Zener Diode Theory & Structure
- a summary or tutorial covering the essentials of the Zener diode or voltage reference diode theory and the structure used for fabrication.
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
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.
- Impact ionisation: 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. 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. 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 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.
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.
By Ian Poole
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