Forward Biasing of PN Junction Diode

Forward Biasing of P‒N Junction Diode

• If an external d.c. voltage is connected in such a way that the p‒region terminal is connected to the positive of the d.c. voltage and the n‒region is connected to the negative of the d.c. voltage, the biasing condition is called forward biasing. The p‒n junction is said to be forward biased.

• Fig. 2.5.1 (a) shows the connection of forward biasing of a p‒n junction.

• To limit the current, practically a current limiting resistor is connected in series with the p‒n junction diode.

• Fig. 2.5.1 (b) shows the symbolic representation of a forward biased diode.

1. Operation of Forward Biased Diode

• When the p‒n junction is forward biased as long as the applied voltage is less than the barrier potential, there cannot be any conduction.

• When the applied voltage becomes more than the barrier potential, the negative terminal of battery pushes the free electrons against barrier potential from n to p‒region.

• Similarly positive terminal pushes the holes from p to n‒region. Thus holes get repelled by positive terminal and cross the junction against barrier potential.

• Thus the applied voltage overcomes the barrier potential. This reduces the width of depletion region.

• As forward voltage is increased, at a particular value the depletion region becomes very much narrow such that large number of majority charge carriers can cross the junction.

• The large number of majority carriers constitute a current called forward current.

• The current in the p‒region is the movement of holes which are majority carriers. This is the hole current. While the current in the n‒region is the movement of free electrons which are majority carriers. This is the electron current.

• Hence the overall forward current is due to the majority charge carriers. The action is shown in Fig. 2.5.2.

• These majority carriers can then travel around the closed circuit and a relatively large current flows.

2. Effect on the Depletion Region

• Due to the forward bias voltage, more electrons flow into the depletion region, which reduces the number of positive ions. Similarly flow of holes reduces the number of negative ions.

• This reduces the width of the depletion region. This is shown in Fig. 2.5.3.

Key Point: Depletion region narrows due to forward bias voltage.

3. Effect of the Barrier Potential

• Under the influence of applied forward bias voltage, the free electrons get the energy equivalent to the barrier potential so that they can easily overcome the barrier, which is a sort of a hill and cross the junction.

• While crossing the junction, the electrons give up the amount of energy equivalent to the barrier potential.

• This loss of energy produces a voltage drop across the p‒n junction which is almost equal to the barrier potential.

Key Point: The polarities of voltage drop across the p‒n junction in forward biased condition are opposite to that of barrier potential but the value is almost equal to the barrier potential.

• Due to the internal resistance, there is additional small voltage drop across the diode.

• Thus the total voltage drop across a p‒n junction diode in a forward biased condition is V_f and it is made up of,

1. Drop due to barrier potential and

2. Drop due to internal resistance.

  V_f = Vγ + I_f r_f

Key Point: The total V_f is of the order of 0.7 V for silicon and 0.3 V for the germanium.

4. Forward VI Characteristics of Diode

• The response of p‒n junction can be easily indicated with the help of characteristics called V-I characteristics of p‒n junction.

• It is the graph of voltage applied across the p‒n junction and the current flowing through the p‒n junction.

• Fig. 2.5.4 shows the forward biased diode.

• The applied voltage is V while the voltage across the diode is V_f.

• The current flowing in the circuit is the forward current I_f.

• The graph of forward current I_f against the forward voltage V_f across the diode is called forward characteristics of a diode.

• The forward characteristics of a diode is shown in Fig. 2.5.5.

• Basically forward characteristics can be divided into two regions:

1. Region O to P: As long as V_f is less than cut‒in voltage (V_γ), the current flowing is very small. Practically this current is assumed to be zero.

2. Region P to Q and onwards: As V_f increases towards V_γ the width of depletion region goes on reducing. When V_f exceeds V_γ i.e. cut‒in voltage, the depletion region becomes very thin and current I_f increases suddenly. This increase in the current is exponential as shown in Fig. 2.5.5 by the region P to Q.

• The point P, after which the forward current starts increasing exponentially is called knee of the curve and the corresponding voltage is called knee voltage.

Key Point: The normal forward biased operation of the diode is above the knee point of the curve. i.e. in the region P‒Q.

• The forward current is the conventional current, hence it is treated as positive and the forward voltage V_f is also treated positive. Hence the forward characteristics is plotted in the first quadrant.

5. Forward Resistance of Diode

• The resistance offered by the p‒n junction diode in forward biased condition is called forward resistance. The forward resistance is defined in two ways:

1) Static forward resistance : This is the forward resistance of p‒n junction diode when p‒n junction is used in d.c. circuit and the applied forward voltage is d.c. This resistance is denoted as R_f and is calculated at a particular point on the forward characteristics.

R_f = Forward d.c. voltage  / Forward d. c. current

= ОА / OC at point E

2) Dynamic forward resistance: The resistance offered by the p‒n junction under a.c. conditions is called dynamic resistance denoted as r_f.

Key Point: The dynamic resistance is reciprocal of the slope of the forward characteristics.

• Consider the change in applied voltage from point A to B shown in Fig. 2.5.5. This is denoted as ΔV_f.

• The corresponding change in the forward current is from point C to D. It is denoted as ΔI_f.

• Thus the slope of the characteristics is ΔI_f/ΔV_f. The reciprocal of the slope is dynamic resistance r_f.

r_f = ΔV_f  / ΔI_f  = 1 / (ΔI_f / ΔV_f)

 = 1 / Slope of forward characteristics

Key Point: Generally the value of r_f is very small of the order of few ohms, in the operating region i.e. above the knee.

Review Questions

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

2. Explain the static and dynamic resistances of the diodes.

3. Explain the V‒I characteristics of diode in forward biased mode.

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