IMPATT diode

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An IMPATT diode (IMPact ionization Avalanche Transit-Time diode) is a form of high-power semiconductor diode used in high-frequency microwave electronics devices. They have negative resistance and are used as oscillators and amplifiers at microwave frequencies. They operate at frequencies of about 3 and 100 GHz, or igher. The main advantage is their high-power capability; single IMPATT diodes can produce continuous microwave outputs of up to 3 kilowatts, and pulsed outputs of much higher power. These diodes are used in a variety of applications from low-power radar systems to proximity alarms. A major drawback of IMPATT diodes is the high level of phase noise they generate. This results from the statistical nature of the avalanche process.

A semiconductor material has an electrical conductivity value falling between that of a metal, like copper, gold, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behaviour opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities ("doping") into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, and gallium arsenide. After silicon, gallium arsenide is the second most common semiconductor used in laser diodes, solar cells, microwave frequency integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits.

Diode electronic component

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.

Microwave form of electromagnetic radiation

Microwaves are a form of electromagnetic radiation with wavelengths ranging from about one meter to one millimeter; with frequencies between 300 MHz (1 m) and 300 GHz (1 mm). Different sources define different frequency ranges as microwaves; the above broad definition includes both UHF and EHF bands. A more common definition in radio engineering is the range between 1 and 100 GHz. In all cases, microwaves include the entire SHF band at minimum. Frequencies in the microwave range are often referred to by their IEEE radar band designations: S, C, X, Ku, K, or Ka band, or by similar NATO or EU designations.

Contents

Device structure

The IMPATT diode family includes many different junctions and metal semiconductor devices. The first IMPATT oscillation was obtained from a simple silicon p–n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity. Because of the strong dependence of the ionization coefficient on the electric field, most of the electron–hole pairs are generated in the high field region. The generated electron immediately moves into the N region, while the generated holes drift across the P region. The time required for the hole to reach the contact constitutes the transit time delay.

An electrical junction may be either a thermoelectricity junction, a metal–semiconductor junction or a p–n junction. Junctions are either rectifying or non-rectifying. Non-rectifying junctions are called ohmic contacts. Electronic components employing rectifying junctions include p–n diodes, Schottky diodes and bipolar junction transistors.

A semiconductor device is an electronic component that exploits the electronic properties of semiconductor material, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced vacuum tubes in most applications. They use electrical conduction in the solid state rather that the gaseous state or thermionic emission in a vacuum.

p–n junction semiconductor–semiconductor junction, formed at the boundary between a p-type and n-type semiconductor

A p–n junction is a boundary or interface between two types of semiconductor materials, p-type and n-type, inside a single crystal of semiconductor. The "p" (positive) side contains an excess of holes, while the "n" (negative) side contains an excess of electrons in the outer shells of the electrically neutral atoms there. This allows electrical current to pass through the junction only in one direction. The p-n junction is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy. If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that would severely inhibit its utility by scattering the electrons and holes.

The original proposal for a microwave device of the IMPATT type was made by Read. The Read diode consists of two regions (i) The Avalanche region (a region with relatively high doping and high field) in which avalanche multiplication occurs and (ii) the drift region (a region with essentially intrinsic doping and constant field) in which the generated holes drift towards the contact. A similar device can be built with the configuration in which electrons generated from the avalanche multiplication drift through the intrinsic region.

In semiconductor production, doping is the intentional introduction of impurities into an intrinsic semiconductor for the purpose of modulating its electrical, optical and structural properties. The doped material is referred to as an extrinsic semiconductor. A semiconductor doped to such high levels that it acts more like a conductor than a semiconductor is referred to as a degenerate semiconductor.

An IMPATT diode generally is mounted in a microwave package. The diode is mounted with its low–field region close to a silicon heat sink so that the heat generated at the diode junction can be readily dissipated. Similar microwave packages are used to house other microwave devices.

Heat sink hardware component

A heat sink is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, thereby allowing regulation of the device's temperature at optimal levels. In computers, heat sinks are used to cool central processing units or graphics processors. Heat sinks are used with high-power semiconductor devices such as power transistors and optoelectronics such as lasers and light emitting diodes (LEDs), where the heat dissipation ability of the component itself is insufficient to moderate its temperature.

The IMPATT diode operates over a narrow frequency band, and diode internal dimensions must correlate with the desired operating frequency. An IMPATT oscillator can be tuned by adjusting the resonant frequency of the coupled circuit, and also by varying the current in the diode; this can be used for frequency modulation.

