ZENER DIODE

                                      ZENER DIODE

                   Zener diode is a pn junction diode, designed to operate in reverse breakdown region.  Actually a zener diode varies from pn junction diode only due to the doping level. In a zener diode the doping level is very high when compared to the pn junction diode. And also the zener diode is used as a voltage regulator in reverse bias condition. The reverse breakdown voltage of a zener diode depends on the doping level. So doping level matters in zener diode.

VI CHARACTERISTICS OF PN JUNCTION VS ZENER DIODE:




                      As we can see during the reverse bias condition the graph shows a slight change! We are going to discuss why this phenomenon occurs.

WORKING:

              Generally a zener diode works in three operating regions or stages, they are,

  1.  FORWARD REGION
  2.  LEAKAGE REGION
  3.  BREAKDOWN REGION
              We are going to see individually about these regions in the upcoming paragraphs!
1. FORWARD REGION:
                           

                                     During the forward bias condition (ie)when the positive terminal of the battery is connected to the anode and the negative terminal of the battery is connected to the cathode of the zener diode, the diode will not conduct until it cross overs the barrier voltage which is approximately equal to 0.7 for sillicon and 0.3v for germanium type material. The barrier voltage is created due to the fact that since the diode consist of a p type and a n type material, the holes from the p type region combine with the electrons of the n type region at the junction layer which causes a layer of immobile ions which acts as a barrier to the flow of the current. Hence this barrier layer is called depletion layer and the voltage which takes to completely demolish this depletion layer is called barrier voltage. When the given voltage is equal and greater than the barrier voltage the depletion layer disappears and the conduction starts. This process of zener diode is same as the pn junction diode.

2.LEAKAGE REGION:
 
                                 During the reverse bias condition where the positive terminal of the battery is connected to the anode end and the positive terminal of the battery is connected to the cathode end, the depletion layer will become much greater and no current will flow according to the theory. But actually a practical current flows during reverse bias during the increase in reverse bias voltage which is denoted as the reverse saturation current.  This small reverse current which flows under reverse bias condition is called reverse saturation current in other words. The region is called as leakage region.

3.BREAKDOWN REGION:
                                 
                                  As we keep on increasing the reverse bias voltage other than normal the zener diode shows a weird phenomenon of maintaining voltage.  Actually due to the high doping nature of the zener diode the raise of voltage to its peak causes interatomic bonds to break causing a release of ions resulting in flow of current while the voltage remains the same. The breakdown voltage of the zener diode totally depends on the doping level. There are two types of breakdown,

  • AVALANCHE BREAKDOWN
  • ZENER BREAKDOWN
AVALANCHE BREAKDOWN:

                                     As the reverse bias voltage gets increases the thermally generated carrier ions due to he higher reverse bias voltage, acquire more kinetic energy than the normal ions fron the reverse bias voltage as a result, the velocity of the carrier ion increases.
                                    These electrons disturb the covalent bond by colliding with the immobile ions and create new electron hole pairs. These new charge carriers acquire sufficient energy from the electric field and collide with other immobile ions making them mobile.  Thereby generating further electrons  holes pairs. This process is stimulative and generates an avalanche of charge carriers in a short time. This process is called as avalanche breakdown.

ZENER BREAKDOWN:
                 
                                     During the reverse bias condition when we keep on increasing the reverse bias voltage the rupture of covalent bonds take place at the strong electric field at the junction.  New electron hole pairs are released with increases the reverse current.  For lightly doped diode we have only avalanche breakdown.

ZENER DIODE AS A VOLTAGE REGULATOR:-
{SHUNT REGULATOR}


                                   



                                                           Consider the above circuit diagram for better understanding.
Actually it consist of two resistances one is the input resistance ri and the other is the output resistance rl or it is also called as load resistance.  The zener diode is connected in parallel with the input source and the load voltage where you can attach the voltmeter and measure the readings.  As the diode is in parallel with the load the regulator or the diode is called as shunt regulator.
                                         
                                                          Iin=Iz +Io 
                             Iin=(Vin  -Vz)/Rin
Since for the zener diode the output voltage equals the output of the zener diode, Vo=Vz;
                                                 THEREFORE
                                                    Io=Vo/Rl
Though the output voltage varies with these two reasons too which include,

  • change in input voltage:
  • change in load current:



CHANGE IN INPUT VOLTAGE:
                     Let us assume the load resistor is fized and the input voltage can be varied. Even that case the output voltage is equal to the output of the zener diode.

case1.
               If Vin increases then the input current I in increases it is constant because Vo=Vz and rl is fixed therefore from kcl equation as Iin increases. nthe corresponding chance will reflect in Iz.
case 2.
               If Vin increases Iin decreases because of which Iz. decreases. Vo will all time never actually a server. But the Vo remains constantly force him to the good certificate.

APPLICATION:

  • THE ZENER DIODE IS USED AS A VOLTAGE REGULATOR AND A RECTIFIER TOO.






















    
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