IMPATT diode operation
- let us consider the effects of avalanche and drift for various points in the cycle of applied voltage.
- We assume that the p region is very narrow and that all the avalanche multiplication takes place in a thin region near the n -p junction.
- If the d-c bias is such that the critical field for avalanche Ea is just met in the n -p space charge region in (Fig a).
- Avalanche multiplication begins at t = 0. Electrons generated in the avalanche move to the n region, and holes enter the i drift region.
- We assume that the device is mounted in a resonant microwave circuit so that an a-c signal can be maintained at a given frequency.
- As the applied a-c voltage goes positive, more and more holes are generated in the avalanche region.
- In fact, the pulse of holes (dotted line) generated by the multiplication process continues to grow as long as the electric field is above Ea (Fig.b).
- It can be shown that the particle current due to avalanche increases exponentially with time while the field is above the critical value.
- The important result of this growth is that the hole pulse reaches its peak value, not at /2, when the voltage is maximum, but at (Fig.c).
- Therefore, there is a phase delay of /2 inherent in the avalanche process itself. A further delay is provided by the drift region.
- Once the avalanche multiplication stops (ωt > ), the pulse of holes simply drifts toward the p contact (Fig.d).
- But during this period, them a-c terminal voltage is negative. Therefore, the dynamic conductance is negative, and energy is supplied to the a-c field.
If the length of the drift region is chosen properly, the pulse of holes is collected at the p contact just as the voltage cycle is completed, and the cycle then repeats itself. The pulse will drift through the length L of the i region during the negative half-cycle if we choose the transit time to be one-half the oscillation period; that is,
where / i s the operating frequency and vd is the drift velocity for holes.