ZENER DIODE

• In  a  general  purpose  PN  diode  the  doping  is  light;  as  a  result  of  this  thebreakdown  voltage  is  high.  If  a  P  and  N  region  are  heavily  doped  then  the breakdown voltage can be reduced.

• When the doping is heavy even the reverse voltage is low, the electric field atbarrier  will  be  so  strong  thus  the  electrons  in the  covalent  bands  can  break away from the bonds. This effect is known as zener effect.

• A  diode  which  exhibits  the  zener  effect  is  called  a  zener  diode.  Hence  it  isdefined as a reverse biased heavily doped PN junction diode which operates in breakdown   region.   The   zener   diodes   have   been   designed   to   operate   at voltages ranging from a few volts to several hundred volts.

• Zener breakdown occurs in junctions which is heavily doped and have narrowdepletion layers. The  breakdown voltage sets up a  very strong  electric field.    This  field  is  strong  enough  to  break  or  rupture  the  covalent  bonds  thereby generating electron hole pairs.

• Even a small reverse voltage is capable of producing large number of currentcarrier, When a zener diode is operated in the breakdown region care must be taken to see that the power dissipation across the junction is within the power rating  of  the  diode  otherwise  heavy  current  flowing  through  the  diode  may destroy it.

Equivalent Circuit of Zener diode

The schematic symbol and its equivalent circuit are shown in figure 14. It is similar to that of normal diode except the line representing cathode is bent both ends are shown in figure 14.

V-I Characteristics of zenerdiode


The forward characteristic of a zener diode is similar to that of a P N Junction diode. The reverse characteristic of zener diode is obtained as follows.

• The reverse current that is present at the origin and the knee of the curve isdue to the reverse leakage current due to the minority carriers. This current is specified by stating its value at 80% of the zener voltage Vz

• As  the  reverse  voltage  is  gradually  increased,  the  breakdown  occurs  at  theknee  and  the  current  increases  rapidly.  To  control  this  current  a  suitable external  resistance  has  to  be  used.  The  maximum  permissible  value  of  the current is denoted by Izmax. The minimum usable current is Izmin

• The  voltage  across  the  terminals  of  the  diode  for  a  current  Iz  which  is  theapproximate  midpoint  of  the  linear  range  of  the  reverse  characteristics  in called the zener voltage Vz. At the knee point, the breakdown voltage remains constant  between  Izmax  and  Izmin.  This  ability  of  a  diode  is  called  regulating ability and is an important feature of a zenerdiode.

Application of Zener Diode

It can be used

a) As voltage regulators

b) As peak clippers

c) For reshaping waveforms

d) For meter protection against damage from accidental application of excessive voltage