The Gunn EffectIn some materials (III-V compounds such as GaAs and InP), after an electric field in the materialreaches a threshold level, the mobility of electrons decrease as the electric field is increased, thereby producing negative resistance. A two-terminal device made from such a material can produce microwave oscillations, the frequency of which is primarily determined by the characteristics of the specimen of the material and not by any external circuit. The Gunn Effect was discovered by J. B. Gunn of IBM in 1963.

The Gunn DiodeIn certain semiconductors, notably GaAs, electrons can exist in a high-mass low velocity state as well as their normal low-mass high-velocity state and they can be forced into the high-mass state by a steady electric field of sufficient strength. In this state they form clusters or domains which cross the field at a constant rate causing current to flow as a series of pulses. This is the Gunn effect and one form of diode which makes use of it consists of an epitaxial layer of n-type GaAs grown on a GaAs substrate. A potential of a few volts applied between ohmic contacts to the n-layer and substrate produces the electric field which causes clusters. The frequency of the current pulses so generated depends on the transit time through the n-layer and hence on its thickness. If the diode is mounted in a suitably tuned cavity resonator, the current pulses cause oscillation by shock excitation and r.f. power up to 1 W at frequencies between 10 and 30 GHz is obtainable.

Gunn Diode TheoryThe Gunn diode is a so-called transferred electron device. Electrons are transferred from one valley in the conduction band to another valley. In order to understand the nature of the transferred electron effect exhibited by Gunn diodes, it is necessary to consider the electron drift velocity versus electric field (or current versus voltage) relationship for GaAs (seeFigure 2). Below the threshold field, Eth, of approximately 0.32 V/mm, the device acts as a passive resistance. However, above Eth the electron velocity (current) decreases as the field (voltage) increases producing a region of negative differential mobility, NDM (resistance, NDR). This is the essential feature that leads to current instabilities and Gunn oscillations in an active device and is due to the special conductance band structure of direct band gap semiconductors such as GaAs

The energy-momentum relationship contains two conduction band energy levels, and L (also known as valleys) with the following properties:In the lower valley, electrons exhibit a small effective mass and very high mobility, 1.In the satellite L valley, electrons exhibit a large effective mass and very low mobility, 2.The two valleys are separated by a small energy gap, E, of approximately 0.31 eV.ApplicationsGunn diodes are reliable, relatively easy to install and the lower output power levels fall well below the safety exposure limits. They are ideally suited for use in low noise sources such as local oscillators, locking oscillators, low and medium power transmitter applications and motion detection systems. Higher power varieties can be used in phase-locked oscillators or as reflection amplifiers in point-to-point communication links and telemetry systems. Microwave sources have the advantages over ultrasonic detectors of size and beamwidth, and over optical systems of working in dusty and adverse environments. The low voltage requirements of Gunn oscillators mean that battery or regulated mains supplies may be used, (battery drain can be reduced by using low current devices or by operation in a pulsed mode). However, microwaves are reflected from metal surfaces and partially reflected from many others e.g. brick, Tarmac and concrete, and they are attenuated by oxygen, water or water vapour.

An impact ionization avalanche transit time (or IMPATT) diode is an NDR device with relatively high power capability. Its basic structure is based on a reversebiased pn junction and an intrinsic (high-resistivity) layer.

When a reverse bias across the pn junction exceeds a certain threshold value, then avalanche breakdown (due to impact ionization) occurs, resulting in a large number of carriers in what is called the avalanche region. In the example above, electrons move through the drift region (intrinsic doping) to the n+ contact in a time known as the transit time delay. An ac signal with a mean value just below avalanche breakdown is applied to the diode. As the voltage increases above threshold, carriers are generated; however, since the rate of carrier generation at avalanche depends not only on the electric field but also on the number of carriers already in existence, the current continues to rise even as the ac voltage decreases (it lags the voltage by 90 deg, known as the injection delay). The length of the diode can be chosen so that the transit time delay results in a further 90-deg phase lag in current and therefore a negative differential resistance device. An external resonant circuit can then be used to sustain the oscillations. Like other devices based on the random avalanche process, IMPATT diodes tend to suffer from phase noise.

TUNNETT DiodesAs the operating frequency of an IMPATT diode increases, the dominant carrier injection mode changes from an avalanche to a mixed avalanchetunnel mechanism. The exploitation of the injection of charge by tunneling led to the development of tunnel injection transit time or TUNNETT diodes.

In these diodes a highly doped, narrow, p++n+ junction (in an overall p++n+nn+ structure) is formed in order to change the breakdown mechanism from avalanche to tunnel injection. Since the carrier generation rate due to tunneling does not depend on the current density, there is no injection delay in this case, and negative differential resistance is achieved by the transit time of carriers through a drift region with a considerably lower electric field than in IMPATT diodes. Tunneling is a quick, low-noise process when compared with impact ionization, so TUNNETT diodes can provide medium power at higher frequencies and with lower noise than IMPATT diodes. Like other NDR devices, TUNNET diodes in an external resonant circuit can be used to generate electromagnetic radiation.

In a resonant tunnel diode (RTD), layers of undoped material are used to create a quantum well between two thin barriers. Quasi-bound, or resonant, energy states are formed in the well.

When an energy level in the quantum well is close to the energy of the electrons in the conduction band, then resonant tunneling through the double-barrier structure occurs. This gives rise to a peak in the current-voltage characteristic. The width of the peak depends on the width of the resonant state in the quantum well. As the bias voltage is increased beyond the resonance peak, the IV curve exhibits negative differential resistance. Like other NDR devices, RTD diodes in an external resonant circuit can be used to generate electromagnetic radiation. Because tunneling is inherently a very fast process, RTDs produce the highest oscillation frequencies of the NTD devices discussed here.