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Diode :

        A pure silicon crystal or germanium crystal is known as an intrinsic semiconductor. For most application, there are not
       free  electrons  and  holes  in  an  intrinsic  semi-conductor  to  produce  an  usable  current.  The  electrical  action  of  these
       modified  by  doping  means  adding  impurity  atoms  to  a  crystal  to  increase  either  the  number  of  free  holes  or  no
       electrons. When a crystal has been doped, it is called a extrinsic semi-conductor. They are of two types n-type semico
        having  free electrons as  majority carriers, p-type semiconductor  having  free holes as  majority carriers. By themselves
        doped  materials  are  of  little  use.  However,  if  a  junction  is  made  by  joining  p-type  semiconductor  to  n-type  semicond
        device  is  produced  which  is  extremely  used  known  as  diode.  It  will  allow  current  to  flow  through  it  only  in  one  directio
        unidirectional  properties  of  a  diode  allow  current  flow  when  forward  biased  and  disallow  current  flow  when  reversed
        This is called rectification process and therefore it is called rectifier.

        The  question  now  exists  how  is  it  possible  that  by  properly  joining  two  semi-conductors  each  of  which,  by  itself,  wi
        conduct the current in any direct refuses to allow conduction in one direction.

        Consider first the condition of p-type and n-type germanium just prior to joining fig1 . The majority and minority carriers
        constant motion.
Fig.1
The minority carriers are thermally produced and they exist only for short time after which they recombine and neutraliz
other.  In  the  mean  time,  other minority  carriers  have  been produced  and  this  process  goes  on  and  on.  The  number  o
electron hole pair that exist at any one time depends upon the temperature. The number of majority carriers is howeve
depending on the number of impurity atoms available. While the electrons and holes are in motion but the atoms are
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place and do not move.

As soon as, the junction is formed, the following processes are initiated: Fig2:
•  Holes from the p-side diffuse into n-side where they recombine with free electrons.

•    Free   electrons   from   n-side   diffuse   into   p-side   where   they   recombine   with   free   holes.   The   diffusion   of   el
   and holes is   due   to   the   fact   that   large   no   of   electrons   are        concentrated   in   one   area   and   large   no   o
   are concentrated in     another area.
•
When these electrons and holes begin to diffuse across the junction then they collide each other and negative charg
in the electrons cancels the positive charge of the hole and both will lose their charges.

The  diffusion  of  holes  and  electrons  is  an  electric  current  referred  to  as  a  recombination  current.  The  recomb
process  decay  exponentially  with  both  time  and        distance  from  the  junction.  Thus  most  of  the  recombination
 just  after  the  junction  is  made  and  very  near  to  junction.  A  measure  of  the  rate  of  recombination  is  the
defined as the time required for the density of carriers to decrease to 37% ot the original concentration.
l
The impurity atoms are of course, fixed in their individual places. The atoms itself is a part of the crystal and so can no
When  the  electrons  and  hole  meet,  their  individual  charge  is  cancelled  and  this  leaves  the  originating  impurity  atoms
net  charge,  the  atom  that  produced  the  electron  now  lack  an  electronic  and  so  becomes  charged  positively,  wher
atoms  that  produced  the  hole  now  lacks  a  positive  charge  and  becomes  negative.  The  electrically  charged  atoms  are
ions since they are no longer neutral. These ions produce an electric field as shown. After several collision occurs, the
field is great enough to repel rest of the majority carriers away of the junction. For example, an electron trying to diffuse
to p side is repelled by the negative charge of the p-side. Thus diffusion process does not continue indefinitely but co
as  long  as  the  field  is  developed.  The  net  result  of  this  field  is  that  it  has  produced  a  region,  immediately  surround
junction that has no majority carriers. The majority carriers have been repelled away from the junction and junction is d
from  carriers.  The  junction  is  known  as  the  barrier  region  or  depletion  region  fig3.  The  electric  field  represents  a  p
difference  across  the  junction  also  called  space  charge  potential  or  barrier  potential.  This  potential  is  0.7v  for  S
degree celcious and 0.3v for Ge.
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 The physical width of the depletion region depends on the doping level. If very heavy doping is used, the depletion re
physically  thin  because  diffusion  charge  need  not  travel  far  across  the  junction  before  recombination  takes  place  (s
time). If doping is light, then depletion is more wide (long life time)
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