Radar Basics

IMPATT- Diode

The IMPATT-Diodes (IMPact ionization Avalanche Transit-Time diode) is a two terminal semiconductor negative conductance device which operates by a combination of avalanche multiplication and transit time effects. Genrally, it has a simple pn structure which is reverse biased to avalanche breakdown. IMPATT-Diodes are prone to oscillate, with the lead inductance from bias circuit connections. They are used as power generating device in semiconductor microwave oscillators and deliver a relatively high RF power to produce carrier signals for microwave transmission systems, particularly airborne and ground-based low-power radar systems. Depending upon the design, IMPATT diodes can operate from a few GHz to a few hundred GHz.

A typical cross-section of a Read-type IMPATT diode (after the Physician William T. Read) is shown in Figure 1. The diode is operated in reverse bias near breakdown, and both the n and n- regions are completely depleted. The internal electric field is shown at the bottom of the figure. Because of the difference in doping between the “drift region” and “avalanche region”, the electric field is highly peaked in the avalanche region and nearly flat in the drift region. In operation, avalanche breakdown occurs at the point of highest electric field, and this generates a large number of hole-electron pairs by impact ionization. The holes are swept into the cathode but the electrons travel across the drift region toward the anode. As they drift, they induce image charges on the anode, giving rise to a displacement current in the external circuit that is 180° out of phase with the nearly sinusoidal voltage waveform.

The main drawback of using an IMPATT-diode is the high level of phase noise that the device generates.