Gunn diode

A rough approximation of the VI curve for a Gunn diode, showing the negative differential resistance region

A Gunn diode, also known as a transferred electron device (TED), is a form of diode used in high-frequency electronics. It is somewhat unusual in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity, preventing further conduction and current actually starts to fall down. In practice, this means a Gunn diode has a region of negative differential resistance.

The negative differential resistance, combined with the timing properties of the intermediate layer, allows construction of an RF relaxation oscillator simply by applying a suitable direct current through the device. In effect, the negative differential resistance created by the diode will negate the real and positive resistance of an actual load and thus create a "zero" resistance circuit which will sustain oscillations indefinitely. The oscillation frequency is determined partly by the properties of the thin middle layer, but can be tuned by external factors. Gunn diodes are therefore used to build oscillators in the 10 GHz and higher (THz) frequency range, where a resonator is usually added to control frequency. This resonator can take the form of a waveguide, microwave cavity or YIG sphere. Tuning is done mechanically, by adjusting the parameters of the resonator, or in case of YIG spheres by changing the magnetic field.

Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz.

The Gunn diode is based on the Gunn effect, and both are named for the physicist J. B. Gunn who, at IBM in 1962, discovered the effect because he refused to accept inconsistent experimental results in Gallium arsenide as "noise", and tracked down the cause. Alan Chynoweth, of Bell Telephone Laboratories, showed in June 1965 that only a transferred-electron mechanism could explain the experimental results. The interpretation refers to the Ridley-Watkins-Hilsum theory.

The Gunn effect, and its relation to the Watkins-Ridley-Hilsum effect entered the monograph literature in the early 1970s, e.g. in books on transferred electron devices and, more recently on nonlinear wave methods for charge transport. Several other books that provided the same coverage were published in the intervening years, and can be found by searching library and bookseller catalogues on Gunn effect.

Microscopic view

Some materials including GaAs have another band or sub-band in addition to the valance and conduction bands usually used in electronic devices. This third band is at a higher energy than the normal conduction band and is empty until energy is supplied to promote electrons to it. The energy stems from the kinetic energy of ballistic electrons. That is, electrons in the conduction band but moving with sufficient kinetic energy can reach the third band.

These electrons either start out below the Fermi level and are given a sufficiently long mean free path to acquire the needed energy by applying a strong electric field, or they are injected by a cathode with the right energy. For the latter, the cathode material has to be chosen carefully; chemical reactions at the interface need to be controlled during fabrication and additional monoatomic layers of other materials inserted. With forward voltage applied, the Fermi level in the cathode is the same as the third band, and reflections of ballistic electrons starting around the Fermi level are minimized by matching the density of states and using the additional interface layers to let the reflected waves interfere destructively.

In GaAs the mobility or drift velocity in the third band is lower than that in the usual conduction band, so with a small increase in the forward voltage, more and more electrons can reach the third band and current decreases. This creates a region of negative incremental resistance in the voltage/current relationship.

Multiple Gunn diodes in a series circuit are unstable, because if one diode has a slightly higher voltage drop across it, it will conduct less current, and the voltage drop will rise further. In fact, even a single diode is internally unstable, and will develop small slices of low conductivity and high field strength which move from the cathode to the anode. It is not possible to balance the population in both bands, so there will always be thin slices of high field strength in a general background of low field strength. So in practice, with a small increase in forward voltage, a slice is created at the cathode, resistance increases, the slice takes off, and when it reaches the anode a new slice is created at the cathode to keep the total voltage constant. If the voltage is lowered, any existing slice is quenched and resistance decreases again.

The laboratory methods that are used to select materials for the manufacture of Gunn diodes include angle-resolved photoemission spectroscopy.

Other kinds of diode include junction diodes, tunnel diodes, which are also fast, avalanche diodes, which are slow, and Zener diodes.

Applications

A Gunn diode can be used to amplify signals because of the apparent "negative resistance". Gunn diodes are commonly used as a source of high frequency and high power signals. A bias tee is needed to isolate the bias current from the high frequency oscillations. Since this is a single-port device, there is no isolation between input and output.

Sensors and measuring instruments

These include: airborne collision avoidance radar, anti-lock brakes, sensors for monitoring the flow of traffic, car radar detectors, pedestrian safety systems, "distance traveled" recorders, motion detectors, "slow-speed" sensors (to detect pedestrian and traffic movement up to 50 m.p.h), traffic signal controllers, automatic door openers, automatic traffic gates, process control equipment to monitor throughput, burglar alarms and equipment to detect trespassers, sensors to avoid derailment of trains, remote vibration detectors, rotational speed tachometers, moisture content monitors.

Radio amateur use

By virtue of their low voltage operation, Gunn diodes can serve as microwave frequency generators for very low powered (few-milliwatt) microwave transmitters. In the late 1970s they were being used by some radio amateurs in Britain. Designs for transmitters were published in journals. They typically consisted simply of an approximately 3 inch waveguide into which the diode was mounted. A low voltage (less than 12 volt) direct current power supply that could be modulated appropriately was used to drive the diode. The waveguide was blocked at one end to form a resonant cavity and the other end ideally fed a parabolic dish.

Oscillators and injectors

Web material includes accounts of a relaxation oscillator, some negative resistance oscillators, and some injectors.