Switching Characteristics

Switching  characteristics  of BJT  are shown  in figure. Due  to the presence of internal  capacitances BJT  cannot be  turned  instantly. Initially when base  emitter  voltage  is applied in CE  configuration leakage current  flows.  The collection current  begins  to rise after  some delay  time  and reaches at its  steady state  value. the time  from which  collector  current  starts  rising  to its  steady  state value  known  as rise time. BJT gets turned on in and collector emitter  voltages falls from  its maximum  value  forward voltage drop.

Till the forward  base  current is  maintained the BJT  remains in  on condition. As  the base emitter  voltage  is removed at a time t1 does not change for a  time ( there  its is  storage time in which the charges are removed from the base).  After  this time I_c begins to fall and collector voltage starts rising  for a time t_f  ( fall time ) . in  fall time  decreases to leakage current  and voltage  rises  to v_cc. Hence the turn off time  of BJT is  t off = t_s +  t_f. In conduction period of BJT a steady state  current  flows and  forward  voltage  drop  appears across the  collection emitter  junctions.