Zener diode

ZENER DIODE

A Zener diode is a t ype of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is lar ger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this el ectrical property.

Figure 4.6 Diode symbol

However, the Zener Diode or "Breakdown Diode" as they are sometimes called, are basically the same as the standard PN junction diode but are specially designed to have a low pre-determined Reverse Brea kdown Voltage that takes advantage of this high reverse voltage. The point at which a zener dio de breaks down or conducts is called the "Ze ner Voltage" (Vz).

The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction. When biased in the forward direction it behaves just like a nor mal signal diode passing the rated current, but when a reverse voltage is applied to it the reverse saturation current remains fairly constant over a wide range of voltages. The reverse vo ltage increases until the

diodes breakdown voltage V B is reached at which point a process called Avalanche Breakdown occurs in the depletion lay er and the current flowing through the zener diode increases dramatically to the maximum circuit value (which is usually limited by a series resistor). This breakdown voltage point is called the "zener voltage" for zener diodes.

Avalanche Breakdo wn: There is a limit for the reverse voltag e. Reverse voltage can increase until the diode brea kdown voltage reaches. This point is called Avalanche Breakdown region. At this stage maximu m current will flow through the zener diode. This breakdown point is referred as “Zener voltage”.

The point at which current flows can be very accurately cont rolled (to less than 1% tolerance) in the doping st age of the diodes construction giving the diode a specific zener breakdown voltage, (Vz) ra nging from a few volts up to a few hundred volts. This zener breakdown voltage on the I-V curve is almost a vertical straight line.

Zener diode characteristics

The Zener Diode is used in its "reverse bias" or reverse breakdo wn mode, i.e. the diodes anode connects to the negativ e supply. From the I-V characteristics curv e above, we can see that the zener diode has a regio n in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the di ode and remains nearly constant even with large cha nges in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max).

Figure 4.7 Zener diode characteristics 

Applications of zener diode

1.The Zener Diode Regulator

Figu re 4.8 Zener diode act as voltage regulator

The constant reverse voltage  of the zener diode makes it a valua ble component for the regulation of the output voltagge against both variations in the input volta ge from an unregulated power supply or variations in t he load resistance. The current through the ze ner will change to keep the voltage at within the limit s of the threshold of zener action and the maximum power it can dissipate.

2. Zener-Controlled Output Switching

This comparator application makes use of the properties of the zener diode to cause the output to switch between voltages determined by the zener diodes when the input voltage difference  changes sign. The output circuit amounts to a zener regulator which switches from one zener voltage to the other on a transition.

3. Zener Limiter

A single Zener diode can limit one side of a sinusoidal waveform to the zener voltage while clamping the other side to near zero. With two opposing zeners, the waveform can be limited to the zener voltage on both polarities.

Figure 4.9 Zener limiter

4. Zener Role in Power Supplies

The zener diode is widely used as a voltage regulator because of its capacity to maintain a constant voltage over a sizeable range of currents. It can be used as a single component across the output of a rectifier or incorporated into one of the variety of one-chip regulators Basically there are two type of regulations such as:

a) Line Regulation

In this type of regulation, series resistance and load resistance are fixed, only input voltage is changing. Output voltage remains the same as long as the input voltage is maintained above a

minimum value.

Percentage of line regulation can be calculated by = 

Where V₀ is the output voltage and V_IN is the input voltage and ΔV₀ is the change in output voltage for a particular change in input voltage ΔV_IN.

b) Load Regulation

In this type of regulation, input voltage is fixed and the load resistance is varying. Output volt remains same, as long as the load resistance is maintained above a minimum value.

Difference between Zener breakdown from avalanche breakdown

Zener Breakdown

1.This occurs at junctions which being heavily doped have narrow depletion layers .

2.This breakdown voltage sets a very strong electric field across this narrow layer.

3.Here electric field is very strong to rupture the covalent bonds thereby generating electron-hole pairs. So even a small increase in reverse voltage is capable of producing large number of current carriers. i.e. why the junction has a very low resistance. This leads to Zener breakdown.

Avalanche breakdown

1.This occurs at junctions which being lightly doped have wide depletion layers.

2.Here electric field is not strong enough to produce Zener breakdown.

3.Her minority carriers collide with semi conductor atoms in the depletion region, which breaks the covalent bonds and electron-hole pairs are generated. Newly generated charge carriers are accelerated by the electric field which results in more collision and generates avalanche of charge Carriers. This results in avalanche breakdown.