Field-effect transistor

The field-effect transistor (FET) is a transistor that uses an electric field to control the shape and hence the electrical conductivity of a channel of one type of charge carrier in a semiconductor material. FETs are also known as unipolar transistors as they involve single-carrier-type operation. The FET has several forms, but all have high input impedance. While the conductivity of a non-FET transistor is regulated by the input current (the emitter to base current) and so has a low input impedance, a FET's conductivity is regulated by a voltage applied to a terminal (the gate) which is insulated from the device. The applied gate voltage imposes an electric field into the device, which in turn attracts or repels charge carriers to or from the region between a source terminal and a drain terminal. The density of charge carriers in turn influences the conductivity between the source and drain.

History

The field-effect transistor was first patented by Julius Edgar Lilienfeld in 1926 and by Oskar Heil in 1934, but practical semiconducting devices (the JFET) were developed only much later after the transistor effect was observed and explained by the team of William Shockley at Bell Labs in 1947, immediately after the 20-year patent period eventually expired. The MOSFET, which largely superseded the JFET and had a profound effect on digital electronic development, was invented by Dawon Kahng and Martin Atalla in 1959.[1]

Basic information

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.[2] 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:[3]

More about terminals

Cross section of an n-type MOSFET

All FETs have source, drain, and gate terminals that correspond roughly to the emitter, collector, and base of BJTs. Most FETs have a fourth terminal called the body, base, bulk, or substrate. This fourth terminal serves to bias the transistor into operation; it is rare to make non-trivial use of the body terminal in circuit designs, but its presence is important when setting up the physical layout of an integrated circuit. The size of the gate, length L in the diagram, is the distance between source and drain. The width is the extension of the transistor, in the direction perpendicular to the cross section in the diagram (i.e., into/out of the screen). Typically the width is much larger than the length of the gate. A gate length of 1 µm limits the upper frequency to about 5 GHz, 0.2 µm to about 30 GHz.

The names of the terminals refer to their functions. The gate terminal may be thought of as controlling the opening and closing of a physical gate. This gate permits electrons to flow through or blocks their passage by creating or eliminating a channel between the source and drain. Electron-flow from the source terminal towards the drain terminal is influenced by an applied voltage. The body simply refers to the bulk of the semiconductor in which the gate, source and drain lie. Usually the body terminal is connected to the highest or lowest voltage within the circuit, depending on the type of the FET. The body terminal and the source terminal are sometimes connected together since the source is often connected to the highest or lowest voltage within the circuit, although there are several uses of FETs which do not have such a configuration, such as transmission gates and cascode circuits.

FET operation

I–V characteristics and output plot of a JFET n-channel transistor.
FET conventional symbol types

The FET controls the flow of electrons (or electron holes) from the source to drain by affecting the size and shape of a "conductive channel" created and influenced by voltage (or lack of voltage) applied across the gate and source terminals. (For simplicity, this discussion assumes that the body and source are connected.) This conductive channel is the "stream" through which electrons flow from source to drain.

n-channel

In an n-channel depletion-mode device, a negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel. If the active region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch. This is called pinch-off, and the voltage at which it occurs is called the pinch-off voltage. Conversely, a positive gate-to-source voltage increases the channel size and allows electrons to flow easily.

In an n-channel enhancement-mode device, a conductive channel does not exist naturally within the transistor, and a positive gate-to-source voltage is necessary to create one. The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. But first, enough electrons must be attracted near the gate to counter the dopant ions added to the body of the FET; this forms a region with no mobile carriers called a depletion region, and the voltage at which this occurs is referred to as the threshold voltage of the FET. Further gate-to-source voltage increase will attract even more electrons towards the gate which are able to create a conductive channel from source to drain; this process is called inversion.

p-channel

In a p-channel depletion-mode device, a positive voltage from gate to body creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions.

