Reverse Bias :

        When  the  diode  is  reverse  biased  then  the  depletion  region  width  increases,  majority  carriers  move  away  from  the  junction
       and  there  is  no  flow  if  current  due  to  majority  carriers  but  there  are  thermally  produced  electron  hole  pair  also.  If  these
       electrons and holes are generated in the vicinity of junction then there is a flow of current. The negative voltage applied to the
        diode will tend to attract the holes thus generated and repel the electrons. At the same time, the positive voltage will attract the
        electrons towards the battery and repel the holes. The electron in the p-material and hole in the n-material are being forced to
        move forward each other and will probably combine. This will cause current to flow in entire circuit. This current is usually very
        small (interms of micro ampher to nano amphere). Since this current is due to minority carriers and these number of minority
        carriers are fixed at a given temperature therefore, the current is almost constant known as reverse saturation current Ico. In
        actual diode, the current is not almost constant but increases slightly with voltage. This is due to surface leakage current. The
        surface  of  diode  follows  ohmic  law  (V=IR).  The  resistance  under  reverse  bias  condition  is  very  high  100k  to  mega  ohms.
        When the reverse voltage is increased, they and certain value, then breakdown to diode takes place and it conducts heavily.
        This is due to avalanche or zener breakdown. Fig4
Fig.4
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Forward bias :

        When the diode is forward bias, then majority carriers are pushed towards junction, when they collide and recombination takes
        place. Number of majority carriers are fixed in semi-conductor. Therefore as each electron is eliminated at the junction, a new
        electron must be introduced, this comes from battery. At the same time, one hole must be created in p-layer. This is formed by
        extracting one electron from p-layer. Therefore, there is a flow of carriers and thus flow of current.

Space Charge Capacitance on Transition Capacitive CT :

       Reverse  bias  causes  majority  carriers  to  move  away  from  the  junction,  thereby  creating  more  ions.  Hence  the  thickness  of
       depletion region increases. This region behaves as the dielectric  material used for making capacitors. The p-type and n-type
       conducting on each side of dielectric act as the plate. The incremental capacitance CT is defined by
Where, dQ is the increase in charge caused by a change dv in voltage.

                                                   i = dQ / dt

                                                 i = CT dv / dt

 CT  is  not  constant,  it  depends  upon  applied  voltage,  therefore  it  is  defined  as  dQ/dV.  When  p-n  junction  is  forward  biased,
then also a capacitance is defined called diffusion capacitance CD (rate of change of injected charge with voltage) to take into
account the time delay in moving the charges across the junction by the diffusion process. Therefore, it cannot be identified in
terms  of  a  dielectric  and  plates  as  space  charge  capacitance.  It  must  be  considered  as  a  fictitious  element  that  allow  us  to
predict  time  delay.  If  the  amount  of  charge  to  be  moved  across  the  junction  is  increased,  the  time  delay  is  greater,  it  follows
that diffusion capacitance varies directly with the magnitude of forward current.
Real Diode: Small Signal Operation: (Load Line)

        Consider the diode circuit shown in Fig.5
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 This equation involves two unknown and can not be solved. The other equation interms of these two variables, is given by the
static  characteristic.  The  straight  line  represented   by  this  above  equation  is  known  as  the  load  line.  The  load  line  pass
through two points, I = 0 , VD = V and VD = 0, I = V /  R  L. The slope of this line is equal to 1/ RL. The point of    inter-section of
straight line and diode characteristic gives the operating point. Fig.6
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Fig.6
Say V=1V, RL=10ohm. (Fig7).
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Fig.7
Therefore, as the diode voltage varies, diode current also varies, sinusoidally (Fig 8).
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Fig.8
 In certain applications only a.c. equivalent circuit is required. Since only a.c. response of the circuit is considered d.c. Source
is  not  shown  (Fig  9).  The  resistance r f  represents  the  dynamic  resistance  or  a.c.  resistance  of  the  diode  .It  is  obtained  by
taking the ratio of at operating point.
Fig. 9
Dynamic Resistance
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