A p-n junction is formed by bringing p-type semiconductor material in contact with the n-type semiconductor material and can be characterized in terms of its depletion region. This is because the width of the depletion region varies in accordance with the bias applied at the terminals, deciding the V-I characteristics of the p-n junction (Figure 1). The span of the depletion region is a function of both the applied bias as well as the level of doping. It is seen that in the forward bias condition, the width of the depletion region reduces with the increase in the applied voltage which eventually leads to an increase in the amount of current flow. On the other hand, if the p-n junction is reversing biased, an increase in the applied voltage increases the width of the depletion region.
However, even then there will be a little amount of current flow through the semiconductor due to the minority charge carriers. Moreover the width of the depletion region is observed to be narrow for heavily doped semiconductors and wide for lightly doped semiconductors.
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Now, consider a heavily doped semiconductor subjected to the reverse bias condition. Here the width of the narrow depletion region (due to high doping) is seen to increase with an increase in the voltage applied across its terminals. This leads to an increase in the electric field developed across the p-n junction as the electric field is nothing but the negative potential gradient. For example, a reverse voltage of 3V across a 100 Ao thick (extremely narrow) depletion region results in the generation of V/m electric field.
Due to this highly intensified electric field, a few of the covalent bonds in the p-n junction break-off releasing their valence electrons. Such free electrons will get excited and move into the conduction band leading to an abrupt increase in the current flow through the device. This phenomenon is referred to as Zener Breakdown and the corresponding voltage is called Zener Breakdown Voltage (VZ), shown in red color in Figure 1. The phenomenon was first observed and explained by Dr. Clarence Zener in 1934 and is thus named after him. Further it is to be noted that the Zener effect is a controllable phenomenon as the number of charge carriers generated can be effectively controlled by controlling the electric field applied. Typically Zener breakdown causes the diode junctions to breakdown below 5V and will not damage the device unless there is no provision made to release the heat generated. Moreover, the Zener breakdown voltage has negative temperature coefficient meaning which the Zener breakdown voltage reduces with the increase in the junction temperature. However, it is to be noted that the voltage at which the Zener breakdown occurs is adjustable during the device manufacture. Lastly it should be kept in mind that the working of widely used Zener diode is based upon the Zener effect.