PN Junction

PN JUNCTION

We join a piece of P-type semiconductor to a piece of N-type semiconductor such that the crystal structure remains continuous at the boundary as shown in fig.3.19 a PN junction is formed. Such a PN junction forms a very useful device and is called a semiconductor diode, PN junction diode (or) simply a crystal diode.

PN junction cannot be formed by simply joining (or) welding the two pieces. together, because it would produce a discontinuous crystal structure. Special fabrication techniques are used to prepare PN junctions.

Formation of Depletion Layer in a PN Junction

The semiconductor consisting of a PN junction it is shown in Figure 3.20. It may be noted that this entire sample is a single crystal. Its left half is a P-type and the right half is N-type.

The P-region has hole (as majority carriers) and negatively charged impurity atoms called negative ions. The majority carriers in P and N region are not shown in figure 3.20 for simplicity.

Holes and electrons are mobile charges, and therefore called as mobile charge carriers. On other hand positive and negative ions are immobile charges and do not take part in the conduction. P-region, the total positive charge on the holes is equal to the total negative charges on free electrons and immobile ions. Similarly, in the N-region, the total negative changes on free electrons is equal to the total positive charge on holes and immobile ions.

No external voltage has been connected to the PN junction of figure 3.20. As soon the junction is formed, the conduction and valence bands of P and N type materials over cap. As a result of this the following processes take place.

(i) The holes, from the P-region diffuse to the N-region, where they combine with the free electrons.

(ii) Free electrons, from the N-region diffuse to the P-region, where they combine with holes.

(iii) The diffusion of holes and free electrons takes place due to the reason that there is a difference of concentrations in the two regions. i.e the P-region has more numbers of free electrons. This difference in concentration creates a concentration gradient across the junction. It results in the diffusion of mobile charge carriers across the junction.

(iv) The diffusion of holes and free electrons across the junction take place for a short time. After a few recombinations of holes and free electrons, in the vicinity of the junction, a restraining force is automatically setup. This force is produced due to depletion region which exists on either side of the junction.

The recombination of free and mobile holes and electrons produce the narrow region at the junction called depletion layer.

Junction (or) Barrier Voltage (V_B)

The depletion layer of a PN junction has no mobile charge carriers. But it contains fixed rows of oppositely charged ions on its two sides. Because of this charge separation, an electric potential (V_B) is establish across the junction, even when the junction is not connected to any external voltage source as shown in Figure 3.21. This electric potential is called as junction (or) potential barrier.

This potential barrier exists a repelling force on the mobile charge carriers, trying to cross over the junction, unless the energy is supplied from an external source. At room temperature, the value of V_B. V_B = 0.7 V for silicon and 0.3 V for germanium.

Effect of Temperature on Barrier Voltage

The barrier voltage of a PN junction depends upon three factors namely density, electronic charge and temperature. For given PN junction, the first two factors are constant. Thus making the value of V_B dependent only on temperature. It has been observed that for both germanium and silicon the value of V_B decrease by 2 mv/°C. ΔV_B - 0.002 × Δt

Where Δt = increase in temperature in °C

Biasing the PN Junction

A PN Junction, connected to an external voltage source is called a biased PN junction. By applying external voltage across a PN junction, we are able to control the width of the depletion layer.

Figure 3.22 Biasing the PN junction

This allows us to control the resistance of the PN junction and also the amount of current that can pass through the device.

There are two ways of connecting voltage source to a PN junction.

(i) Forward bias: In this case, positive terminal of voltage source is connected to the P-side and negative terminal to the N-side as shown in Figure 3.22 (a). A large amount of current flows through the junction under this condition.

(ii) Reverse bias: In this case, positive terminal of the voltage source is connected to the N-side and negative terminal to the P-side as shown in Figure 3.22 (b). Practically, no current flows through the junction under this condition.