Operation

For either enhancement- or depletion-mode devices, at drain-to-source voltages much less than gate-to-source voltages, changing the gate voltage will alter the channel resistance, and drain current will be proportional to drain voltage (referenced to source voltage). In this mode the FET operates like a variable resistor and the FET is said to be operating in a linear mode or ohmic mode.[4][5]

If drain-to-source voltage is increased, this creates a significant asymmetrical change in the shape of the channel due to a gradient of voltage potential from source to drain. The shape of the inversion region becomes "pinched-off" near the drain end of the channel. If drain-to-source voltage is increased further, the pinch-off point of the channel begins to move away from the drain towards the source. The FET is said to be in saturation mode;[6] although some authors refer to it as active mode, for a better analogy with bipolar transistor operating regions.[7][8] The saturation mode, or the region between ohmic and saturation, is used when amplification is needed. The in-between region is sometimes considered to be part of the ohmic or linear region, even where drain current is not approximately linear with drain voltage.

Even though the conductive channel formed by gate-to-source voltage no longer connects source to drain during saturation mode, carriers are not blocked from flowing. Considering again an n-channel enhancement-mode device, a depletion region exists in the p-type body, surrounding the conductive channel and drain and source regions. The electrons which comprise the channel are free to move out of the channel through the depletion region if attracted to the drain by drain-to-source voltage. The depletion region is free of carriers and has a resistance similar to silicon. Any increase of the drain-to-source voltage will increase the distance from drain to the pinch-off point, increasing the resistance of the depletion region in proportion to the drain-to-source voltage applied. This proportional change causes the drain-to-source current to remain relatively fixed, independent of changes to the drain-to-source voltage, quite unlike its ohmic behavior in the linear mode of operation. Thus, in saturation mode, the FET behaves as a constant-current source rather than as a resistor, and can effectively be used as a voltage amplifier. In this case, the gate-to-source voltage determines the level of constant current through the channel.

Composition

FETs can be constructed from various semiconductors, with silicon being by far the most common. Most FETs are made by using conventional bulk semiconductor processing techniques, using a single crystal semiconductor wafer as the active region, or channel.

Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field-effect transistors (OFETs) that are based on organic semiconductors; often, OFET gate insulators and electrodes are made of organic materials, as well. Such FETs are manufactured using a variety of materials such as silicon carbide (SiC), gallium arsenide (GaAs), gallium nitride (GaN), and indium gallium arsenide (InGaAs). In June 2011, IBM announced that it had successfully used graphene-based FETs in an integrated circuit.[9][10] These transistors are capable of about 2.23 GHz cutoff frequency, much higher than standard silicon FETs.[11]

Types of field-effect transistors

Depletion-type FETs under typical voltages: JFET, poly-silicon MOSFET, double-gate MOSFET, metal-gate MOSFET, MESFET.
  Depletion
  Electrons
  Holes
  Metal
  Insulator
Top: source, bottom: drain, left: gate, right: bulk. Voltages that lead to channel formation are not shown.

The channel of a FET is doped to produce either an n-type semiconductor or a p-type semiconductor. The drain and source may be doped of opposite type to the channel, in the case of enhancement mode FETs, or doped of similar type to the channel as in depletion mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel and gate. Types of FETs include:

Advantages of FET

One advantage of the FET is its high gate to main current resistance, on the order of 100 MΩ or more, thus providing a high degree of isolation between control and flow. Because base current noise will increase with shaping time,[19] a FET typically produces less noise than a bipolar junction transistor (BJT), and is thus found in noise sensitive electronics such as tuners and low-noise amplifiers for VHF and satellite receivers. It is relatively immune to radiation. It exhibits no offset voltage at zero drain current and hence makes an excellent signal chopper. It typically has better thermal stability than a BJT.[3] Because they are controlled by gate charge, once the gate is closed or opened, there is no additional power draw, as there would be with a bipolar junction transistor or with non-latching relays in some states. This allows extremely low-power switching, which in turn allows greater miniaturization of circuits because heat dissipation needs are reduced compared to other types of switches.

Disadvantages of FET

It has a relatively low gain-bandwidth product compared to a BJT. The MOSFET has a drawback of being very susceptible to overload voltages, thus requiring special handling during installation.[20] The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage or changes to threshold voltage during handling. This is not usually a problem after the device has been installed in a properly designed circuit.

FETs often have a very low 'on' resistance and have a high 'off' resistance. However the intermediate resistances are significant, and so FETs can dissipate large amounts of power while switching. Thus efficiency can put a premium on switching quickly, but this can cause transients that can excite stray inductances and generate significant voltages that can couple to the gate and cause unintentional switching. FET circuits can therefore require very careful layout and can involve trades between switching speed and power dissipation. There is also a trade-off between voltage rating and 'on' resistance, so high voltage FETs have a relatively high 'on' resistance and hence conduction losses.

