have stumbled across a successful field effect transistor".^{[9][10][11][12][13]}

In 1955, Ian Munro Ross filed a patent for a FeFET or MFSFET. Its structure was like that of a MOSFET, but ferroelectric material was used as a dielectric/insulator instead of oxide. He envisioned it as a form of memory, years before the floating gate MOSFET. In February 1957, John Wallmark filed a patent for FET in which germanium monoxide was used as a gate dielectric, but he didn't pursue the idea. In his other patent filed the same year he described a double gate FET. In March 1957, in his laboratory notebook, Ernesto Labate, a research scientist at Bell Labs, conceived of a device similar to the later proposed MOSFET, although Labate's device didn't explicitly use silicon dioxide as an insulator.^{[14][15][16][17]}

Metal-oxide-semiconductor FET (MOSFET)[edit]

Main article: MOSFET

Mohamed M. Atalla invented the MOSFET (MOS field-effect transistor) in 1959

A breakthrough in FET research came with the work of Egyptian engineer Mohamed Atalla in the late 1950s.^[4] He investigated the surface properties of silicon semiconductors at Bell Labs, where he adopted a new method of semiconductor device fabrication, coating a silicon wafer with an insulating layer of silicon oxide, so that electricity could reliably penetrate to the conducting silicon below, overcoming the surface states that prevented electricity from reaching the semiconducting layer. This is known as surface passivation, a method that became critical to the semiconductor industry as it made mass-production of silicon integrated circuits possible.^[18][19] Building on his surface passivation method, he developed the metal–oxide–semiconductor (MOS) process,^[18] which he presented in 1957.^[20] He later proposed the MOS process could be used to build the first working silicon FET, which he began working on building with the help of his Korean colleague Dawon Kahng.^[18]

The metal–oxide–semiconductor field-effect transistor (MOSFET) was invented by Mohamed Atalla and Dawon Kahng in 1959.^[21][22] The MOSFET largely superseded both the bipolar transistor and the JFET,^[3] and had a profound effect on digital electronic development.^[23][22] With its high scalability,^[24] and much lower power consumption and higher density than bipolar junction transistors,^[25] the MOSFET made it possible to build high-density integrated circuits.^[26] The MOSFET is also capable of handling higher power than the JFET.^[27] The MOSFET was the first truly compact transistor that could be miniaturised and mass-produced for a wide range of uses.^[7] The MOSFET thus became the most common type of transistor in computers, electronics,^[19] and communications technology (such as smartphones).^[28] The US Patent and Trademark Office calls it a "groundbreaking invention that transformed life and culture around the world".^[28]

CMOS (complementary MOS), a semiconductor device fabrication process for MOSFETs, was developed by Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor in 1963.^[29][30] The first report of a floating-gate MOSFET was made by Dawon Kahng and Simon Sze in 1967.^[31] A double-gate MOSFET was first demonstrated in 1984 by Electrotechnical Laboratory researchers Toshihiro Sekigawa and Yutaka Hayashi.^[32][33] FinFET (fin field-effect transistor), a type of 3D non-planar multi-gate MOSFET, originated from the research of Digh Hisamoto and his team at Hitachi Central Research Laboratory in 1989.^[34][35]

See also[edit]

• Chemical field-effect transistor
• CMOS
• FET amplifier
• FinFET
• FlowFET
• Multigate device

References[edit]

External links[edit]

• PBS The Field Effect Transistor
• How Semiconductors and Transistors Work (MOSFETs) WeCanFigureThisOut.org
• Junction Field Effect Transistor
• CMOS gate circuitry
• Winning the Battle Against Latchup in CMOS Analog Switches
• Field Effect Transistors in Theory and Practice
• The Field Effect Transistor as a Voltage Controlled Resistor
• The FET (field effect transistor)