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CAN ANY OF YOU DO IT ? REPLY SOON.
1. Silicon has an atomic weight of 28.1g per mole and a density of 2329kgm -3. How many Si atoms are contained in a transistor of a microprocessor of dimensions 15x80x7nm 3? [1 mole of a substance contains Avogadro’s number N=6.02×10 23 mol -1 atoms].
2. Two identical shaped rods of silicon and copper have length 0.01m and circular cross section of radius 0.05mm. If a voltage of 0.1V is applied between the ends of the copper rod a current of 4.5A flows. When the experiment is repeated for the silicon rod a current of 24nA flows. Calculate the resistances of the copper and silicon rods and hence find their electrical conductivities.
3. When an electric field of 800Vm -1is applied to a semiconductor the electrons are found to have a drift velocity of 7.5mms -1. What is the mobility of the electrons? If the effective mass of the electrons is 0.06m0 what is the mean time between collisions? If the free electron density is 2.5×10 23 m-3 what is the conductivity of the semiconductor?
4. A semiconductor quantum well has an electron effective mass of 0.06m0. If the electron Fermi energy is measured to be 45meV calculate the free electron density. Assume only one sub-band is occupied.
5. In a Hall experiment a semiconductor bar has length 12mm and sides each of 1.2mm. When a voltage of 6V is applied across the ends of the bar a current of 0.55mA flows and in an applied field of 1.2T a Hall voltage of 0.18V is developed. Calculate the conductivity, mobility and free carrier density of the semiconductor.
6. A copper wire has a radius of 0.8mm and carries a current of 0.45A. If 8.5×10 28 m-3 electrons contribute to the electrical conductivity calculate their drift velocity? How long does it take a given electron to travel 1m? What is the kinetic energy of the electrons corresponding to this velocity? How does this energy compare with a typical spacing between energy bands of 1eV, could this energy cause an electron to be excited between the bands?
7. In a certain semiconductor, at temperatures close to room temperature, the intrinsic electron density is found to increase by 8% per degree rise in temperature and the mobility is found to decrease by 3% per degree rise in temperature. If the temperature is increased from 0 to 10 °C calculate the percentage change in (i) the intrinsic carrier density, (ii) the mobility and (iii) the conductivity.
