Effects of tunneling on an IMPATT oscillator

Abstract

A phenomenological formulation which incorporates both avalanche and tunneling mechanisms in an IMPATT diode is given. Here tunneling is viewed as a field‐dependent carrier source. An electron after being field emitted may gain sufficient energy from the field to cause ionization. In this formulation, pure avalanche and pure tunneling appear as the two extreme cases of the general problem. The resultant general dc I‐ V characteristic shows the dominance of tunneling at low voltages and the onset of the multiplication at higher voltages as observed experimentally. A small‐signal admittance of an IMPATT oscillator with tunneling has been calculated. Under some conditions tunneling may increase the negative conductance. However as tunneling dominates, the negative conductance deteriorates and the oscillator will operate in the tunnel transit‐time mode. Tunneling invariably shifts the frequency for optimum negative conductance upwards. The threshold frequency for negative conductance varies as the square root of current density for large multiplication factors as expected. However, for small ones it converges to a value determined only by the drift transit time. The general admittance expression reduces to that of pure avalanche and pure tunneling under the prescription ωa → ω as M → ∞ and ωa → ωaz as M → 1, respectively. ωa, ωa0, and ωaz are the modified avalanche frequencies for the general case, pure avalanche, and pure tunneling, respectively.