A major drawback of IMPATT diodes is the high level of phase noise they generate.
The first IMPATT oscillation was obtained from a simple silicon p–n junction diode biased into a reverse avalanche break down and mounted in a microwave cavity.
A similar device can be built with the configuration in which electrons generated from the avalanche multiplication drift through the intrinsic region.
The high resistivity region is the drift zone through which the avalanche generated electrons move toward the anode.
Then, the field in the avalanche region reaches its maximum value and the population of the electron-hole pairs starts building up.
The generated electron concentration does not follow the electric field instantaneously because it also depends on the number of electron-hole pairs already present in the avalanche region.
With a further increase in t, the AC voltage becomes negative, and the field in the avalanche region drops below its critical value.
The electrons in the avalanche region are then injected into the drift zone which induces a current in the external circuit which has a phase opposite to that of the AC voltage.
This condition is achieved by making the length of the drift region equal to the wavelength of the signal.
This situation produces an additional phase shift of 90° between the AC voltage and the diode current.