Field-effect transistor

Field-effect transistors or FET (English field - effect transistor) are a group of unipolar transistors in which, unlike the bipolar transistors only one type of charge to the electric power is involved - depending on the type: electrons or holes or defect electrons. They are at low frequencies - high- switched largely without loss or - in contrast to the bipolar transistors. The most widely used type of the field effect transistor is of the MOSFET ( metal-oxide- semiconductor field effect transistor).

Was discovered the principle of the field-effect transistor in 1925 by Julius Lilienfeld. At that time it was not yet possible, such a FET actually produce. Semiconductor material of the required purity as the starting material does not occur in nature and methods for producing high-purity semiconductor material were not known. To that extent, the special properties of semiconductors were not yet sufficiently researched. Only with the production of highly pure semiconductor crystals ( germanium ) in the early 1950s, this problem was solved. But only the silicon semiconductor technology (among thermal oxidation of silicon) in the 1960s were first laboratory model of the FET can be produced.


The first concrete description of a component with properties similar to those of an electron tube goes back to Julius Lilienfeld in 1925. At this time, however, lacked the necessary technologies to implement these proposals. In the aftermath, there are similar attempts by Joseph Weber ( 1930), Holst and Geal (1936 ) and especially Rudolf Hilsch and Robert Wichard Pohl (1938 ), the lattice of electron tubes in solids, recreate, especially in crystals, but also none of which realizations are known.

After Lilienfeld 1928 then notwithstanding, proposed a design and patented, which came very close to today's IGFET, designed the German physicist Oskar Heil in 1934 the first field effect transistor, which he also filed for a patent.

The following description of the first JFET with a pn junction as a control by Herbert Mataré, Heinrich Welker and parallel to William B. Shockley and Walter Brattain H. was back in 1945 and thus before the invention of the bipolar transistor in 1948. Due to the rapid progress, however, the one made with these transistors, and because field-effect transistors were not yet economically produced with the former technologies and the then state of knowledge, field-effect transistors have been used to in the 1960s not outside of laboratories. Only it because of problems with the bipolar transistors focused its attention from circa 1955 more closely at the semiconductor surfaces and developed manufacturing processes which brought the field-effect transistors for series production. This includes in particular the planar technology.


In contrast to the current-controlled bipolar field-effect transistors are voltage-controlled circuit elements. It is controlled by the gate -source voltage, which is used for regulating the passage cross -section, or the charge carrier density, i.e., the semiconductor resistor, in order to shift or to control the strength of an electric current.

The FET has three terminals:

  • Source ( English for " source ", " inflow " )
  • Gate (English for "gate", " gate " )
  • Drain ( English for " sink", " drain " )

When MOSFET also a fourth Bulk (substrate) connection is available. This is already connected with individual transistors internally connected to the source terminal and are not connected.

The control or gain of the current flow between the drain and source is done by selectively zooming in and out conducting and non-conducting regions of the semiconductor material ( substrate). The prior p- and n- doped semiconductor material is thus enriched by the applied voltage and the resulting electric field either impoverished or with charge carriers.

The crucial difference to the bipolar transistor circuit design is the virtually powerless at low frequencies driving the FET, it merely require a control voltage.

Another difference is the charge transport in the unipolar source-drain channel. This fact makes it possible, in principle a reverse operation of the FET, i.e., the drain and source can be exchanged. However, this only applies to very few FETs, because most types, inputs bulk and source are both asymmetrically constructed as internally connected. In addition, the unipolar channel can be used as bi-directional resistance, thereby affecting not only DC but also alternating current, which is used for example, in snubber circuits ( attenuator muting ).

Depending on the type of FET, different effects are used to control the conductivity of the regions. FETs also have a lower slope ΔIAusgang / ΔUsteuer than comparable bipolar transistors.

Due to the different properties of bipolar and field effect transistors, the bipolar transistor 1984 are (English Insulated Gate Bipolar Transistor, IGBT), developed on the basis of MISFETs with insulated gate. It represents a combination of field-effect transistor and bipolar transistor, but is limited in the area of ​​application to higher operating voltages.


