Volt-Ampere Characteristics of pn Junction

Volt-ampere or V-I characteristic of a  pn  junction (also called a  crystal or semiconductor diode) is the

curve between voltage across the junction and the circuit current.  Usually, voltage is taken along  x-

axis and current along  y-axis. The below  fig.1  shows the  circuit arrangement for determining the  V-I

characteristics of a  pn  junction. The characteristics can be studied under three heads, namely;    zero     external voltage ,   forward bias and   reverse bias .

                                             Figure:1

( i) Zero external voltage.   When the external voltage is zero, i.e. circuit is open at  K, the

potential barrier at the junction does not permit current flow.  Therefore, the circuit current is zero as

indicated by point  O in Fig.2

                                                   Figure:2                         

(ii ) Forward bias:

  With forward bias to the  pn  junction  i.e. p -type connected to positive terminal

and  n-type connected to negative terminal, the potential barrier is reduced.  At some forward voltage

(0.7 V for Si and 0.3 V for Ge), the potential barrier is altogether eliminated and current starts flowing

in the circuit.  From now onwards, the current increases with the increase in forward voltage. Thus, a

rising curve  OB is obtained with forward bias as shown in Fig. 2.  From the forward characteristic, it

is seen that at first (region OA),the current increases very slowly and the curve is non-linear.  It is

because the external applied voltage is used up in overcoming the potential barrier. However, once the

external voltage exceeds the potential barrier voltage, the pn  junction behaves like an ordinary conduc-

tor. 

Therefore, the current rises very sharply with increase in external voltage (region AB on the curve ).The curve is almost linear.

( iii ) Reverse bias.    With reverse bias to the pn  junction  i.e.p -type connected to negative terminal and n -type connected to positive terminal, potential barrier at the junction is increased.  Therefore, the junction resistance becomes very high and practically no current flows through the circuit .However, in practice, a very small current (of the order of µ A) flows in the circuit with reverse bias as shown in the reverse

                                                                  Figure:3

characteristic.  This is called  reverse  saturation current  ( Is)  and is due to the minority carriers. It may be recalled that there are a few free electrons in p -type material and a few holes in n-type material.  These undesirable free electrons in p -type and holes in n -type are called minority carriers .As shown in Fig. 3, to these minority carriers, the applied reverse bias appears as forward bias.

Therefore, a  small current flows in the reverse direction.If reverse voltage is increased continuously, the kinetic energy of electrons (minority carriers) may become high enough to knock out electrons from the semiconductor atoms.  At this stage break-down of the junction occurs, characterised by a sudden rise of reverse current and a sudden fall of the

resistance of barrier region. This may destroy the junction permanently.