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.
|
