Reverse Biasing of PN Junction Diode

Reverse Biasing of P‒N Junction Diode

• 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.

• Fig. 2.6.1 (a) shows the connection of a reverse biasing of a p‒n junction while Fig. 2.6.1 (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 Fig. 2.6.2.

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 Fig. 2.6.3.

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

2. Breakdown Mechanisms

• If reverse voltage is increased beyond particular value, large reverse current can flow damaging the diode. This is called reverse breakdown of a diode.

• Such a reverse breakdown of a diode can take place due to the following two effects,

1. Avalanche effect and

2. Zener effect

Breakdown due to the Avalanche Effect:

• If reverse voltage is increased, at a particular value, velocity of minority carriers increases.

• Due to the kinetic energy associated with the minority carriers, more minority carriers are generated when there is collision of minority carriers with the atoms. The collision make the electrons to break the co‒valent bonds.

• These electrons are available as minority carriers and get accelerated due to high reverse voltage. They again collide with another atoms to generate more minority carriers. This is called carrier multiplication.

• Finally large number of minority carriers move across the junction, breaking the p‒n junction. These large number of minority carriers give rise to a very high reverse current.

• This effect is called avalanche effect and the mechanism of destroying the junction is called reverse breakdown of a p‒n junction. The voltage at which the breakdown of a p‒n junction occurs is called reverse breakdown voltage.

Breakdown due to the Zener Effect:

• Electric field is voltage per distance and due to narrow depletion region and high reverse voltage, it is intense.

• Such an intense field is enough to pull the electrons out of the valence bands of the stable atoms.

• Such a creation of free electrons is called zener effect which is different than the avalanche effect. These minority carriers constitute very large current and mechanism is called zener breakdown.

• These effects are required to be considered for special diodes such as Zener diode as such diodes are always always operated in reverse breakdown condition.

3. Reverse Characteristics of P‒N Junction Diode

• Fig. 2.6.4 shows the reverse biased diode. The reverse voltage across the diode is V_R while the current flowing is reverse current I_R flowing due to minority charge carriers.

• The graph of I_R against V_R 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 Fig. 2.6.5.

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 I₀ though reverse voltage is increased.

• The point A where breakdown occurs and reverse current increases rapidly is called knee of the reverse characteristics.

• The voltage corresponding to point A is called reverse breakdown voltage V_BR of diode.

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 R_r It is the ratio of applied reverse voltage to the reverse saturation current I₀.

R_r = Applied reverse voltage / Reverse saturation current

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

 r_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 Questions

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

2. Explain the V‒I charactersics of diode in reverse biased mode.

3. Explain briefly : i) Avalanche breakdown ii) Zener breakdown.

4. With necessary diagram, describe the characteristics of a reverse biased PN junction diode.