Avalanche Region of IMPATT Diodes

This paper considers the avalanche region of IMPATT* diodes,1'2'3 especially of Read ! -type diodes in which the avalanche region is localized. * I M P a c t ionization Avalanche Transit Time. 1797 1802 T H E BELL SYSTEM T E C H N I C A L J O U R N A L , D E C E M B E R 196(5 Recent theoretical4 and experimental5 results for the impedance of Read diodes show considerable structure in the current and frequency dependence. This paper attempts to enhance the understanding of the smallsignal negative resistance of IMPATT diodes by isolating the role of the avalanche region. Using the calculated parameters by which the avalanche region is characterized, the admittance for typical IMPATT diodes is calculated and exponential growth rate for oscillations is studied. An application of the results to the design of Read diode oscillators is made. At a given angular frequency w and ac terminal current density itot through the diode, the avalanche region can be characterized by three complex numbers: Zr, Fh , Fe defined below. In Fig. 1, to the right of the avalanche region the small-signal particle current consists of a plane wave of holes. If we assume a constant drift velocity, then the magnitude is constant and the phase changes linearly with distance. Let Ah be the complex amplitude, extrapolated to a phase reference plane at x0 in the avalanche region. Then we define the hole current fraction Fh Fh = Ah/it ot. (1)