PHYSICS EXPERIMENT 6

Experiment No.6

Object:-  To draw the characteristic curves of a  forward & reverse Biased  P-N junction diode and  to determine the  static resistance of the given diode.

Apparatus Required:  Semiconductor P-N junction diode kit and connecting wires.

Theory: A Semiconductor diode is prepared by joining P and N sections of a semi conducting material like germanium or silicon. The P type has excess number of holes while the N type has excess number of electrons. Holes and electrons are respectively the charge carriers in P and N type. They are called the majority carriers. Near about the junction, holes and electrons recombine giving rise to a charge free space called depletion region or barrier region. In this Process, (+)ve charge gets accumulated at the barrier of the N section, and (-)ve charge at the barrier of the P section, creating a potential barrier. A sort of a fictitious battery with it’s (+)ve pole on the N section and (–)ve pole on the P section is formed. It is shown in fig. 1. This barrier stops further motion of holes towards N side and electrons towards P side.

Figure: 1

FORWARD BIASING:        When the anode of an external battery is connected to the P side and cathode to the N side, it is called forward biasing. It is shown in fig.(2).  When the applied potential is lower than barrier potential, negligible current flow through the junction. As the applied potential higher than that of the barrier potentials, holes would be forced to move towards N side and electrons towards P side. Thus, current starts flowing through the junction in the external circuit. The voltage at which current start increasing called knee voltage. The forward current is increases with the applied potential difference.

REVERSE BIASING: When the anode of an external battery is connected to the N side and the cathode to the P side. It is called reverse biasing. In this case the polarity of the fictitious battery is the same as that of the external battery, as shown fig.(3). Thus, it leads to the increase in the potential barrier, and electron in N type and hole in P type [both majority carriers] are drawn away from the junction, and hence hardly there is any current flowing in the external circuit. However, the small reverse current which flows through the junction is due to minority carriers i.e. electrons in P section and holes in N section. The minority carriers are created due to the action of light and thermal agitation.

If the potential difference in the reverse direction is increased beyond a certain critical limit, the reverse current abruptly increases. This is known as AVALANCHE BREAK DOWN, and the maximum reverse voltage is called Break Down. It is clear from above discussion that a diode offers a negligible resistance when forward biased and a very high resistance when reverse biased.

Procedure:

FORWARD BIAS CHARACTERISTICS

Make the circuit according to fig.(2).[ use 1V range for voltmeter and 10 mA range for   ammeter]

1.         By increasing the voltage across the diode in steps of 0.1 volts, note down corresponding current in the  table no.1

2.        Calculation of static resistance: Using the forward bias curve, take the points on the curve beyond the knee voltage and calculate R  

       R _static = Potential at a point beyond the knee voltage. / Current at that point

REVERSE BIASED CHARACTERRISTICS

1.    Make the circuit according to fig.3 [use 10V range for voltmeter and 50 µA range for current meter.]

2.    By increasing the voltage across the diode in steps of 1.0 volts, note down corresponding current in the table no 2.

OBSERVATION TABLE-1: FOR FORWARD BIASED

S. No.	Voltage in volts	Current in mA
1
2
3
4
5
6
7
8
9
10

                       

OBSERVATION TABLE -2: FOR REVERSE  BIASED

S. No.	Voltage (volts)	Current in µA
1
2
3
4
5
6
7
8
9
10

                                                                                                                              

Result:-  The I/V Characteristic of P-N Junction diode is shown in the graph.

Precaution and source of error:

1.        Voltmeter and ammeter of appropriate ranges should be selected.

2.        The variation in V should be done in steps of 0.1 V.

3.        The battery connections of p-n junction diode should be checked and it should be ensured that p is connected to positive and n to the negative of the battery.

4.        Never cross the limits specified by the manufacturer otherwise the diode will get damaged.

                                                     

Viva Voce:

Q.1 What is diode?

Ans:A diode is a two-terminal electronic component with asymmetric transfer characteristic, with low (ideally zero) resistance to current in one direction, and high (ideally infinite) resistance in the other. 

Q.2 What do you understand by P-N Junction diode?

Ans: A Semiconductor diode is prepared by joining P and N sections of a semi conducting material like germanium or silicon. The P type has excess number of holes while the N type has excess number of electrons. Holes and electrons are respectively the charge carriers in P and N type.

Q.3. Explain forward and reverse biasing in diode?

Ans: When the anode of an external battery is connected to the P side and cathode to the N side, it is called forward biasing. When the anode of an external battery is connected to the N side and the cathode to the P side, It is called reverse biasing.

Q.4. What do you mean by breakdown phenomenon?

Ans: If the potential difference in the reverse direction is increased beyond a certain critical limit, the reverse current abruptly increases. This is known as BREAK DOWN.

Q.5 What is the difference between Zener diode and P-N Junction diode?.

Ans: A P-N Junction diode will let current go in only one direction (forward bias). If current is applied in the opposite direction (reverse bias) then we would not have current on the other end unless the voltage that you apply to it surpasses its Breakdown Voltage, but then diode will be burn out. Zener diodes would act the same way as a P-N Junction diode except that in reverse bias it would allow current to pass when the voltage surpasses its Breakdown Voltage (Zener Voltage).

Q.6 What is the static resistance?   

Ans: The static resistance of diode is defined as diode resistance measure when DC voltage is applied above the knee voltage.