THE P-N JUNCTION DIODE

• The PN junction is formed of the P type and N type semiconductor material.

• In P type, the holes are the majority charge carriers

• In N type material, the electrons are the majority charge carriers.

• Therefore at the junction there is a tendency for the free electrons to diffuse over to the P-side and holes to the N-side. This process is called diffusion.

• The diffusion of holes and free electrons   is due   to the difference   in concentration of the two regions.

• This  difference  in  concentration  creates  a  concentration  gradient  across  the junction.

• Due to the process of diffusion the negative acceptor ions in the P region and positive  donor  ions  in  the  N region  are  left  uncovered  in  the  vicinity  of  the junction as seen in

• The  additional  holes,  trying  to  diffuse  to  the  N-region,  are  repelled  by  the uncovered positive charge of the donor ions.

• Similarly, the electrons trying to diffuse into the P-region are repelled by the uncovered negative charge of the acceptor ions.

• Thus   a   barrier  is   set   up   near  the   junction,  which   prevents   the   further movement  of  charge  carriers.  This  is  called  as  potential  barrier  or  junction barrier V0.

• As a result, further diffusion of free electrons and holes across the junction is stopped.

• The region containing the uncovered acceptor and donor ions, in the vicinity of the junction is called depletion region.

• Since  this  region  has  immobile  ions,  which  are  electrically  charged,  the depletion region is also known as space-charge region.

• The  width  of  the  depletion  layer  depends  upon  the  doping  level  of  the impurity in N-type or P-type semiconductor.

• The higher the doping level, the thinner will b e the depletion layer and vice versa.

• The  depletion  layer  consists  of  fixed  rows  of  oppositely  charged  ions  on  its two sides.

• Because  of this  charge separation,  an electric potential is  established  across the junction, even when no external voltage being applied.

• This electric potential is called junction or potential barrier.

WORKING AND VI CHARACTERISTICS: UNDER FORWARD BIAS CONDITION:

When  positive  terminal  of  the  battery  is  connected  to  P-type  and  negative terminal  to  N-type  of  the  PN  junction  diode,  the  bias  applied  is  known  as  forward bias.

• Under  forward bias,  the  applied  positive  potential  repels  the holes  in  P-type region so that the holes move towards the junction.

• The  applied  negative  potential  repels  the  electrons  in the N-type  region  and the electrons also move towards the junction.

• Eventually, when the potential applied exceeds the internal barrier potential, the depletion region and internal potential barrier disappear.

                                               V/I characteristics under forward bias condition

As  forward  voltage  increases,  for  VF  <  V0  (potential  barrier),  the  forward current  I  is  almost  zero,  because  the  potential  barrier  the  potential  barrier prevents the flow of electrons and holes across the depletion region from N

and P regions respectively.

For VF >V0, the potential barrier is overcome and current increases rapidly.
Cut-in or threshold voltage: Below which the current is very small and at the cut-
in  voltage  the  potential  barrier  is  overcome  and  the  current  through  the  junction starts to increase rapidly




UNDER REVERSE BIAS CONDITION:

When  the  negative  terminal  of  the  battery  is  connected  to  the  P-type  and positive terminal of the battery is connected to N-type of the PN junction, the    bias applied is known as reverse bias.

                                                            Under reverse bias condition


                          

• Under  reverse  bias  condition, holes  from  P  side  move  towards the  negative terminal of the battery.

• The electrons  from N  side  are  attracted towards  the  positive  terminal of the battery.

• Hence the width of the depletion region increases.

• The  resultant  potential  barrier  also  increases,  which  prevents  the  flow  of majority charge carriers in both directions.

• Theoretically, no current should flow in the external circuit.

• But in practice, a very small current flows under reverse bias condition.

• Due to the  absorption of energy by the electrons cause the breaking of the covalent bonds.

• This results in the generation of electron-hole pairs.

• The thermally generated charge carriers cross the junction and giving rise to what is known as reverse saturation current.

• For large  applied  reverse  bias,  avalanche  effect  takes place, leading  to  very large reverse current.

• This leads to the breakdown of the junction.

• Breakdown voltage: The reverse voltage at which the junction breakdown occurs is called breakdown voltage.

                                            V/I characteristics under reverse bias condition



BREAKDOWN IN REVERSE BIASED

Though   the   reverse   saturation   current   is   not   dependent   on   the   applied reverse voltage, if reverse voltage is increased beyond particular value, large reverse    current  can  flow  damaging  the  diode.  This  is  called  reverse  breakdown  of  a diode. Such a reverse breakdown of a diode can take place due to the following two effects,

1. Avalanche effect and
2. Zener effect


BREAKDOWN DUE TO THE AVALANCHE EFFECT

Though  reverse  current  is  not  dependent  on  reverse  voltage,  if  reverse voltage is increased, at a particular value, velocity of minority carriers increases. Due
to the kinetic energy associated with the minority carriers, more minority carriers are generated  when  there  is  collision  of  minority  carriers  with  the  atoms.  The  collision makes  the  electrons  to  break  the  covalent  bonds.  These  electrons  are  available  as minority carriers and get accelerated due to high reverse voltage. They again collide

with another atom to generate more minority carriers. This is called caner
multiplication.  Finally  large  number  of  minority  carriers  move  across  the  junction, breaking the p-n junction. These large number of minority carriers give rise to a very     high  reverse  current.  This  effect  is  called  avalanche  effect  and  the  mechanism  of destroying the junction is called reverse breakdown of a p-n junction. The voltage at    which the  breakdown  of  a p-n  junction  occurs is  called reverse  breakdown  voltage. The  series  resistance  must  be  used  to  avoid  breakdown  condition,  limiting  the reverse current.



BREAKDOWN DUE TO THE ZENER EFFECT

The breakdown of a p-n junction may occur because of one more effect called zener  effect.  When  a  p-n  junction  is  heavily  doped  the  depletion  region  is  very narrow. So under reverse bias conditions, the electric field across the depletion layer
is  very  intense.  Electric  field  is  voltage  per  distance  and  due  to  narrow  depletion region and high reverse voltage, it is intense. Such an intense field is enough to pull
the electrons out of the valence bands of the stable atoms. So this is not due to the collision of carriers with atoms. Such a creation of free electrons is called zener effect which is different than the avalanche effect. These minority carriers constitute very     large current and mechanism is called zener breakdown.


The  breakdown  effects  are  not  required  to  be  considered  for  p-n  junction diode. These effects  are  required to be  considered  for special  diodes such as  zener diode as such diodes are always operated in reverse breakdown condition.