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Aim: To determine the Energy Band Gap of a Semiconductor by using PN Junction Diode. Apparatus: Energy band gap kit containing a PN junction diode placed inside the temperature controlled electric oven, microammeter, voltmeter and connections brought out at the socket, a mercury thermometer to mount on the front panel to measure the temperature of oven. Formula Used: The reverse saturation current, Is is the function of temperature (T) of the junction diode. For a small range of temperatures, the relation is expressed as,
Where, T is temperature in Kelvin (K) and Eg is the band gap in electron volts (eV). Graph between 103/T as abscissa and log10 Is as ordinate will be a straight line having slope = 5.036 Eg Hence band gap,
Theory: A semi-conductor (either doped or intrinsic) always possesses an energy gap between its valence and conduction bands (fig.1). For the conduction of electricity, a certain amount of energy is to be given to the electron so that it can jump from the valence band to the conduction band. The energy so needed is the measure of the energy gap (Eg) between the top and bottom of valence and conduction bands respectively. In case of insulators, the value of Eg varies from 3 to 7 eV. However, for semiconductors, it is quite small. For example, in case of germanium, Eg = 0.72 eV and in case of silicon, Eg = 1.1 eV.
In semi-conductors at low temperatures, there are few charge carriers to move, so conductivity is quite low. However, with increase in temperature, more number of charge carriers get sufficient energy to be excited to the conduction band. This lead to increase in the number of free charge carriers and hence increase in conductivity. In addition to the dependence of the electrical conductivity on the number of free charges, it also depends on their mobility. The mobility of the charge carriers, however decreases with increasing temperature. But on the average, the conductivity of the semiconductors rises with rise in temperature. To determine the energy band gap of a semi-conducting material, we study the variation of its conductance with temperature. In reverse bias, the current flowing through the PN junction is quite small and internal heating of the junction does not take place. When PN junction is placed in reverse bias as shown in fig.2(a), the current flows through the junction due to minority charge carriers only. The concentration of these charge carriers depend on band gap Eg. The saturation value, Is of reverse current depends on the temperature of junction diode and it is given by the following equation,
Precautions: The following precautions should be taken while performing the experiment: 1. The diode must be reverse biased. 2. Do not exceed the temperature of the oven above 100℃ to avoid over heating of the diode. 3. The voltmeter and ammeter reading should initially be at zero mark. 4. Bulb of the thermometer should be inserted well in the oven. 5. Readings of microammeter should be taken when the temperature is decreasing. 6. Readings of current and temperature must be taken simultaneously. Sample viva voce questions: 1. What is PN junction diode? 2. What do you understand by band gap of a semi-conductor? 3. What do you mean by valence band, conduction band and forbidden band? 4. How many types of semi-conductors are there? 5. What are P-type and N-type semi-conductors? 6. Define doping and dopant. 7. Why P-type (N-type) semi-conductor is called Acceptor (Donor)? 8. What do you mean by Fermi energy level? 9. What is the position of Fermi level in an intrinsic semi-conductor and in a p-type or n-type semi-conductor with respect to the positions of valence and conduction bands? 10. What do you mean by forward biasing and reverse biasing? 11. Why diode is reverse biased in determining the band gap of semi-conductor? 12. What is the shape of graph between log10 Is and 103/ T? How do you find band gap energy from this graph? 13. Why conductivity of metals decreases with increase in temperature? 14. Why conductivity of a semi-conductor increases with increase in temperature? References: Solid State Electronic Devices by Streetman and Banerjee B.sc Practical Physics by Geeta Sanon
