## LESSON 3. V I characteristics of diode-Zener diode- Zener and breakdown- V I characteristics of a Zener diode

Volt –ampere characteristics of semiconductor Diode

The figure shows the circuit for determining the volt-ampere characteristics of a semiconductor diode.  By the use of a potential divider, the applied voltage can be varied both in the forward and revere direction.

Whom the applied voltage in few, a small equalization potential Vo exists across the depletion region.  As the applied voltage in increased more and more, the correct goes an increasing.  The current is bipolar the character since it consultant –ve and the carrier (electron and holes).  The total current in constant throughout the circuit but the properties of current due to holes and electron varies with distance along the P-N bar.

Whom a reverse voltage is applied (b main the slide r s below 0), only a small current flows.  This small current is the reverse ** current Io.  IF the operating temp in increased, Io increase.  When the reverse voltage reaches a critical value, the reverse current  through the diode increase abruptly and a relatively large current can flow with little increase in voltage.  This phenomenon,  known as reverse break down, occurs because electron gain  enough energy so that ion zeta by collision takes  place and the covalent  bonds are distributed. The release a large number of electron and a large reverse current occurs.  This breakdown happens at the zener breakdown voltage and the diode in destroyed.  However, same diodes are built to operate specifically in the zener breakdown region and are known as zener diodes.

EQUIVALENT CIRCUIT OF A PN JUNCTION DIODE

When forward biased the junction offers ac resistance rac and possesses diffusion capacitance Cd.  Cd is taken into account at high frequencies only.  Hence the equivalent circuit as shown in Fig.  can represent it.

An opposing battery has been connected in series with rac to account for the junction barrier potential.  Resistance RR connected in parallel with capacitance CT can be represent a reverse biased junction.

ZENER DIODE

It is a reversed biased, heavily doped silicon or germanium PN junction diode that is operated in the breakdown region where current is limited by both external resistance and power dissipation of the diode.  Silicon is preferred to germanium because of its higher temperature and current capabilities.

If the reverse bias applied to a PN junction is increased, a point is reached when the junction breaks down and reverse current rises to a value limited only by the external resistance  connected in series with the junction.  This critical voltage is known as break down voltage VBR.   Once  break down has occurred, very little further increase in voltage is required to increase the current to relatively high values.  The junction offers almost zero resistance at this point.  The breakdown voltage depends on the width of the depletion region, which in turn depends on the doping level.  Two mechanisms are responsible for break down under increasing reverse voltage.

Zener break down

This form of break down occurs in junctions which being heavily doped have narrow depletion layers.  The break down voltage sets a very strong electric field about 108 V/m across this narrow layer.  This field is strong enough to break or rupture the covalent bonds thereby generating electron –hole pairs. Even a small further increase in reverse voltage is capable of producing large number of current carriers.  That is why the junction has very low resistance in the break down region.

Avalanche break down

This form of break down occurs in junctions which being lightly doped have wide depletion layers where the electric field is not strong enough to produce Zener break down.  Instead the minority collide with the semiconductor atoms in the depletion region.  Upon collision with valance electrons, covalent bonds are broken and electron-hole pairs are generated.  These newly generated charge carriers are also accelerated by the electric field resulting in more collision and hence further production of charge carriers.  This leads to an avalanche of charge carriers and subsequently, to a very low resistance.

V-I CHARACTERSTICS OF A ZENER DIODE

The forward characteristic is simply that of a ordinary forward biased junction diode.

The reverse characteristics indicates certain points.

1. Zener break down voltage

2. Minimum current to sustain break down.

3. Maximum Zener current limited by maximum power dissipation.

Since its reverse characteristics is not exactly vertical, the diode posse some resistance zener dynamic impedance.

It is negligible compared to the large external resistance connected in the circuit.