P-N-Junction slide for class 12 neb.pptx

Note slide

1. P.N. Junction By sks (SHYAM)

2. P.N. Junction If a piece of P- type semiconductor is placed in contact with a piece of N- type semiconductor, then their exist a common contact surface between them which is known as P- N junction. At the junction, there is a tendency of free electron to jump from N-type to the holes in p-type. This process is called recombination or diffusion. After some recombination, N-region gains excess positive charge and P-region gains excess negative charge. These charges set up an electric field which produces a potential difference is known as potential barrier denoted by V B . This p.d prevents further diffusion of free electrons and holes. Inside this region, there is no mobile charge carrier so this region is called depletion region.

3. P-N Junction Diode The device in which P-type semiconductor is in contact with N-type semiconductor is called P-N junction diode. It has two terminals namely anode and cathode. The anode refers to the P- type region and the cathode refers to N-type region.

4. Biasing of P-N junction Diode Applying the potential difference across the P-N junction diode in order to get it ready for operation is called biasing. The junction diode can be biased in following two ways. 1. Forward biasing 2. Reverse biasing 1. Forward biasing : - The process of connecting P-side of the diode with positive terminal and N-side with negative terminal of a battery is called forward biasing.

5. Features: Potential barrier is reduced and is eliminated at some forward voltage 0.3V to 0.7 V Junction offers low resistance (forward resistance) to current. Inside the diode, current is due to both type of charge carriers ( i.e electrons and holes) but outside diode it is due to flow of electrons only. Depletion layer decreases. Current increases with applied voltage but do not obey Ohm’s Law

6. 2. Reverse biasing :- The process of connecting P-side of the diode with negative terminal and N-side with positive terminal of a battery is called reverse biasing.

7. In the reverse biasing , the electrons from the N-region are attracted towards the positive terminal and holes in the P-region are attracted towards the negative terminal of the battery. The departing electrons and holes leave more positive ions and negative ions respectively near the junction. Therefore , the depletion layer gets wider . The depletion layer stops growing when potential difference across it equals the applied voltage and then free electrons and holes stop growing. So, the flow of negligible current inside the diode is due to minority charge carriers. Features: Potential barrier is increased. Depletion layer is increased The junction offers very high resistance (reverse resistance) Reverse current is very small and is called leakage current.

8. Characteristics of a p-n junction diode The graphical relationship between current and potential difference across the junction diode is called characteristics of a diode. It is also known as I-V characteristics of junction diode. Usually, the voltage is taken along x-axis and the current along y-axis. These are of two types. Forward characteristics :- fig It is the graphical relation between forward current and forward voltage . The circuit diagram for studying the forward characteristics of p-n junction diode is as shown in figure.

9. Fig: b Forward characteristics of PN junction diode Fig: a Circuit diagram for forward biasing

10. The voltmeter V measures the potential difference across the diode and milliammeter mA measures the current flows through it. The forward voltage across the diode V is changed with the help of rheostat R H and corresponding forward current I is noted. The forward voltage at which the current through the diode starts to increase rapidly is called knee voltage and is denoted by V k . The knee voltage for silicon and germanium is . V k = 0.7V for silicon V k = 0.3V for germanium

11. 2. Reverse characteristics It is the graphical relation between reverse current and reverse voltage. The circuit diagram for studying reverse characteristics is shown in fig. The voltmeter V measures the reverse voltage and microammeter μ A measures the reverse current. A graph is then plotted between reverse voltage and reverse current. This graph represents the reverse characteristics of the diode. The reverse voltage at which the diode current starts to increase sharply is called breakdown voltage v B Fig: a Reverse biasing circuit Fig: b Reverse biasing characteristics

12. Rectification and rectifier The process of conversion of a.c . into d.c. is called rectification. The device which is used for rectification is called a rectifier. The p-n junction diode conducts in forward bias and doesn’t conduct in reverse bias. This unidirectional current conduction property of diode is used in rectification. There are two types of rectifier Half wave rectifier Full wave rectifier Half wave rectifier: A rectifier which converts only half cycle of a.c . into d.c . is called half wave rectifier. The circuit diagram of a half –wave rectifier is as shown in figure .

13. Two ends of the primary coil of transformer are connected to a.c . source. Secondary coil of transformer is connected to the load resistor through a diode . The a.c . voltage V across the secondary winding S 1 S 2 changes polarity in every half cycle. Let during the positive half cycle of the input a.c . supply, the end S 1 becomes positive and S 2 becomes negative. When the end S 1 becomes positive the diode is forward biased and hence it conducts the current. So, we get output across the load R L .

14. During the negative half cycle of the input a.c . supply, the end S 2 becomes positive and end S 1 becomes negative .When the end S 1 becomes negative the diode is reverse biased and it does not conduct . Hence, we do not get output as the – ve half cycle of the input a.c . supply is blocked. Thus, the direction of current across the load after every half cycle is same and the current in the same direction is d.c.

15. 2. Full wave rectifier (Centre tapped) The full wave rectifier is a device which converts full cycle of a.c . into d.c. In the full wave rectification, the rectifier utilize the both + ve half cycle and negative half cycle of the input ac supply. The circuit diagram of centre tapped full wave rectifier is as shown below. Two ends of primary coil of transformer are connected to a.c . source. The ends of the secondary coil are connected to the P-ends of the diodes D 1 and D 2 . The central tapping of the secondary is connected to the N ends of the diodes through the load resistance R L .

16. As we know, the A.C voltage changes its polarity in every half cycle. During the positive half cycle, say the end S 1 becomes + ve and end S 2 becomes – ve . When the end S 1 becomes + ve , the diode D 1 is forward biased and D 2 is reverse biased. So, the diode D 1 conduct the current and diode D 2 doesnot conduct. The direction of current through the load is as shown by arrow head in the upper half of the secondary winding. During the – ve half cycle of input a.c . supply , the end S 2 becomes + ve and end S 1 becomes – Ve . When the end S 2 becomes + Ve the diode D 2 is forward biased and diode D 1 is reverse biased. The diode D 2 conduct the current and diode D 1 doesnot conduct. The direction of Current is shown by arrow head in the lower half of the secondary winding. So, in both half cycle , the direction of current through the load is in the same direction which is d.c. In this way, by working alternately two diode utilize both half cycle of a.c . input supply and we get d.c.

17. BRIDGE FULL WAVE RECTIFIER

18. Wave form of bridge full wave rectifier