Field effect transistor

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Field effect transistors ( FETs ) are a group of transistors in which, in contrast to bipolar transistors, only one type of charge is involved in the electric current - depending on the type: electrons or holes or defect electrons . At low frequencies - in contrast to the bipolar transistors - they are switched largely without power or loss. The most common type of field effect transistor is the MOSFET (metal-oxide-semiconductor field effect transistor).

Connections and doping in the substrate of an n-channel MOSFET

The principle of the field effect transistor was discovered in 1925 by Julius Lilienfeld . At that time, however, it was not yet possible to actually manufacture such an FET. Semiconductor material of the necessary purity as a starting material does not occur in nature and methods for producing high-purity semiconductor material were not yet known. In this respect, the special properties of semiconductors have not yet been adequately researched. This problem was only solved with the production of high-purity semiconductor crystals ( germanium ) in the early 1950s. But only the silicon - semiconductor technology (.. And a thermal oxidation of silicon ) in the 1960s, the first laboratory prototype could be made of the FET.


Laboratory tests

The first concrete description of an unheated component with properties similar to an electron tube goes back to Julius Lilienfeld in 1925. At that time, however, there was a lack of technology to implement these proposals. In 1928, Lilienfeld received a patent for a design that came close to today's IGFET , and there were similar attempts by Joseph Weber in 1930.

In 1934, the German physicist Oskar Heil applied for a patent for the first field effect transistor. Further attempts were made by Holst and Geal in 1936 and by Rudolf Hilsch and Robert Wichard Pohl in 1938, but no realizations are known.

The description of the first JFET with pn junction by Herbert Mataré , Heinrich Welker and parallel to it William B. Shockley and Walter H. Brattain took place in 1945 before the invention of the bipolar transistor in 1948. Until the 1960s, field effect transistors were only available in laboratories for technological reasons .

Series production

Due to problems with bipolar transistors that initially still occurred, more in-depth research into semiconductor surfaces and the development of manufacturing processes that brought the first field effect transistors to series production began around 1955 . Several scientists and engineers did pioneering work here, u. a. the South Korean Dawon Kahng and the Egyptian Martin M. Atalla . Her work at Bell Telephone Laboratories resulted in several patents from 1960. The first patent application in German-speaking countries for the manufacture of field-effect transistors ready for series production took place on May 19, 1961 at the DPMA with the title: Semiconductor device (later called reinforcing semiconductor component).

Today's manufacturing processes for field effect transistors include, in particular, planar technology and FinFET technology.


The basic principle

In contrast to the current- controlled bipolar transistors , field effect transistors are voltage- controlled circuit elements. The control takes place via the gate-source voltage, which is used to regulate the channel cross-section or the charge carrier density , i.e. H. the semiconductor - resistance so the strength of, the electric current to switch or control.

The FET has three connections:

  • Source ( English for "source", "inflow")
  • Gate (English for "gate", "gate") - the control electrode
  • Drain (English for "sink", "drain")

The MOSFET also has a fourth bulk connection (substrate). In the case of single transistors, this is already connected internally to the source connection and is not connected separately.

The control or amplification of the current flow between drain and source is done by targeted enlargement and reduction of conductive and non-conductive areas of the semiconductor material (substrate). The semiconductor material , which is p- and n- doped in advance , is either depleted or enriched with charge carriers by the applied voltage or the resulting electric field .

The decisive circuit- related difference to the bipolar transistor is the fact that the FET is actuated with practically no power at low frequencies; only a control voltage is required.

Another difference is the charge transport in the unipolar source-drain channel. This fact enables, in principle, an inverse operation of the FET, i. i.e., drain and source can be exchanged. However, this only applies to very few FETs, because most types are both asymmetrical and the bulk and source connections are internally connected. In addition, the unipolar channel can be used as a bidirectional resistor and thus affect not only direct, but also alternating currents, which z. B. is used for damping circuits (attenuator, muting).

Depending on the type of FET, different effects are used to control the conductivity of the areas. FETs also have a lower slope Δ I output / Δ U control over comparable bipolar transistors.

Due to the different properties of bipolar and field effect transistors, the bipolar transistor with an insulated gate electrode (English insulated-gate bipolar transistor , IGBT) was developed in 1984 on the basis of MISFETs . It is a combination of field effect transistor and bipolar transistor, but is limited to higher operating voltages.

Junction field effect transistor (JFET)

Scheme of an n-channel JFET

When barrier layer or junction field effect transistor (JFET or SFET) the current flow through between the drain and source lying flow channel by using a barrier layer (see FIG. Pn junction , engl. Junction ) between the gate and the channel controlled. This is possible because the expansion of the barrier layer and thus the size of the constriction of the current channel depends on the gate voltage (see also space charge zone ).

Analogous to the insulating-layer field-effect transistor (IGFET, MISFET, MOSFET), the group of junction field-effect transistors (JFET) is also known as NIGFET ( non-insulated-gate field-effect transistor ), i.e. field-effect transistor without an insulated gate . A distinction is essentially made between the following types of field effect transistors (without insulated gate, NIGFETs):

Insulated gate field effect transistor (IGFET, MISFET, MOSFET)

Scheme of an n-channel MOSFET (with a conductive channel already formed between source and drain )

Main article: MOSFET , currently the most widely used insulated gate field effect transistor

In an insulating layer field effect transistor (IGFET, from English insulated gate FET , also called field effect transistor with insulated gate ), an electrically non-conductive layer separates the control electrode ( gate ) from the so-called channel, the actual semiconductor area in which the transistor current later flows between source and drain . The usual structure of such a transistor consists of a control electrode made of metal, an electrically insulating intermediate layer and the semiconductor, i.e. a metal-insulator-semiconductor structure . Transistors of the structure are therefore called metal-insulator-semiconductor (MISFET, English metal insulator semiconductor FET ) or - if an oxide is used as a non-conductor - metal-oxide-semiconductor field effect transistor (MOSFET, English metal oxide semiconductor FET ).

