Gunn diode
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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. Eventually, this layer starts to conduct, reducing the gradient across it, preventing further conduction. 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. The oscillation frequency is determined partly by the properties of the thin middle layer, but can be adjusted by external factors. Gunn diodes are therefore used to build oscillators in the 10 GHz and higher (THz) frequency range, where a resonant cavity is usually added to control frequency. The resonator can be based on a waveguide, coaxial cavity, YIG resonator, etc. Tuning is done mechanically, by adjusting the parameters of the resonator, or in case of YIG resonators by electric current.
Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 terahertz.
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[edit] Microscopic view
(For the experts: Domain formation is in the next paragraph) GaAs has a third band above the conduction band. The gap is indirect, so a phonon is needed or created to deliver the impulse for the transition. The energy stems from the kinetic energy of ballistic electrons. They either start out at the high energy region of the Fermi-Dirac equation and a have a long enough mean free path and a strong electric field is applied. Or they are injected by a the cathode with the right energy. For this the cathode material has to be chosen carefully, chemical reactions at the interface need to be controlled at fabrication and additional mono atomic layers of other materials are inserted. In the end with applied forward voltage the Fermi level in the cathode is at the same level 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 drift velocity in the third band is lower than in the usual conduction band. If fading in the forward voltage more and more electrons can reach the third band and current decreases. This means a negative differential resistance. At the anode is a ohmic contact with a metal.
Multiple Gunn diodes in series circuit are unstable, because if one diode has slightly higher voltage across itself, it will conduct less current and the voltage will further rise. Therefor even a single diode is unstable and will develop small slices of low conductivity and high field strength moving from the cathode to the anode. It is not possible to balance the population in both bands, it will always be short high field strength slices in a large low field strength background. So in reality if fading in the forward voltage a slice is created at the cathode, resistance increases, the slice takes off, and when reaching the anode a new slice is created to keep the total voltage constant. If the voltage is lowered, any existing slice is quenched and resistance decreases again.
[edit] Applications
- thin diodes act as amplifiers
- thick diodes show time of flight effects for the electrons and are narrow-band
- a bias tee is needed to isolate the bias current from the high frequency oscillations.
[edit] 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 (and presumably elsewhere). 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.
[edit] See also
- tunnel diode is also fast
- Avalanche diode is slow
- Zener diode is a combination of the above two for temperature compensation
- ARPES allows to find materials suitable for Gunn diodes
