Forward bias characteristics:- First of all make the connection according to the circuit shown in figure. By changing the forward voltages note down the corresponding forward current and plot the graph between forward voltage and forward current. The graph so obtained is called as forward characteristic curve of p-n junction.
From the graph it is clear that initially there is no current. When the applied voltage is less than potential barrier, the current flow through the junction is very small. As the forward voltage increases above the potential barrier, current increases approximately linearly. When the forward voltage is equal to the potential barrier then the curve becomes like a knee and called as knee voltage. At this voltage the thickness of depletion of layer becomes intelligibility and the diffusion of electron and holes across the junction takes place easily i.e. thep-n junction offers low resistance when it is forward bias and the resistance is of the order of 100 ohm.
Reverse characteristic curve– Make the connection according to the circuit shown in the following figure. Change the reverse voltage and note down the corresponding reverse current. The graph plotted between reverse voltage and the reverse current is called as reverse bias curve. Practically in reverse bias there is no current if the applied voltage is low but a very small current due to minority carriers.
On increasing the reverse voltage to a very high value, the current increases abruptly, which is clear from the graph. It is due to the fact that at very high voltage, the entire covalent bond near the junction is broken. Due to which large no. of electrons and holes liberate and the corresponding voltage is called as zener voltage. In the reverse bias the thickness of the depletion layer increases due to which the further diffusion of charge carriers stops and no current flows through the junction. Thus in reverse bias the junction offers a very high resistance.
Dynamic resistance:The ratio of small change in voltage to the small change in current is called as dynamic or a.c. resistance of the junction diode. It is represented by Vd.
The region of the characteristic curve where dynamic resistance is almost independent of the applied voltage is called the liner region of junction diode.
Junction diode as rectifier:An electronic device, which covert a.c. power to d.c. power is called as rectifier.
Half wave rectifier: A rectifier, which rectify only one half of each a.c. input supply cycle is called half wave rectifier.
Principle: It is based on the principle that the diode offers low resistance when it is forward bias and high resistance when it is reverse bias i.e. the current will flow through the diode when it is forward biased.
Arrangements: The p-region of the junction diodeis joined to the one terminal of the secondary coil of a step down transformer and the load resistance is joined between n-region and the II terminal of the secondary coil.
Working: Let during the first half cycle of the input a.c. upper end i.e. point A of secondary is at +ve potential and the lower end i.e. point B is at –ve potential. Thus the diode is forward bias. During the first half cycle and current flow through diode in load resistance from C to D.
During the next half cycle the upper end becomes –ve and lower end becomes +ve and thus the diode gets reversed biased and no current flow through it. In the next half cycle diode gets forward bias and current flows through it from C to D and this process repeated again and again. The current obtain in output is discontinuous and pulsating d.c. due to which there is a huge loss of energy.
Full wave rectifier:A rectifier which rectify both halves of the a.c. input is called as full wave rectifier.
Principle: It is based on the principle that the diode offers low resistance when is is forward bias and high resistance what it is reverse biased.
Arrangement: The a.c. supply is fed across the primary coil P of a step down transformer. Two ends of the secondary coilS of the transformer are connected to the p-regions of the junction diode D1 and D2. A load resistance Rl is connected between the n-region between the two diodes and the central tapping of the secondary coil. The output d.c. is obtained across the load resistance.
Working:Suppose that during the first half of the input, the upper end A of the secondary coil is at +ve potential and the lower end B is at –ve potential. So the diode D1 gets forward bias andD2 gets reverse bias hence the current flow through D1 in load resistance from Cto D. During the next half cycle A becomes -ve and B becomes +ve and henced1 gets reverse bias and D2 gets forward bias. Thus the current flow through D2 from C to D in load resistance.
Hence the full wave rectifier, rectifies the both halves of the a.c. The output d.c. is continuous but pulsating. To reduce the fluctuations, filter circuits is used in output circuits. Electrolytic condenser and zener diode is used to reduce fluctuations of d.c.