For lightly doped junctions electron tunneling is negligible, and instead, the breakdown mechanism involves the impact ionization of host atoms by energetic
carriers. Normal lattice-scattering events can result in the creation of EHPs if the carrier being scattered has sufficient energy.
- For example, if the electric field ξ in the transition region is large, an electron entering from the p side may be accelerated to high enough kinetic energy to cause an ionizing collision with the lattice (Fig. a).
- A single such interaction results in carrier multiplication; the original electron and the generated electron are both swept to the n side of the junction, and the generated hole is swept to the p side (Fig. b).
- The degree of multiplication can become very high if carriers generated within the transition region also have ionizing collisions with the lattice.
- For example, an incoming electron may have a collision with the lattice and create an EHP; each of these carriers has a chance of creating a new EHP, and each of those can also create an EHP, and so forth (Fig. c).
- This is an avalanche process, since each incoming carrier can initiate the creation of a large number of new carriers.
We can make an approximate analysis of avalanche multiplication by assuming that a carrier of either type has a probability P of having an ionizing
collision with the lattice while being accelerated a distance W through the transition region.
- Thus for nm electrons entering from the p side, there will be Pnin ionizing collisions and an EHP (secondary carriers) for each collision.
- After the Pnin collisions by the primary electrons, we have the primary plus the secondary electrons nin(l P).
- After a collision, each EHP moves effectively a distance of W within the transition region.
- For example, if an EHP is created at the center of the region, the electron drifts a distance W/2 to n and the hole W/2 to p.
- Thus the probability that an ionizing collision will occur due to the motion of the secondary carriers is still P in this simplified model.