What Is a Gunn Diode?
A Gunn diode is a type of diode used in microwave band oscillators and other devices.
Gunn diodes are diodes that utilize the Gunn effect. N-type gallium arsenide (GaAs) is mainly used, and physicist J.B. Gunn discovered that when a DC electric field is applied to a GaAs crystal and exceeds a certain threshold value, oscillation occurs in the microwave region. For this reason, this phenomenon is called the Gunn effect.
Gunn diodes have a region of negative resistance, and this effect is used in microwave oscillators. Negative resistance here refers to the electrical characteristic that current decreases as voltage increases.
Uses of Gunn Diodes
Gunn diodes are commonly used in the microwave region for oscillator radar applications using the Gunn effect.
When a Gunn diode is mounted in a cavity or waveguide and a DC voltage is applied, it oscillates with an intrinsic vibration that depends on the thickness of the crystal. Although the structure is relatively simple, the frequency stability is not as good as that of the usual microwave control method using a VCO or PLL, so it is often used for speed sensors using the Doppler effect in addition to communications.
Familiar applications include speed guns for baseball and radar for speed enforcement.
Principle of Gunn Diodes
Gunn diodes take advantage of the microwave band oscillation phenomenon that occurs when electrons rapidly transition to the band with the higher energy level when a voltage exceeding the critical electric field is applied to a semiconductor crystal with two energy bands. When a graph of energy is drawn, it has two conduction bands with different bottom energies and wave numbers.
When voltage is applied, an electric current flows, and the electrons responsible for this current are those in the conduction band. Normally, more electrons exist in the conduction band with lower energy at the bottom, and as the voltage increases, more electrons are in the conduction band with higher energy. When a certain voltage is exceeded, electrons from the lower level conduction band move to the higher level conduction band, resulting in a decrease in mobility.
This phenomenon causes a decrease in the apparent mobility of electrons, or current, when the voltage is increased above a certain voltage, and thus has the characteristic of negative resistance. As the voltage is further increased, the electrons in the lower level conduction band are transferred to the higher level conduction band, and the current increases again.
At this point, an avalanche of rapid, high-speed electron transfer occurs in the microwave band, resulting in the oscillation phenomenon. While general VCOs (voltage-controlled oscillators) utilize a negative resistance that takes advantage of the impedance matching of transistors, Gunn diodes make good use of the inherent energy bands of semiconductor crystals.
Other Information on Gunn Diodes
1. Application to Speed Sensors
Velocity sensors, a typical application of Gunn diodes, generally use a physical phenomenon called the Doppler effect. The Doppler effect is the principle that the reflected wave of an electromagnetic wave irradiated onto an object moving at high speed is observed at a different apparent frequency than the original frequency of the irradiation.
The frequency of a Gunn diode in a GaAs crystal is about 10 GHz, and the speed of the irradiated object is calculated from the difference in frequency variation. Incidentally, this difference in frequency variation is described as a “whirr” in physical terms, and the variation relative to a frequency of 10 GHz is a very small percentage for speed measurements in the 100 km range for baseball speed guns and automobile speed measurements.
2. Expansion to Sub-Terahertz Frequencies
Gunn diodes and similar diodes, as well as resonant tunneling diodes, are currently the focus of attention in research institutions as research materials for sub-terahertz frequency oscillators.