P-N Junction
When a p-type semiconductor is brought into a close contact with n-type semiconductor crystal, the resulting arrangement is a PN junction or junction diode. On account of difference in concentration of charge carriers in the two sections, the electrons from n-region diffuses through the junction into p region and the holes from p-region diffuse into n-region. Due to this the electron falls into the vacancy i.e., it completes the covalent bond. This process is called electron-hole recombination. As a result of the migration of charge carriers across the junction, the electrons leave ionised donor atoms which are bound and cannot move. Similarly, the p-region of the junction will have ionised acceptor atoms which are immobile. The accumulation of electric charges of opposite polarities in the two regions gives rise to an electric field. This is like a fictitious battery and prevents the further migration of charges. This battery is otherwise the potential barrier VB. This region which is devoid of any free charges is called depletion region.
The width of the depletion region and VB depends on the semiconductor and its doping concentration. Symbolically the p-n junction is shown as where P-side is known as anode and n-side the cathode. This junction is also called a semiconductor diode.
Biasing of the P-N junction
Forward biasing
A p-n junction is said to be forward biased, if the positive terminal of the external battery B is connected to p-side and the negative terminal to the n-side of the p-n junction. Here the forward bias opposes the potential barrier VB and so the depletion layer becomes thin. The majority charge carriers in the P type and N types are repelled by their respective terminals due to battery B and hence cross the junction. On crossing the junction, recombination process takes place. For every electron hole combination, a covalent bond near the +ve terminal of the battery B is broken and this liberates an electron which enters the +ve terminal of B through connecting wires. This in turn creates more holes in P-region. At the other end, the electrons from -ve terminal of B enter n-region to replace electron lost due to recombination process. Thus a large current will flow to migration of majority carriers across the p-n junction which is called forward current.
Reverse biasing
A p-n junction is said to be reverse biased if the positive terminal of the battery B is connected to N-side and the negative terminal to p-side of the p-n junction. The majority carriers are pulled away from the junction and the depletion region becomes thick. The resistance becomes high when reverse biased and so there is no conduction across the junction due to majority carriers. The minority carriers however cross the junction and they constitute a current that flows in the opposite direction. This is the reverse current.
The V-I characteristics of a p-n junction diode
The axes of the graph show both positive and negative values and so intersect at the centre. The intersection has a value of zero for both current (the Y axis) and voltage (the X axis). The axes +I and +V (top right) show the current rising steeply after an initial zero current area. This is the forward
conduction of the diode when the anode is positive and cathode negative. Initially no current flows until the applied voltage is at about the forward junction potential, after which current rises steeply showing that the forward resistance (I/V) of the diode is very low; a small increase in voltage giving a large increase in current.
The -V and -I axes show the reverse biased condition (bottom left). Here we see that although the voltage increases hardly any current flows. This small current is called the leakage current of the diode and is typically only a few micro-amps with germanium diodes and even less in silicon. If a high enough reverse voltage is applied however there is a point (called the reverse breakdown voltage) where the insulation of the depletion layer breaks down and a very high current suddenly flows. In most diodes this breakdown is permanent and a diode subjected to this high reverse voltage will be destroyed. In Zener diodes however, this point is used to give the diode its special ability to stabilise the applied voltage: If the voltage increases at this point heavy current flows and reduces the voltage. The breakdown in a Zener diode is not destructive due to its special construction.