Frequency modulation encoding of information in a carrier wave by varying the instantaneous frequency of the wave

In telecommunications and signal processing, frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave.

Principle of operation

If a free electron with a sufficient energy strikes a silicon atom, it can break the covalent bond of silicon and liberate an electron from the covalent bond. If the electron liberated gains energy by being in an electric field and liberates other electrons from other covalent bonds then this process can cascade very quickly into a chain reaction, producing a large number of electrons and a large current flow. This phenomenon is called avalanche breakdown.

Covalent bond chemical bond that involves the sharing of electron pairs between atoms

A covalent bond, also called a molecular bond, is a chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. For many molecules, the sharing of electrons allows each atom to attain the equivalent of a full outer shell, corresponding to a stable electronic configuration.

At breakdown, the n– region is punched through and forms the avalanche region of the diode. The high resistivity region is the drift zone through which the avalanche generated electrons move toward the anode.

Consider a dc bias VB, just short of that required to cause breakdown, applied to the diode. Let an AC voltage of sufficiently large magnitude be superimposed on the dc bias, such that during the positive cycle of the AC voltage, the diode is driven deep into the avalanche breakdown. At t=0, the AC voltage is zero, and only a small pre-breakdown current flows through the diode. As t increases, the voltage goes above the breakdown voltage and secondary electron-hole pairs are produced by impact ionization. As long as the field in the avalanche region is maintained above the breakdown field, the electron-hole concentration grows exponentially with t. Similarly this concentration decays exponentially with time when the field is reduced below breakdown voltage during the negative swing of the AC voltage. The holes generated in the avalanche region disappear in the p+ region and are collected by the cathode. The electrons are injected into the i – zone where they drift toward the n+ region. Then, the field in the avalanche region reaches its maximum value and the population of the electron-hole pairs starts building up. At this time, the ionization coefficients have their maximum values. The generated electron concentration does not follow the electric field instantaneously because it also depends on the number of electron-hole pairs already present in the avalanche region. Hence, the electron concentration at this point will have a small value. Even after the field has passed its maximum value, the electron-hole concentration continues to grow because the secondary carrier generation rate still remains above its average value. For this reason, the electron concentration in the avalanche region attains its maximum value at, when the field has dropped to its average value. Thus, it is clear that the avalanche region introduces a 90° phase shift between the AC signal and the electron concentration in this region.

With a further increase in t, the AC voltage becomes negative, and the field in the avalanche region drops below its critical value. The electrons in the avalanche region are then injected into the drift zone which induces a current in the external circuit which has a phase opposite to that of the AC voltage. The AC field, therefore, absorbs energy from the drifting electrons as they are decelerated by the decreasing field. It is clear that an ideal phase shift between the diode current and the AC signal is achieved if the thickness of the drift zone is such that the bunch of electron is collected at the n+ – anode at the moment the AC voltage goes to zero. This condition is achieved by making the length of the drift region equal to the wavelength of the signal. This situation produces an additional phase shift of 90° between the AC voltage and the diode current.

Origins

In 1956 W. T. Read and Ralph L. Johnston of Bell Telephone Laboratories proposed that an avalanche diode that exhibited significant transit time delay might exhibit a negative resistance characteristic. The effect was soon demonstrated in ordinary silicon diodes and by the late 1960s oscillators at 340 GHz had been produced. Silicon IMPATT diodes can produce up to 3 kilowatts of power continuously, with higher power available in pulses. [1]

TRAPATT

A microwave oscillator device with a similar structure to the IMPATT diode is the TRAPATT diode, which stands for "trapped plasma avalanche triggered transit". This mode of operation produces relatively high power and efficiency, but at lower frequency than a device operated in IMPATT mode. [2]

See also

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The BARITT diode is a high frequency semiconductor component of microelectronics. A related component is the DOVETT diode. The BARITT diode uses injection and transit-time properties of minority carriers to produce a negative resistance at microwave frequencies. About the biased forward boundary layer, the minority carriers are injected. There is no avalanche breakdown instead. Consequently, both the phase shift and the output power is substantially lower than in a IMPATT diode.

References

  1. Thomas H. Lee Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits Cambridge University Press 2004 , ISBN   0521835267, pp. 296
  2. Sitesh Kumar Roy, Monojit Mitra, Microwave Semiconductor Devices PHI Learning Pvt. Ltd., 2003, ISBN   8120324188, page 86

Further reading