p - n junction diode

            p-n junction diode

 It is a single crystal of Ge or Si doped  in such a manner that one half portion of it acts a p-type semiconductor and the other half a n-type semiconductor. 

We take a thin p- type silicon (padi) semiconductor wafer and add to it a small quantity  of pentavalent impurity. A part of kisi wafer gets converted into n-Si  wafer. The wafer now contains p- region and n-region with Metallurgical junction between the two regions. 

Formation of depletion region and potential barrier in a p-n junction

As soon as p-n junction is formed, the majority charge carriers begin to diffuse from the regions of higher concentration to the regions of lower concentrations. Thus the electrons from the n- region diffuse into the p-region and where they combine with the holes and get neutralised. 

Similarly, the holes from the p-region diffuse into the n-region where they combine with the electrons and get neutralised. This process is called electron-hole recombination. 

The p-region near the junction is left with immobile -ve ions and n-region near the junction is left with +ve ions, as shown in diagram. The small region in the vicinity of the junction which is depleted of free charge carriers and has only immobile ions is called the depletion layer. 

The accumulation of negative charges in the p-region and positive charges in the n-region sets up a potential difference across the junction. This acts a a barrier and is called barrier potential V(B) which opposes the further diffusion of electrons and holes across the junction. The barrier potential sets up a barrier field E(B)  in the direction from n-region to p-region. 

The barrier potential V(B) depends on 

(1). The nature of the semiconductor

(2). Temperature

(3). The amount of doping. 

Circuit symbol for a p-n junction diode

Working of a p-n junction

An external potential difference can be applied to a p-n junction in two ways: 

(1). Forward biasing 

       If the positive terminal of a battery is connected to the p-side and negative terminal to the n-region side, then the p-n junction is said to be forward biased. 

As shown in diagram, here the applied voltage V opposes the barrier voltage V(B) in which V towards n - semiconductor and V(B) towards p - semiconductor. As a result of this 

(1). The effective barrier potential decreases to [ V(B) - V]  and hence the energy barrier across the junction decreases. 

(2). The majority charge carries holes from p- side and electrons from n-region side begin to flow towards the junction. 

(3). The diffusion of electrons and holes into the depletion layer decreases it's width, and the effective resistance across the p-n junction decreases. 

When V exceeds V(B) , the majority charge carriers start flowing easily across the junction and set up a large current  called forward current(mA), in the circuit. The current increases with the increase in applied voltage. 

Characteristics curve

It is the graph between applied potential and current through the p-n junction diode in forward bias

(1). The V-I graph is not a straight line  a junction diode does not obey ohm's law. 

(2). Intially, the current increases very slowly almost Negligibally, till the voltage across the diode crosses a certain value, called the threshold-voltage or cut-in-voltage. Before this voltage, the depletion layer plays a dominant role in controlling the motion of charge carriers. 

(3). After the cut-in-voltage, the diode current increases rapidly(exponentially), even for a very small increase in the diode bias voltage. The resistance across the junction becomes quite low.

(2). Reverse biasing

         If the positive terminal of a battery is connected to the n-side  and negative terminal  to the p- side, then the p-n junction is said to be reverse biased. 

As shown in diagram, the applied voltage V and barrier potential V(B) are in the same direction which is towards p - semiconductor. As a result of this 

(1). The barrier potential increases to [ V(B) + V] and hence the energy barrier across the junction increases. 

(2). The majority charge carriers move away from the junction, increasing the width of the depletion layer. 

(3). The resistance of the p-n junction becomes very large, and no current flows across the junction due to majority charge carriers. 

However, at room temperature there are always present some minority charge carriers like holes in n-region and electrons in p- region. The reverse biasing pushes them towards junction setting a current, called reverse or leakage current, in the external circuit in the opposite direction. As the minority charge carriers are much less in number than the majority charge carriers, hence the reverse current is small (MICROAMPERE). 

Characteristics curve

It is the graph between applied potential and current passes through the p-n junction diode in forward bias. 

(1). When the diode is reverse biased, a very small current, about a few MICROAMPERES flows, which almost remains constant with bias. This small current is called reverse saturation current. It is due to the drift of minority charge carriers across the junction. 

(2). When the reverse voltage across the p-n junction reaches a sufficiently high value, the reverse current suddenly increases to a large value. This voltage at which breakdown of the junction diode occurs is called zener breakdown voltage or peak-inverse  voltage of the diode.