Section 6.24 explained that normally no significant current will pass through a p-n junction in the reverse direction. The basic reason can be readily explained in terms of the schematic of the p-n junction figure 6.33. A significant reverse current would require that the majority n-side conduction electrons and p-side holes both move away from the junction. That would require the creation of significant amounts of electron-hole pairs at the junction to replenish those that leave. Normally that will not happen.
But if the reverse voltage is increased enough, the diode can break down and a significant reverse current can indeed start to flow. That can be useful for voltage stabilization purposes.
Consider figure 6.33. One thing that can happen is that
electrons in the valence band on the p side end up in the
conduction band on the n side simply because of their quantum
uncertainty in position. That process is called
tunneling. Diodes in which tunneling happens are
The process requires that the energy spectrum at one location is raised sufficiently that its valence band reaches the level of the conduction band at another location. And the two locations must be extremely close together, as the quantum uncertainty in position is very small. Now it is the electrostatic potential, shown in green in figure 6.33, that raises the p-side spectra relative to the n-side ones. To raise a spectrum significantly relative to one very nearby requires a very steep slope to the electrostatic potential. And that in turn requires heavy doping and a sufficiently large reverse voltage to boost the built-in potential.
Once tunneling becomes a measurable effect, the current increases extremely rapidly with further voltage increases. That is a consequence of the fact that the strength of tunneling involves an exponential function, chapter 7.13 (7.74). The fast blow-up of current allows Zener diodes to provide a very stable voltage difference. The diode is put into a circuit that puts a nonzero tunneling current through the diode. Even if the voltage source in the circuit gets perturbed, the voltage drop across the Zener will stay virtually unchanged. Changes in voltage drops will remain restricted to other parts of the circuit; a corresponding change over the Zener would need a much larger change in current.
There is another way that diodes can break down under a sufficiently large reverse voltage. Recall that even under a reverse voltage there is still a tiny current through the junction. That current is due to the minority carriers, holes from the n side and conduction electrons from the p side. However, there are very few holes in the n side and conduction electrons in the p side. So normally this current can be ignored.
But that can change. When the minority carriers pass through the space charge region at the junction, they get accelerated by the strong electric field that exists there. If the reverse voltage is big enough, the space charge region can accelerate the minority carriers so much that they can knock electrons out of the valence band. The created electrons and holes will then add to the current.
Now consider the following scenario. A minority electron passes through the space charge region. Near the end of it, the electron has picked up enough energy to knock a fellow electron out of the valence band. The two electrons continue on into the n side. But the created hole is swept by the electric field in the opposite direction. It goes back into the space charge region. Traveling almost all the way through it, near the end the hole has picked up enough energy to knock an electron out of the valence band. The created conduction electron is swept by the electric field in the opposite direction of the two holes, back into the space charge region... The single original minority electron has set off an avalanche of new conduction electrons and holes. The current explodes.
A diode designed to survive this is an “avalanche diode.” Avalanche diodes are often loosely called Zener diodes, because the current explodes in a similar way. However, the physics is completely different.
- Unlike the idealized theory suggests, under suitable conditions significant reverse currents can be made to pass through p-n junctions.
- It allows voltage stabilization.