Zener breakdown and avalanche breakdown are processes which cause reverse currents to flow through p-n junctions in diodes when large reverse-bias voltages are applied.
Background of Zener Breakdown and Avalanche Breakdown
The p–n Junction Under Equilibrium
A p-n junction consists of a p-type semiconductor in contact with an n-type semiconductor. When they are put in contact, electrons and holes diffuse from the side that they are more concentrated to the side that they are less concentrated. This flow of majority carriers due to a concentration gradient is called a diffusion current.
The majority carriers on the n side are electrons, and so these diffuse across to the p side, leaving the n side positively charged. Similarly holes, which are majority carriers of the p side, diffuse across to the n side, leaving the p side with a negative charge. These charged regions form the space charge region (or the depletion region).
Eventually, the charged regions give rise to an electric field, which acts as a potential barrier to the diffusion current. This electric field also sweeps minority carriers across the space charge region—i.e. electrons from the p side to n side, and holes from n side to p side. This flow of minority carriers is called the drift current, and it is in the direction opposite to that of the diffusion current. An equilibrium is established, where the drift current is equal to the diffusion current, making the net current flow across the junction zero.
The p–n Junction under Forward Bias
A p–n junction is in forward bias when a voltage is applied across the p–n junction externally, with the p side connected to the more positive potential than the n side. Connecting in forward bias reduces the potential barrier to the diffusion current and also reduces the space charge width. The diffusion current increases substantially as a result of the reduced potential barrier. The drift current, however, remains virtually unchanged. The overall result is a net current that flows from p side to n side.
As the forward voltage across the diode is increased further, the current increases exponentially. At very high forward voltages, the forward current saturates, and heating effects may cause the diode to break.
The p–n Junction under Reverse Bias
The p–n junction is in reverse bias when voltage is applied across the junction, with the n side is connected to the more positive potential. Here, the potential barrier to the diffusion current and the space charge width are increased. Since the potential barrier is now large, the diffusion current drops. The drift current does not change significantly. The overall result is a small net current flowing from n side to p side, which is called the reverse saturation current (). Increasing the reverse voltage across the junction further causes no change to the current until, at large reverse voltages, Zener and avalanche breakdown processes cause large reverse currents to flow.
For a typical diode, these effects are summarised in the following current vs. voltage graph:
Diodes only allow a considerable current to flow when they are connected in forward bias. Therefore, they can be used to ensure that current in a circuit flows along a given direction. For instance, diodes can be used to convert alternating current to direct current. However, as mentioned above, a large reverse voltage can cause reverse currents to flow. This is referred to as breakdown, and can take place either as “Zener breakdown” or as “avalanche breakdown”. The differences between the two types of breakdown are outlined below.
In Zener breakdown, electrons “tunnel” from the valence band of the p side to the conduction band on the n side. In classical physics, electrons should not have been able to cross over in this way. Tunnelling is, in fact, a quantum mechanical phenomenon, which comes about from electrons having wave properties.
The probability for an electron to tunnel across is higher when the space charge region is narrower, and when the electric field is larger. Typically, Zener breakdown occurs where materials used to construct the p–n junction are heavily doped. In these junctions, due to heavy doping, the space charge region is quite narrow even when the junction is under reverse bias.
In avalanche breakdown, charge carriers in the space charge region gain so much kinetic energy from being accelerated electric field that, they can collide with lattice atoms and tear electrons away from them, creating electron-hole pairs. This is also known as impact ionisation. These newly-separated electrons and holes, too, are then accelerated by the electric field, giving them large amounts of kinetic energy. In the meantime, the original charge carriers, which lost energy during the collision, are also accelerated. Consequently, both original charge carriers as well as the recently-separated ones now have the capacity to cause impact ionisation. The process is called “avalanche” breakdown because, with each collision, more and more charge carriers are made available to cause future impact ionisations.
In terms of energy bands, the incoming charge carrier’s kinetic energy must be larger than the energy “gap” between conduction and valence bands for impact ionisation to take place. Then, once the collision takes place and the electron-hole pair is formed, this electron and the hole are essentially in the conduction and valance bands respectively.
For most diodes, avalanche breakdown is the dominant effect. For a given diode, the dominant effect is determined by the material used to construct the junction and also by the level of doping.
Difference Between Zener and Avalanche Breakdown
- Zener breakdown and avalanche breakdown are processes by which diodes begin to conduct significant currents, when they are subject to a high reverse voltage.
- Zener breakdown occurs when the doping levels are high, and involves electrons tunnelling from the valence band of the p side to the conduction band on the n side.
- Avalanche breakdown occurs when charge carriers which are accelerated by the electric field gain enough kinetic energy such that, when they collide with lattice atoms, they ionise the lattice atoms to produce electron-ion pairs. These pairs, in turn, cause further ionisations, leading to an “avalanche” effect.
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