A gunn diode is made up of only N-type semiconductor which is typically sandwiched between two metal conductors. The central section is usually lightly doped N– surrounded by heavily doped N+ layers. The central section is N-gallium arsenide whereas the two outer sections undergo epitaxial growth from GaAs with increased doping and higher conductivity.
Consider the oscillator circuit below that includes an N-type gallium arsenide gunn diode:
A voltage applied across the N-type gallium arsenide gunn diode creates a strong electric field across the lightly doped N– layer. As the voltage is increased, conduction increases due to electrons in low energy conduction band. As voltage is increased beyond the threshold of approximately 1 V, electrons move from the lower conduction band to the higher energy conduction band where they no longer contribute to conduction. To put it in other words, as voltage increases, current decreases, a negative resistance condition. The oscillation frequency is determined by the transit time of the conduction electrons, which is inversely related to the thickness of the N– layer. The frequency can be controlled to some extent by embedding the gunn diode into a resonant circuit as illustrated in the circuit above.
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