IMPATT diodes

Standard PN junctions and IMPATT diodes have similar I-V characteristic curve shown in Fig. 1a. As indicated in the figure, when the forward bias voltage reaches the “turn on” level, the diode starts to conduct in the forward direction while preventing the reverse current. As soon as the bias voltage applied in reverse exceeds the breakdown point, the diode starts conducting in reverse direction due to the avalanche breakdown phenomenon.

For the purpose of microwave signal generation, there are different types of IMPATT diode structures that can be used. Figures 1b-1e illustrate its basic operating principle considering a p⁺-n-i-n⁺ semiconductor structure as an example.

In this case, there can be identified two distinct spatially separated regions shown in Fig.1b, the avalanche multiplication region (the forward-biased ‘p⁺-n’ junction) and the carrier drift region (the i-region which becomes completely depleted at high reverse voltages). This particular arrangement creates the carrier distribution profile indicated in Fig. 1c and Fig. 1d.

The former region is where the injection of carriers (the rapid increase in the number of free charges) occurs. Due to maximally high electric field intensity across the junction (Fig. 1e), the newly formed holes are accelerated into the heavily doped p⁺-region resulting in impact ionization which, in turn, leads to avalanche carrier multiplication.

The electrons, on the other hand, are forced through the latter region, where they drift in a constant electric filed (Fig. 1e) and then into the n⁺-region. Provided that the electric field strength in the lightly doped n-region is high, the electron drift velocity remains almost constant. Therefore, with the differential electron mobility being close to zero, the volume charge density does not decrease with electron flow.

When the reverse-bias voltage is increased to the point where the electric field in the p⁺-n junction exceeds the breakdown level, the impact ionization coefficient α reaches high values. Hence, the depth of the impact ionization region is kept small because of strong dependence of α on the electric field intensity level.