Gunn diode
A Gunn diode, also known as a transferred electron device (TED), is a form of
Its internal construction is unlike other diodes in that it consists only of
Gunn diode oscillators
The negative differential resistance, combined with the timing properties of the intermediate layer, is responsible for the diode's largest use: in
History
The Gunn diode is based on the Gunn effect, and both are named for physicist
The Gunn effect and its relation to the Watkins–Ridley–Hilsum effect entered electronics literature in the early 1970s, e.g., in books on transferred electron devices[4] and, more recently, on nonlinear wave methods for charge transport.[5]
Principle
The
These electrons either start 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. With forward voltage applied, the Fermi level in the cathode moves into 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 effective mass of the electrons in the third band is higher than those in the usual conduction band, so the mobility or drift velocity of the electrons in that band is lower. As the forward voltage increases, more and more electrons can reach the third band, causing them to move slower, and the current through the device decreases. This creates a region of negative differential resistance in the voltage/current relationship.
When a high enough potential is applied to the diode, the charge carrier density along the cathode becomes unstable and will develop small segments of low conductivity, with the rest of the cathode having high conductivity. Most of the cathode voltage drop will occur across the segment so that it will have a high electric field. Under the influence of this electric field, it will move along the cathode to the anode. It is impossible to balance the population in both bands, so thin slices of high-field strength will always be in a background of low-field strength. So in practice, with a slight increase in forward voltage, a low conductivity segment is created at the cathode, resistance increases, the segment moves along the bar to the anode, and when it reaches the anode, it is absorbed, and a new segment is created at the cathode to keep the total voltage constant. Any existing slice is quenched if the voltage is lowered and resistance decreases again.
The laboratory methods used to select materials for manufacturing Gunn diodes include
Applications
Because of their high-frequency capability, Gunn diodes are mainly used at microwave frequencies and above. They can produce some of the highest output power of any semiconductor device at these frequencies. Their most common use is in
Sensors and measuring instruments
Gunn diode oscillators generate microwave power for:
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 transceivers called Gunnplexers. British radio amateurs first used them in the late 1970s, and many Gunnplexer designs have been published in journals. They typically consist of an approximately 3 inch waveguide into which the diode is mounted. A low voltage (less than 12 volt) direct current power supply that can be modulated appropriately is used to drive the diode. The waveguide is blocked at one end to form a resonant cavity, and the other end usually feeds a horn antenna. An additional "mixer diode" is inserted into the waveguide, and it is often connected to a modified FM broadcast receiver to enable listening of other amateur stations. Gunnplexers are most commonly used in the 10 GHz and 24 GHz ham bands, and sometimes 22 GHz security alarms are modified as the diode(s) can be put in a slightly detuned cavity with layers of copper or aluminium foil on opposite edges for moving to the licensed amateur band. If intact, the mixer diode is reused in its existing waveguide, and these parts are well known for being extremely static sensitive. On most commercial units, this part is protected with a parallel resistor and other components, and a variant is used in some Rb atomic clocks. The mixer diode is useful for lower frequency applications even if the Gunn diode is weakened from use, and some amateur radio enthusiasts have used them in conjunction with an external oscillator or n/2 wavelength Gunn diode for satellite finding and other applications.
Radio astronomy
Gunn oscillators are used as local oscillators for millimeter-wave and submillimeter-wave radio astronomy receivers. The Gunn diode is mounted in a cavity tuned to resonate at twice the fundamental frequency of the diode. The cavity length is changed by a micrometer adjustment. Gunn oscillators capable of generating over 50 mW over a 50% tuning range (one waveguide band) are available.[7]
The Gunn oscillator frequency is multiplied by a diode frequency multiplier for submillimeter-wave applications.
References
- ^ V. Gružinskis, J.H. Zhao, O.Shiktorov and E. Starikov, Gunn Effect and the THz Frequency Power Generation in n(+)-n-n(+) GaN Structures, Materials Science Forum, 297--298, 34--344, 1999. [1]
- ^ Gribnikov, Z. S., Bashirov, R. R., & Mitin, V. V. (2001). Negative effective mass mechanism of negative differential drift velocity and terahertz generation. IEEE Journal of Selected Topics in Quantum Electronics, 7(4), 630-640.
- ISSN 0018-9235.
- ^ P. J. Bulman, G. S. Hobson and B. C. Taylor. Transferred electron devices, Academic Press, New York, 1972
- ^ Luis L. Bonilla and Stephen W. Teitsworth, Nonlinear Wave Methods for Charge Transport, Wiley-VCH, 2010.
- ^ The Gunn effect, University of Oklahoma, Department of Physics and Astronomy, course notes.[2]
- ^ J.E. Carlstrom, R.L. Plambeck, and D. D. Thornton. A Continuously Tunable 65-115 GHz Gunn Oscillator, IEEE, 1985 [3]