Avalanche Region of IMPATT Diodes

The avalanche region of an IMPATT (IMPact ionization Avalanche Transit Time) diode under small signal conditions is characterized by the fraction of the total alternating current that is carried by holes and electrons in their respective drift spaces and by a residual impedance. The current fractions are roughly in phase with the total current below, and nearly 180° out of phase above a resonance frequency that is proportional to the square root of direct current density. This paper extends the calculations of Gilden and Hines for the current fraction to include phase shifts in the avalanche region so that extended avalanche regions can be considered. Realistic values (α ≠ β in Si) for the ionization coefficients are used. Results of detailed numerical calculations for the current fractions as a function of frequency and direct current density are presented. For typical frequencies and current densities, the residual impedance is negligible and hole and electron current fractions are equal. The avalanche region at a given frequency and current density is then characterized by one complex number and the admittance of a diode containing the avalanche region and adjacent drift regions is easily calculated. Plots showing the admittance as a function of frequency and current density for typical structures are given. It is found that an optimal exponential growth rate of oscillations is obtained when the current density is such that the resonance frequency is about equal to one half the reciprocal transit time through the longest drift region. If the assumption is made that conditions giving the largest small-signal exponential growth rate give the best large-signal performance, an optimum Read-diode design is obtained for which the avalanche region width is a substantial fraction (≍ 1/3) of the drift region width.