Uses of FET

The most commonly used FET is the MOSFET. The CMOS (complementary metal oxide semiconductor) process technology is the basis for modern digital integrated circuits. This process technology uses an arrangement where the (usually "enhancement-mode") p-channel MOSFET and n-channel MOSFET are connected in series such that when one is ON, the other is OFF.

In FETs, electrons can flow in either direction through the channel when operated in the linear mode. The naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrically from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example.

A common use of the FET is as an amplifier. For example, due to its large input resistance and low output resistance, it is effective as a buffer in common-drain (source follower) configuration.

IGBTs are used in switching internal combustion engine ignition coils, where fast switching and voltage blocking capabilities are important.

Source Gated Transistor

Source Gated Transistors are more robust vs manufacturing and environmental issues, but are slower in operation than FETs.[21]

See also

References

  1. ↑ 1960 - Metal Oxide Semiconductor (MOS) Transistor Demonstrated - The Silicon Engine | Computer History Museum
  2. ↑ Jacob Millman (1985). Electronic devices and circuits. Singapore: McGraw-Hill International. p. 397. ISBN 0-07-085505-6.
  3. 1 2 Millman (1985). Electronic devices and circuits. Singapore: McGraw-Hill. pp. 384–385. ISBN 0-07-085505-6.
  4. ↑ C Galup-Montoro & Schneider MC (2007). MOSFET modeling for circuit analysis and design. London/Singapore: World Scientific. p. 83. ISBN 981-256-810-7.
  5. ↑ Norbert R Malik (1995). Electronic circuits: analysis, simulation, and design. Englewood Cliffs, NJ: Prentice Hall. pp. 315–316. ISBN 0-02-374910-5.
  6. ↑ RR Spencer & Ghausi MS (2001). Microelectronic circuits. Upper Saddle River NJ: Pearson Education/Prentice-Hall. p. 102. ISBN 0-201-36183-3.
  7. ↑ A. S. Sedra and K.C. Smith (2004). Microelectronic circuits (Fifth ed.). New York: Oxford. p. 552. ISBN 0-19-514251-9.
  8. ↑ PR Gray, PJ Hurst, SH Lewis & RG Meyer (2001). Analysis and design of analog integrated circuits (Fourth ed.). New York: Wiley. pp. §1.5.2 p. 45. ISBN 0-471-32168-0.
  9. ↑ IBM creates first graphene based integrated circuit
  10. ↑ Lin, Y.-M., Valdes-Garcia, A., Han, S.-J., Farmer, D. B., Sun, Y, Wu, Y, Dimitrakopoulos, C., Grill, A, Avouris, P, and Jenkins, K. A. (2011). "Wafer-Scale Graphene Integrated Circuit". Science 332: 1294–1297. doi:10.1126/science.1204428. PMID 21659599.
  11. ↑ Flexible graphene transistor sets new records - physicsworld.com
  12. ↑ HIGFET and method - Motorola
  13. ↑ Ionescu, A. M.; Riel, H. (2011). "Tunnel field-effect transistors as energy-efficient electronic switches". Nature 479 (7373): 329–337. doi:10.1038/nature10679. PMID 22094693.
  14. ↑ Poghossianb, Arshak (2002). "Recent advances in biologically sensitive field-effect transistors (BioFETs)". Analyst 127: 1137–1151. doi:10.1039/B204444G.
  15. ↑ http://www.sciencedaily.com/releases/2010/01/100125122101.htm
  16. ↑ Sarvari H, et al. (2011). "Frequency analysis of graphene nanoribbon FET by Non-Equilibrium Green's Function in mode space". Elsevier Physica E 43 (8): 1509–1513. doi:10.1016/j.physe.2011.04.018.
  17. ↑ Vertical Slit Integrated Circuits
  18. ↑
  19. ↑ http://www-physics.lbl.gov/~spieler/physics_198_notes/PDF/VIII-5-noise.pdf
  20. ↑ Allen Mottershead (2004). Electronic devices and circuits. New Delhi: Prentice-Hall of India. ISBN 81-203-0124-2.
  21. ↑ "Source-gated transistors for order-of-magnitude performance improvements in thin-film digital circuits"

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