The current flow in the channel is controlled via the electrical potential at the gate, more precisely the voltage between the gate and the bulk or substrate. The gate potential influences the concentration of the types of charge carriers ( electrons , holes ) in the semiconductor, cf. Inversion , and enables or prevents the flow of current between source and drain depending on the structure. For example, in an enhancement type n-channel IGFET, as the voltage increases , the holes, i.e. H. the former majority charge carriers are displaced and a non-conductive area is formed due to charge carrier depletion. If the voltage continues to rise, inversion occurs, the p-doped substrate below the gate becomes n-conductive and forms a conductive channel between the source and drain , the majority of which are now electrons. In this way, the voltage between gate and bulk controls the flow of current between source and drain .

For technological reasons, the material combination silicon dioxide - silicon has established itself here . That is why the term MOSFET was widely used in the early years of microelectronics and is still used today as a synonym for the more general designation MISFET or even IGFET.

A distinction is made between the following types of field effect transistors (with insulated gate, IGFETs):

Types and symbols

Basic types of field effect transistors

Basically four different types of MOSFETs can be constructed, normally on and normally off with a p- or n-channel. The symbols n and p, which are usually used to identify doping, do not stand for doping (for example for the channel), but rather identify the type of majority charge carriers, i.e. the charge carriers that are used to transport the electrical current. Here n stands for electrons and p for holes as majority charge carriers.

In German-speaking countries, the symbols shown on the left with the connections for gate, source, drain and body / bulk (central connection with arrow) are usually used as symbols . The direction of the arrow on the body / bulk connection indicates the type of channel, i.e. the type of majority charge carrier. Here, an arrow to the channel indicates an n-channel transistor and an arrow away from the channel indicates a p-channel transistor. Whether the transistor is normally-off or normally-on is shown by a dashed ("channel must first be inverted" → enrichment type, normally-off) or a continuous ("current can flow" → depletion type, normally on) channel line. In addition, especially in the international environment, other characters are also common in which the body / bulk connection usually connected to source is not shown.

Basic circuits

As with bipolar transistors with their basic emitter, collector and base circuits, there are basic circuits in FETs in which one of the connections is connected to ground and the other two function as an input or output.

application areas

The use of the different designs of the field effect transistors is primarily dependent on the demands on stability and noise behavior. Basically there are field effect transistors for all areas of application, but IGFETs are used more in digital technology, JFETs more in high-frequency technology .

Power MOSFETs are superior to bipolar transistors in terms of switching speed and losses, especially at voltages up to approx. 950 V (super mesh V technology). They are therefore used in switching power supplies and switching regulators. Because of the high switching frequencies (up to approx. 1 MHz) that this makes possible, smaller inductive components can be used.

They are also widespread in the automotive sector in the form of so-called “intelligent” circuit breakers, that is, with integrated protective circuits . In addition, they are used as RF power amplifiers, mostly manufactured in designs with special characteristics and housings. Class D audio amplifiers work with MOSFETs in the PWM switching stages.

See also


  • Reinhold Paul: MOS field effect transistors . Springer, Berlin 2002, ISBN 3-540-55867-5 .
  • Stefan Goßner: Basics of electronics (semiconductors, components and circuits) . 11th edition Shaker, 2019, ISBN 978-3-8440-6784-2 .

Web links

Commons : Field-Effect Transistors  - Collection of images, videos and audio files
Wiktionary: field effect transistor  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. GK Teal, JB Little: Growth of germanium single crystals . In: Phys. Rev. Band 78 , 1950, pp. 647 , doi : 10.1103 / PhysRev.78.637 (Proceedings of the American Physical Society; Minutes of the Meeting at Oak Ridge, March 16-18, 1950).
  2. D. Kahng: A historical perspective on the development of MOS transistors and related devices . In: Electron Devices, IEEE Transactions on . tape 23 , no. 7 , 1976, p. 655-657 , doi : 10.1109 / T-ED.1976.18468 .
  3. SM Sze, Kwok Kwok Ng: Physics of semiconductor devices . John Wiley and Sons, 2007, ISBN 978-0-471-14323-9 ( limited preview in Google Book Search).
  4. Patent US1745175 : Method and Apparatus For Controlling Electric Currents. Registered on October 22, 1925 , inventor: JE Lilienfeld.
  5. Reinhold Paul, field effect transistors - physical principles and properties. Verlag Berliner Union [u. a.], Stuttgart 1972, ISBN 3-408-53050-5 .
  6. Patent GB439457 : Improvements in or relating to electrical amplifiers and other control arrangements and devices. Inventor: Oskar Heil (registered in Germany on March 2, 1934).
  7. ^ Bo Lojek: The history of semiconductor engineering . Jumper. Berlin / Heidelberg, 2007, ISBN 978-3-540-34257-1 , p. 321 f.
  8. Patent DE1439921A : Semiconductor device. Filed May 19, 1961 , published November 28, 1968 , inventor: Dawon Kahng (Priority: US3102230, filed May 19, 1960).
  9. a b cf. Michael Reisch: Semiconductor components . Springer, 2007, ISBN 978-3-540-73200-6 , pp. 219 ( limited preview in Google Book search).
  10. Heinz Beneking: field effect transistors . Springer Verlag, Berlin 1973, ISBN 3-540-06377-3 .