• When  diode  is  switched  from  forward  biased  to  the  reverse  biased  state  or viceversa, it takes finite time to attain a steady state.

  • This  time  consists  of  a  transient  and  an  interval  of  time  before  the  diode attains  a  steady  state.  The  behavior  of  the  diode  during  this  time  is  called switching characteristics of the diode.

  • In  the forward-bias state, there are a large number of electrons from the n
  • side  diffusing  into  p  side  and  a  large  number  of  holes  diffusing  into  n  side from  p  side.  This  diffusion  process  establishes  a  large  number  of  minority carriers in each material.

  • When forward biased, let n is concentration of electrons on p side at thermal equilibrium and p is concentration of holes on n side thermal equilibrium. This is concentration level far away from the junction.

  • It  increases  towards  the  junction  and  becomes  n  and  Pn  on  p  and  n  side respectively in steady state. These minority charge carriers are supplied from  other side of the junction, where those carriers are majority in number.

  • When  the  diode  is  reverse  biased,  again  far  from  the  junction  the  minority charge concentration is n on p side and Pno on n side.

  • In reverse biased condition, as they approach the junction, they quickly cross     the  junction.  Hence  minority  carrier  concentration  decreases  to  zero  at  the junction in steady state. This is .shown in the Fig.

  • Now when  a  forward  biased  diode is  suddenly reverse  biased, it  takes  finite time  to  change  the  minority  charge  carrier  concentration  and  to  attain  new steady state value.

  • The   diode   can   not   attain   steady   state   till   the   minority   charge   carrier concentration changes from that corresponding to the forward biased to that corresponding to the reverse biased.

  • Till the excess charge carrier concentration pn-pno and np-npo reduces to zero, the diode continues to conduct. This current is decided by the current limiting external resistance connected in the circuit.

  • Hence in switching applications, the time required by the diode to attain new steady state, plays an important role.

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