→ Main article: JFET

The junction or junction field effect transistor (JFET or SFET ) of the current flow through the underlying flow channel between the drain and source by means of a barrier layer is controlled (see, pn-junction ) between the gate and the channel. This is possible because the extension of the barrier layer, that is the size of the zone having the opposite conductivity type of the channel material, depending on the gate voltage (see also the space charge region ).

Analogous to the field-effect transistor with insulated gate ( IGFET see MISFET ), the group of the junction field- effect transistors ( JFET) as NIGFET (English non insulated -gate field - effect transistor) that is referred to insulated gate field effect transistor no. We distinguish the following main field effect transistor types (excluding insulated gate NIGFETs ):

  • The junction field effect transistor ( JFET) high - electron -mobility transistor (HEMT )
  • Metal-semiconductor field effect transistor ( MESFET and the Schottky gate field effect transistor known )


→ Main article: MOSFET, the MISFET is currently most widely used

In a MISFET ( metal insulator semiconductor FET Sheet ) and IGFET ( insulated gate FET Sheet ), a metal-insulator- semiconductor structure is utilized to produce a conducting channel between source and drain by means of inversion. Here, with increasing voltage between gate and bulk or substrate first, the hole, that is, the former majority charge carriers, displaced and it is formed by charge carrier depletion a non-conductive area. If the voltage rises further, it comes to inversion, the p-type substrate under the gate is n-type and forms a channel between source and drain, the majority carriers are electrons now. In this way the voltage between the gate and the bulk controls the current flow between source and drain.

For technological reasons, here the material combination of silica - silicon has prevailed. Therefore took place in the early years of microelectronics, the term MOSFET widespread, and is still used today as a synonym for the more general term MISFET or even IGFET.

We distinguish the following main types of field-effect transistor ( insulated gate IGFET ):

  • Metal-insulator- semiconductor field effect transistor ( MISFET ) Metal -oxide-semiconductor field effect transistor (MOSFET)
  • Organic field effect transistor ( OFET)
  • The ion sensitive field effect transistor ( ISFET)
  • Enzyme field-effect transistor ( ENFET )
  • Electrolyte - oxide-semiconductor field - effect transistor ( EOSFET )

Types and circuit symbols

Basically four different types can be constructed of MOSFETs itself conducting and self-locking with a p- or n-channel. Commonly used for the labeling of dopants characters n and p are, however, not able to find a dopant (for example, for the channel ), but indicates the type of the majority charge carriers, that is charge carriers which are used for the transport of electrical current. Where n is electrons and p for electron holes as majority charge carriers.

When switching characters usually the circuit symbol shown opposite with the connections for the gate, source, drain and body / bulk ( central connector with arrow ) is used in German-speaking countries. The direction of the arrow marks on the body / bulk terminal, the channel - type, ie the Majoritätsladungsträgerart. Here, an arrow to the channel is an n -channel, and an arrow off the channel comprises a p-channel transistor. Whether the transistor is self-locking or self-conducting, in turn, is ( " are inverted channel must only " → enhancement type, self-blocking) via a dashed or a continuous ( " current can flow " → depletion mode normally-on ) represented channel line. In addition, however, other characters are especially common in the international environment in which the body / bulk terminal usually connected to the source is not displayed.

Areas of application

The use of different types of field effect transistors is mainly dependent on the demands of stability and noise performance. Basically, there are field-effect transistors for all applications, while maintaining the IGFET will tend to be used in digital technology, JFETs rather in the high frequency technology.

Power MOSFET are superior to bipolar transistors with respect to switching speed and losses especially at voltages up to 950 V (Super -Mesh -V technology ). They are therefore used in switching power supplies and switching regulators. Encouraged by the possible high switching frequencies (up to about 1 MHz) is smaller inductive components can be employed.

Furthermore, they are provided in the form of so-called "smart", ie, with built-in protection circuits, distributed power switches in the automotive sector. In addition, they are used as an RF power amplifier usually made in designs with special characteristics and housings. Class -D audio amplifier working in the PWM switching stages with MOSFETs.