Q6Microwave Theory And Techniques

Structure and biasing: an IMPATT diode is a p-n (or p-i-n) junction diode reverse-biased close to its avalanche breakdown voltage, consisting of a thin, heavily-doped avalanche (multiplication) region near the junction followed by a longer, lightly-doped drift region.

Avalanche delay: when the applied RF voltage swings the diode above its breakdown threshold, impact-ionization avalanche multiplication generates a rapidly growing population of electron-hole pairs; because avalanche multiplication builds up cumulatively rather than instantaneously, the resulting avalanche current pulse lags the applied RF voltage by approximately 90° in phase — the avalanche process acts like an integrator, so the current peaks a quarter-cycle after the voltage peak that triggered it.

Transit-time delay: the avalanche-generated charge carriers (holes, in a typical n+-p-i-p+ or similar structure) are then swept by the strong reverse-bias field across the drift region at their saturated drift velocity; the finite time required to cross this drift region introduces a further phase delay, ideally chosen (by design of the drift-region length) to be an additional 90°, so that the induced terminal current lags the RF voltage by approximately 90° (avalanche) + 90° (transit) = 180° total.

Negative resistance and oscillation: a current that lags the voltage by close to 180° is equivalent to a current that is essentially in phase with the negative of the voltage — precisely the condition for negative dynamic resistance, meaning the diode delivers RF power to an external circuit rather than absorbing it (opposite to a normal resistor). Embedded in a resonant cavity or waveguide circuit tuned to the desired microwave frequency, this negative resistance sustains continuous oscillation (or provides amplification when used as a negative-resistance reflection amplifier) at frequencies typically from a few GHz up to over 100 GHz, making IMPATT diodes among the highest-power solid-state sources available at millimetre-wave frequencies, though at the cost of relatively high phase noise arising from the inherently statistical (noisy) nature of the avalanche multiplication process itself.