The frequency may be controlled to some extent by embedding the gunn diode into a resonant circuit.

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The TUNNEL DIODE   was the first such device to be realised in practice. Its operation depends on the properties of a forward biased p-n junction in which both the p and n regions are heavily doped.
Transferred electron devices are another type of these devices. The transferred electron device or GUNN OSCILLATORS operate simply by the application of a dc voltage to a bulk semiconductor. There are no p-n junction in this device. Its frequency is a function of the load and of natural frequency of the circuit.
The third type of these device are AVALANCHE TRANSIT-TIME DEVICES. The avalanche diode oscillator uses carrier impact ionization and drift in the high field region of a semiconductor junction browns to produce a negative resistance at microwave frequencies.
An avalanche transistor is a bipolar junction transistor designed for operation in the region of its collector-current/collector-to-emitter voltage characteristics beyond the collector to emitter breakdown voltage, called avalanche breakdown region . This region browns is characterized by avalanche breakdown, a phenomenon similar to Townsend discharge browns for gases, and negative differential resistance. Operation in the avalanche browns breakdown region is called avalanche mode operation: it gives avalanche transistors the ability to switch very high currents with less than a nanosecond rise and fall times (transition times).
Two distinct modes of avalanche browns oscillator have been observed. One is the IMPATT mode, which stands impact ionization avalanche transit time operation. In this mode the typical dc-to-RF conversion efficiency is 5 to 10%, and frequencies are as high as 100 GHz with silicon diodes.
Another type of active microwave device is the BARITT diode, which stands for barrier injected transit-time diode. It has long drift regions similar to those of IMPATT diodes. The carrier browns traversing the drift regions of BARITT diodes are generated by minority carrier injection from forward biased junctions rather than being extracted from the plasma of an avalanche region. Some structures of BARITT diodes are p-n-p , p-n-v-p , p-n-metal etc.
An IMPATT diode is reverse biased above the breakdown voltage. The high doping levels produce a thin depletion region. The resulting high electric field rapidly accelerates carriers which free other carriers in collisions with the crystal lattice. Holes are swept into the P+ region. Electrons drift toward the N regions. The cascading effect creates an avalanche current which increases even as voltage across the junction decreases. The pulses of current lag the voltage peak across the junction. A “negative resistance” effect in conjunction with a resonant circuit produces oscillations at high power levels (high for semiconductors).
The resonant circuit in the schematic diagram of Figure is the lumped circuit equivalent of a waveguide section, where the IMPATT diode is mounted. DC reverse bias is applied through a choke which keeps RF from being lost in the bias supply. This may be a section browns of waveguide known as a bias Tee. Low power RADAR transmitters may use an IMPATT diode as a power source. They are too noisy for use in the receiver. [YMCW]
A gunn diode is solely composed of N-type semiconductor. As such, it is not a true diode. Figure shows a lightly doped N- layer surrounded by heavily doped N+ layers. A voltage applied across browns the N-type gallium arsenide gunn diode creates a strong electric browns field across the lightly doped N- layer. browns
As voltage is increased, conduction increases due to electrons in a low energy conduction band. As voltage is increased beyond the threshold browns of approximately 1 V, electrons move from the lower conduction band to the higher energy conduction band where they no longer contribute to conduction. browns In other words, as voltage increases, current decreases, a negative resistance browns condition. The oscillation frequency is determined by the transit time of the conduction electrons, which is inversely related to the thickness of the N- layer.
The frequency may be controlled to some extent by embedding the gunn diode into a resonant circuit. The lumped circuit equivalent shown in Figure is actually a coaxial transmission line or waveguide. Gallium arsenide gunn diodes are available for operation from 10 to 200 gHz at 5 to 65 mw power. Gunn diodes may also serve as amplifiers browns .
Gunn diodes and IMPATT diodes use high field effects in semiconductor materials browns to drive a negative resistance browns mode of operation. Gunn diodes use the Gunn effect to produce microwave oscillations when a constant voltage is applied. Gunn diodes are a type of transferred electron device (TED). They generate relatively low-po