When a semiconductor p-n junction diode is reverse biased and the reverse voltage is increased, the breakdown voltage is reached where the reverse current increased sharply. This breakdown was first explained by the American scientist C. Zener. The breakdown voltage and current are also called zener voltage and zener current respectively. The zener voltage depends on the amount of doping. When an ordinary semiconductor diode is heavily and properly doped, it has a sharp breakdown voltage and such a device is called "zener diode".
In other words, we can define zener diode as "A properly doped crystal diode which has a sharp breakdown voltage is known as a Zener Diode. The symbol of zener diode is shown in below fig.
Fabrication of Zener Diode
A zener diode is fabricated by heavily doping both P- and N- side of the P-N junction diode. Due to heavy dopping, the thickness of the depletion region is very thin (< 10-6 m). As a result, in reverse bias configuration, the electric field E of the junction is extremely high even for small reverse bias voltage of about 5 volt. So, the breakdown voltage of a zener diode is comparatively small and sharp.
I-V characteristics of a Zener Diode
The I-V characteristics of a zener diode is shown in figure. The forward bias characteristics is almost same as that of a simple P-N junction diode. In reverse bias region, the initial reverse bias saturation current is small. However, when the applied reverse bias voltage (V) reach the breakdown voltage (Vz) of the zener diode, there is a large and sudden change in the current.
Under the effect of reverse biasing the current remains almost constant over a wide voltage range upto a limit when the current increases sharply due to a sudden increase in the number of electron-hole pairs. This limit of the reverse voltage can be adjusted by varying the doping levels as the zener potential (Vz) decreases by increasing the doping. If the diode is heavily doped, depletion layer becomes thin and consequently the breakdown voltage will be lower.
The mechanism of breakdown can be explained as follows. Leakage current in P-N junction diode is largely due to the drift of minority carriers across the junction of under the influence of the reverse bias. If the electric field in the region of junction is sufficiently great, the electrons are accelerated and acquire energy high enough to ionize atoms by collision and generate electron-hole pairs in the depletion layer.
Thus, when the reverse voltage applied across a P-N junction becomes high enough to impart sufficient energy to the minority charge carriers, some of the covalent bonds are broken due to their collision with the semiconductor atoms. The large number of electrons and holes has produced is responsible for the rapid rise in the saturation current. This phenomenon is known as avalanche breakdown or zener breakdown.
Use of Zener Diode
A zener diode can be used as voltage stabilizer to provide a constant voltage from a source whose voltage varies over a wide range.