What is a Gunn diode
A Gunn Diode is a microwave semiconductor diode designed to work in devices having microwave frequencies. Microwave Frequencies are electromagnetic radiation of tremendously high frequencies (THF), (maximum working frequency up to 3 THz, this depends on the material used in its structure). The most common materials used in their manufacturing process are GaAs (Gallium Arsenide), and GaN (Gallium Nitride). In that process, microwave diodes require a unique environment (from lead due to sensitivity to electromagnetic pulses).
The symbol of this diode is as shown below:
Gunn diodes have short switching times due to their design. When a low threshold voltage VT (0.3-0.4V) is required, microwave diodes are used as substitutes for Ge (Germanium) semiconductor diodes.
Gunn Diode structure:
The internal structure of Gunn diode is divided into three areas: two areas with high concentration of N-dopants and a thin layer with the low concentration of dopants that separates them from themselves. Although it is called a ‘diode,’ it does not consist of the P-N junction of two semiconductors like a zener diode, Schottky diodes, and other diodes, but only constructed from the N-doped semiconductor. That is why Gunn diodes cannot be used for rectifying alternating current.
Because of the way this element was built, Gunn diode is more often called TED – Transferred Electron Device.
The Gunn diode operating principle:
The Gunn diode operating principle is based on the so-called ‘Gunn effect’ (from the name of its inventor: J. B. Gunn). In materials such as GaAs (Gallium Arsenide) and InP (Indium Phosphide) after reaching a certain threshold value of the material’s electric field, the electron motion begins to decrease in proportion to the increase in the electric field (increasing the voltage decreases the current value, so-called “negative resistance”).
From the above, it follows that Gunn diode has a negative resistance area on its current-voltage characteristics. When the critical value is achieved with the negative GaAs electrode, there will be an area with low electron mobility. Then it will move towards the positive electrode. When it will encounter a domain of strong electric field with the positive electrode at the negative electrode, a cyclic form of the area of low electron mobility and high electric field will start to re-create. The cyclical nature of this phenomenon generates oscillations with frequencies up to 100 GHz. After exceeding this value, the oscillations will start to quickly disappear.
Gunn diode applications:
Thanks to their properties, the Gunn diodes are very well suited for the construction of electronic microwave oscillators in the frequency ranges from GHz’s to THz’s. Like mentioned in this article electronic oscillator is simply an electronic circuit that produces a cyclical, oscillating electronic signal, commonly a sine wave or a square wave. They convert direct current (DC) from a power supply to an alternating current (AC) signal. After biasing the diode with DC Voltage into its negative resistance region, it will be self-generating oscillations.
The resonator is commonly added to it to make frequency controllable in the form of waveguide or microwave cavity (Gunn diode is very often mounted inside the cavity). The aforementioned oscillation’s frequency depends mostly on the type of the Gunn diode’s middle layer. The similar solution is Tunnel diode oscillator, but it is operating at lower frequencies than the oscillator with the Gunn diode. Gunn diodes are most commonly used as e.g., amplifiers (also microwave amplifiers), oscillators, radar speed-guns, detectors, relays or microwave trackers.
