Reverse Recovery Time

When a conventional pn diode is forward biased some of the majority carriers crossing the junction are neutralised by combination with majority carriers of opposite polarity. Others remain (as minority carriers) and, when the applied voltage is reversed, return across the junction in the form of a substantial pulse of reverse current which takes a significant time to decay to the normal value of reverse current. This delay is a serious disadvantage in diodes required for operation at microwave frequencies or in high-speed switching.

Schottky Diodes

A schottky diode uses a metal-semiconductor contact instead of a pn junction, and this gives a diode with superior reverse recovery. For example, in one form of construction a region of epitaxial n-type GaAs is grown on a GaAs substrate and a metallic layer is deposited on this. Ohmic connections are made to the substrate and the metallic layer. Only one type of charge carrier is involved in operation of the diode. When the metal is biased positively electrons from the n-region are attracted to it to neutralise the charge so giving rise to the forward current. When the metal is negatively charged electrons are repelled and there is no reverse current. There is no p-layer in which electrons could be stored and the resulting diode is highly efficient at frequencies as high as 20GHz.

The Gunn Diode

In 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 electirc 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.

The Pin Diode

As its name suggests, this is a junction diode with a region of intrinsic semiconductor between the n- and p- regions. When such a diode is reverse-biased the intrinsic layer is depleted of carriers and the diode behaves as a capacitor. When it is forward-biased carriers are injected into the intrinsic region to give a forward resistance which varies linearly between, say, 1 ohm and 10kOhms with the current through the device. This property makes the diode useful as a modulator or switch in microwave systems and at frequencies between 1 MHz and 20 GHz.

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