Summary of the zener diode

Zener diodes are semiconductor diodes that are widely used as voltage reference sources in the electronics industry. Providing a stable voltage they are often used in power supplies when regulated outputs are needed. Being cheap and easy to use they are ideal for many applications.

Operation
Zener diodes or as they may sometimes be called, reference diodes operate like an ordinary diode in the forward bias direction. They have the normal turn on voltage of 0.6 volts for a silicon diode. However in the reverse direction their operation is rather different. For very low voltages, like a normal diode they do not conduct at all. However once a certain voltage is reached the diode “breaks down” and current flows. It can be seen by looking at the curves for Zener diodes that the voltage is almost constant regardless of the current carried. This means that the diode provides a stable known voltage across it. However for normal operation, a resistor must be placed in series with the diode to limit the amount of current flowing, and the top connection of the diode (i.e. at the connection between the resistor and diode) is used for the stable reference voltage point.

The actual reverse voltage is repeatable for a given diode and is dependent upon the internal geometry and characteristics of the diode.

Modes of operation
There are two effects that can be used in Zener diodes. One is called Zener breakdown, and the other, impact ionisation. The Zener effect predominates above about 5.5 volts whereas impact ionisation is the major effect below this voltage.

The two effects are totally different, although they produce almost identical effects. Impact ionisation occurs when a high electric field is present in a semiconductor. Electrons are strongly attracted and move towards the positive potential. In view of the high electric field their velocity increases, and often these high energy electrons will collide with the semiconductor lattice.

When this occurs a hole-electron pair is created. This newly created electron moves towards the positive voltage and is accelerated under the high electric field, and it to may collide with the lattice. The hole, being positively charged moves in the opposite direction to the electron. If the field is sufficiently strong sufficient numbers of collisions occur so that an effect known as avalanche breakdown occurs. This happens only when a specific field is exceeded, i.e. when a certain reverse voltage is exceeded for that diode, making it conduct in the reverse direction for a given voltage, just what is required for a voltage reference diode.

The Zener effect occurs in a totally different manner. Under most conditions electrons are contained within atoms in the crystal lattice. In this state they are in what is called the valence band. If a large electric field is placed across the semiconductor this may be sufficient to pull the electrons out the atom into what is called the conduction band. When they are free from the atom they are able to conduct electricity, and this gives rise to the name of the conduction band. For them to pass from the valence band into the conduction band there must be a certain force to pull them free. It is found that once a certain level of electric field is present a large number of electrons are pulled free creating allowing current to suddenly start to flow once a certain reverse voltage is reached.

The reverse conduction effects, in common with many other aspects of semiconductor technology are subject to temperature variations. It is found that the impact ionisation and Zener effects have temperature coefficient in opposite directions. As a result Zener diodes with reverse voltages of around 5.5 volts where the two effects occur almost equally have the most stable overall temperature coefficient as they tend to balance each other out for the optimum performance.