Reverse Biasing of P-N Junction Diode

Reverse Biasing of P-N Junction Diode

AU : May-06, 07, 08, 09, 10, Dec.-05, 06, 12, 14, 15

• If an external d.c. voltage is connected in such a way that the p-region terminal of a p-n junction is connected to the negative of the battery and the n-region terminal of a p-n junction is connected to the positive terminal of the battery, the biasing condition is called reverse biasing of a p-n junction.

Key Point : Reverse biasing means connecting p-region to -ve and n-region to +ve of the battery.

• The Fig. 1.7.1 (a) shows the connection of a reverse biasing of a p-n junction while the Fig. 1.7.1 (b) shows the symbolic representation of a reverse biased diode.

1. Operation of Reverse Biased Diode

• When the p-n junction is reverse biased the negative terminal attracts the holes in the p-region, away from the junction. The positive terminal attracts the free electrons in the n-region away from the junction. No charge carrier is able to cross the junction. As electrons and holes both move away from the junction, the depletion region widens. This creates more positive ions and hence more positive charge in the n-region and more negative ions and hence more negative charge in the n-region. This is because the applied voltage helps the barrier potential. This is shown in the Fig. 1.7.2.

Key Point : Reverse biasing increases the width of the depletion region.

• As depletion region widens, barrier potential across the junction also increases. However, this process cannot continue for long time. In the steady state, majority current ceases as holes and electrons stop moving away from the junction.

• The polarities of barrier potential are same as that of the applied voltage. Due to increased barrier potential, the positive side drags the electrons from p-region towards the positive of battery. Similarly negative side of barrier potential drags the holes from n-region towards the negative of battery. The electrons on p side and holes on n side are minority charge carriers, which constitute the current in reverse biased condition. Thus reverse conduction takes place.

• The reverse current flows due to minority charge carriers which are small in number. Hence reverse current is always very small.

Key Point : The generation of minority charge carriers depends on the temperature and not on the applied reverse bias voltage. Thus the reverse current depends on the temperature i.e. thermal generation and not on the reverse voltage applied.

• For a constant temperature, the reverse current is almost constant though reverse voltage is increased upto a certain limit. Hence it is called reverse saturation current and denoted as I₀.

Key Point : Reverse saturation current is very small of the order of few microamperes for germanium and few nanoamperes for silicon p-n junction diodes.

• The reverse current and its direction is shown in the Fig. 1.7.3.

• The reverse biasing produces a voltage drop across the diode denoted as V_R which is almost equal to applied reverse voltage. 

2. Breakdown in Reverse Biased

• Though the reverse saturation current is not dependent on the applied reverse voltage, if reverse voltage is increased beyond particular value, large reverse current can flow damaging the diode. This is called reverse breakdown of a diode. The corresponding voltage is called reverse breakdown voltage of a diode denoted as V_BR.

Why to avoid reverse breakdown ?

1. Large reverse voltage appears across the diode and large current flows through the diode in reverse breakdown condition.

2. So large power gets dissipated which appears in the form of heat at the junction.

3. This increases junction temperature beyond the safe limits and this may damage the diode permanently.

• So reverse breakdown must be avoided for conventional diodes.

• Some special diodes are manufactured to be operated in the reverse breakdown region and are called zener diodes.

3. Reverse Characteristics of P-N Junction Diode

• The Fig. 1.7.4 shows the reverse biased diode. The reverse voltage across the diode is VR while the current flowing is reverse current IR flowing due to minority charge carriers. The graph of IR against VR is called reverse characteristics of a diode.

• The polarity of reverse voltage applied is opposite to that of forward voltage. Hence in practice reverse voltage is taken as negative. Similarly the reverse saturation current is due to minority carriers and is opposite to the forward current. Hence in practice reverse saturation current is also taken as negative. Hence the reverse characteristics is plotted in the third quadrant as shown in the  Fig. 1.7.5

Key Point : Typically the reverse breakdown voltage is greater than 50 V for normal p-n junctions.

• As reverse voltage is increased, reverse current increases initially but after a certain voltage, the current remains constant equal to reverse saturation current IQ though reverse voltage is increased. The point A where breakdown occurs and reverse current increases rapidly is called knee of the reverse characteristics. 

4. Reverse Resistance of Diode

• The p-n junction offers large resistance in the reverse biased condition called reverse resistance. This is also defined in two ways.

1. Reverse static resistance : This is reverse resistance under d.c. conditions, denoted as Rr. It is the ratio of applied reverse voltage to the reverse saturation current I₀.

R_r = OQ / I₀ =  Applied reverse voltage / Reverse saturation current

2. Reverse dynamic resistance : This is the reverse resistance under the a.c. conditions, denoted as rr. It is the ratio of incremental change in the reverse voltage applied to the corresponding change in the reverse current.

T_r  = ΔV_R / ΔI_R       = Change in reverse voltage / Change in reverse current

• The dynamic resistance is most important in practice whether the junction is forward or reverse biased.

Review Question

1. With a neat diagram explain the working of a PN junction diode in reverse bias condition.

AU : May-06,07,08,09,10, Dec.-05,06,12,14,15, Marks 5