Forward Biased PN Junction

FORWARD BIASED PN JUNCTION

In unbiased PN junction, there is no flow of current, when forward bias is applied to the PN junction, the applied forward voltage creates an electric field opposite to the potential barrier.

i. Thus the potential barrier is reduced at the junction. As the potential barrier is very small (0.7 V for Si and 0.3 V for Ge), a small forward voltage is sufficient to eliminate the barrier potential.

ii. The applied positive potential repels the holes in P-type region towards the junction. The electrons in the n-type region are also repelled towards the junction due to the negative voltage applied in n-region.

iii. When the applied potential is more than the internal barrier potential, the depletion region completely disappears and the junction resistance becomes zero.

iv. The junction provides a low resistance path and thus current flows in the circuit. This current is called forward current.

v. When the forward voltage is increased, the large number of majority charge carriers can cross the junction. These large number of majority charge carriers constitute the forward current.

The current in P - region is due to the movement of holes which are majority carriers. This is called hole current.

The current in the n-regions is due to the movement of free electrons which are majority carriers. This is called electron current. The overall forward current is due to the majority charge carriers.

These majority carriers can travel around the closed circuit and large amount of current flows from negative to positive of the battery.

The direction of conventional current is from positive to negative of the battery which is opposite to the direction of flow of electrons.

The voltage drop across a p- n junction diode in forward biased condition is made up of

(i) drop due to barrier potential

(ii) drop due to internal resistance

REVERSE BIASED P-N JUNCTION

The P-region is connected to negative terminal of the battery and n -region is connected to positive terminal of the battery.

When p-n junction is reverse biased, the negative terminal attracts the holes in p - region and the positive terminal attracts the free electrons in n - region away from the junction.

The electrons and holes move away from the junction and the width of depletion region increases.

The charge carriers are unable to cross the junction.

Thus there are more positive ions in the n- region and more negative ions in the p- region.

When the depletion region widens, the barrier potential across the junction also increases.

The applied reverse voltage creates an electric field which is in same direction of potential barrier and thus the barrier width is increased.

This increased potential barrier prevents the flow of charge carriers across the junction.

When PN junction is reverse biased, it has high resistance path and no current flows in the circuit.

After certain extent, the junction breakdown occurs. Then a small amount of current flows through it due to minority charge carriers i.e., the electrons on p-side and holes on n- -side constitute current in reverse biased condition. So, the reverse current is always very small. The reverse current remains constant even if the reverse voltage is increased upto a certain limit. Hence, it is called Reverse saturation current. The magnitude of reverse saturation current depends on junction temperature.

PN DIODE SYMBOL

V-I CHARACTERISTICS OF PN DIODE

Fig.1.12 shows the circuit for V-I characteristics of PN junction diode.

V-I characteristics is used for studying the response of p - n junction.

It is defined as the graph of voltage applied across the p-n junction and the current flowing through the p-n junction.

As shown in Fig.1.13, the applied voltage is V and 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.

As voltage is applied, the current flow is very small upto the cut-in voltage V_C or threshold voltage V_Th. or knee voltage.

When the applied voltage exceeds the threshold voltage the width of the depletion region is further reduced and the forward current rises exponentially as shown in Fig.1.13. If the forward voltage is increased beyond a certain value, extremely large amount of current is produced which may destroy the diode due to overheating.

The point after which the current starts increasing exponentially is called Knee of the curve.

Reverse Bias

The Fig. 1.14 shows the reverse biased p- n diode. The reverse voltage across the diode is V_R and the current flowing through the diode is reverse current I_R. The reverse current is mainly due to minority charge carriers. The graph of reverse current I_R versus reverse voltage V_R is called reverse characteristics of a diode.

During reverse bias, all the majority carriers are attracted by the battery i.e., holes from P-type move towards negative terminal of battery and electrons from n-type move towards positive terminal of battery. Thus only immobile ions near the junction form a strong depletion region which offers very high resistance for majority carriers.

Hence only very small current flows in the circuit. The polarity of reverse voltage applied is opposite to that of forward voltage. So, the reverse saturation current is opposite to the forward current and its value is negative.

i. When reverse voltage is increased, reverse current increases initially upto certain voltage. After some extent, the current remains constant although reverse voltage is increased. This current is called reverse saturation current I_o the point at which breakdown occurs and reverse saturation current increases rapidly is called knee of the reverse characteristics.