Other diodes: Diode types
The Zener diode utilises the same basic structure as an ordinary diode, but the concept of operation of the reverse breakdown effects are not normally wanted or used for normal diode operation.
The Zener diode structure is optimised to ensure the required performance - this entails some differences to the structure of an ordinary diode.
Zener diode theory and operation
The basis of the operation of Zener diodes or voltage regulator diodes can be seen fromt heir characteristic IV curve.
In the forward direction they behave like a normal PN junction diode, but in the reverse direction their operation revolves around their characteristic in this section of the curve.
It can be seen that as the reverse voltage increases, the voltage is contained across the diode . . . until a certain point is reached.
Here they undergo a form of reverse breakdown. The slope of the reverse breakdown is such that over a wide range of current levels flowing through the diode, the voltage across it is held remarkably constant.
This makes these diodes very good as voltage regulators.
There are two effects that can be used in Zener diodes. One is called Zener breakdown, and the other, impact or avalanche ionisation. The Zener effect predominates below 5.5 volts whereas impact ionisation is the major effect above this voltage.
The two effects are totally different, although they produce almost identical effects.
• Zener breakdown effect:
Zener breakdown effect is the one from which the diode gains its popular name. It is the quantum mechanical effect tunnelling effect, but when applied to the voltage reference diode, it retains the Zener name after the man who discovered it.
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 Zener effect was first proposed by Dr. Clarence Zener in 1934 from whom it gains its name.
• Avalanche breakdown:
Avalanche breakdown, sometimes also called impact ionisation is very different to Zener breakdown and it 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 in a process known as impact ionisation. This newly created electron moves towards the positive voltage and is accelerated under the high electric field, and it 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, as the process grows as in an avalanche.
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.
It is worth noting that there is some hysteresis associated with avalanche breakdown. It is found that once the breakdown has commenced, the effect will continue even when the potential applied across the diode is reduced a litte.
The two reverse breakdown effects in the diode have very similar characteristics, but they are not the same. In most cases it is possible to ignore the difference between the two effects and use a diode in the same manner.
Temperature coefficients of breakdown mechanisms
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.
The Zener effect which predominates below 5.5 volts exhibits a negative temperature coefficient. However the avalanche effect which is the major effect above 5.5 volts has a positive temperature coefficient.
As a result Zener diodes or voltage reference 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.
The actual reverse voltage is repeatable for a given diode and is dependent upon the internal geometry and characteristics of the diode.
Design of voltage reference diodes
Zener voltage reference diodes are able to run in reverse breakdown condition within their specifications very reliably. If a conventional PN junction diode is run into reverse breakdown it is most likely to be irreparably damaged.
When a reverse voltage is applied to a conventional PN junction diode and the voltage is increased it will eventually breakdown and it will be subject to high current due to avalanche breakdown. If this current is not limited by the surrounding circuitry, the diode is very likely to be destroyed because of overheating.
A Zener diode exhibits almost the same properties, except the device is specially designed to have a reduced breakdown voltage. Also the Zener diode or voltage reference diode topology is designed in a way that the diode exhibits a controlled breakdown and allows the current to keep the voltage across the Zener diode close to the breakdown voltage.
In addition to this, the circuits in which Zener or voltage reference diodes are incorporated normally have series resistors to limit the current, thereby preventing catastrophic damage to the device.
Zener diodes are widely used for a host of different applications where a stable voltage is needed. Although there are two effects that exhibit the stable reverse breakdown voltage, the diodes are universally referred to as Zener diodes.
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