Sub: Physics of Semiconductor Devices- 3rd Sem (Preparatory Q’S for University Exam-2010) 3rd semester 2010 (B.Tech) Model Set-I 1. (a) What is meant by effective mass of electron in a semiconductor? Explain why we do not come across the term in a metal? (b) What is Einstein relation ship ? What is about its significance ? (c) What is difference between a Sehottky barrier diode and o-n junction diode ? (d) Calculate the drift current density in an extrinsic n-type semiconductor at T = 300K, if the applied electric field is 10V/cm. Assume under completer ionization, doping concentration of nd = 1016cm-3 and electron mobility of 1350 cm2/v.s. (e) Calculate the built in potential barrier in a silicon p-n junction at T = 300K with doping concentration Na = 1 x 1016 cm-3, Nd = 1 x 1015 cm-3. Assume ni = 1.5 x 1010 cm-3. (f) How does the nature of threshold voltage changes between MOS devices with n-type substrate and p-type substrate. (g) What is meant by the flat band voltage of a MOS structure ? Draw the energy-band diagram of a MOS capacitor. (h) Represent graphically, the minority carrier distribution in an npn bipolar transistor operating in the forward active mode. (i) What are advantages of CMOS logic circuit and other MOS circuit? IjP Write the expression for total drift density in a semiconductor. Label the terms used. 2. (a) Draw a diagram to indicate the location of inherent resistances and capacitances in the n-channel MOSFET structure. (b) Develop the small-signal equivalent circuit of a common source n-channel MOSFET. Derive an expression for the drain current in LOW frequency operation. (c) In an n-channel MOSFET, the constant electron mobility n = 500 cm2-v/s, the channel length L = 3m VT = 1V and VGS = 3V. Calculate the cut off frequency of an ideal MOSFET. 3. (a) Plot the minority carrier concentration as a function of distance form the p-n junction, under different bias conditions. Indicate the excess concentration in the diagram. What is the Law of junction? (b) What do you mean by minority carrier storage time of a diode ? (c) Why the collector region of a BJT is large in size ? 4. (a) Draw the basic construction of a p-channel depletion type MOSFET. In what ways the construction of a depletion type MOSFET similar and different to that of a JFET ? (b) What is the significant difference between the construction of an enhancement type and a depletion type of MOSFET ? 5. (a) Explain the features for different modes of operation of an npn bipolar transistor. Indicate graphically the minority carrier distribution in all these modes. (b) Explain the amplifying action of bipolar transistor. 6. (a) What do you mean by intrinsic carrier concentration in semiconductor ? explain with mathematical expression how the charge densities change when do pants are – added ? (b) What do you mean by mobility of charge carriers ? How conductivity and mobility are related to each others ? (c) Explain the terms diffusion length, diffusion constant and mean life time of change carriers in a semiconductor state any relationship that exists between them. 7. (a) Sketch the output characteristic plot for a p-n-p CE transistor configuration. Indicate the active, saturation and cut-off region in the above plot. Derive the expression for IC ~ IB for the active region of transistor operation. (b) Write the Eber-Moll equations for a BJT. Sketch the circuit model which satisfies these equations. (c) What is meant by inverted mode of operation of a BJT ? 8. (a) Derive the continuity equations for the excess carriers in a semiconductor. (b) Show that under low level injection, the excess minority carriers exponentially. (c) What is meant by ionization energy for an extrinsic semiconductor ? Sub: Physics of Semiconductor Devices- 3rd Sem (Preparatory Q’S for University Exam-2010) 3rd semester 2010 (B.Tech) Model Set-II 1. Derive the Einstein relation between carrier mobility and diffusion coefficient in a semiconductor. 2. Derive an expression for thermal equilibrium electron concentration in the conduction band for an intrinsic semiconductor. 3. What is the significant difference between the construction of an enhance type MOSFET and depletion type MOSFET ? 4. Derive an expression for the built in potential barrier for p-n junction diode. 5. Derive an expression for metal-semiconductor work function difference in a MOS structure. How it is linked to doping level ? 6. Explain the features for different modes of operation of an N+ PN transistor. Indicate graphically the minority carrier distribution in all these modes. 7. Derive the continuity equation for the excess carriers in a semi conductor. 8. What is meant by ionisation energy for an extrinsic semiconductor ? Show that the IE for Si is much less than the band gap energy of Si. Derive the necessary expression. 9. Derive the expression for flat band voltage in a MOS device. 10. Derive the expression for drift current using both electrons & holes. 11. Draw the energy band diagram of p-n junction in thermal equilibrium & explain. 12. Develop the basic Ebers-Moll equivalent circuit of a BJT. Derive necessary expressions. 13. Graphically show the variation of Fermi energy with temperature. 14. What do you mean by holes and its effective mass in a semiconductor ? 15. What do you mean by Built -in potential in a Schottky diodes. Write the expression when no voltage applied to semiconductor. 16. Draw the energy band diagram of Schottky diode in reverse bais and in forward bias. 17. Write the expression for deplection region width in Schottky diode. 18. Find the expression for specific contact resistance in Schottky diode. 19. Draw the energy band diagram at a MOS capacitor with a p-type substrate and negative gate bias. 20. Define Thiushold voltage in a MOS. 21. Distinguish between surface depiction and surface inversion. 22. What do you mean by accumulation of charge ? 23. Short Notes: (a) Fermi Energy (b) Schottky diode (c) Diffusion current & Drift current (d) MOSFET (e) Early potential (f) MOS Capacitor Sub: Physics of Semiconductor Devices- 3rd Sem (Preparatory Q’S for University Exam-2010) 3rd semester 2010 (B.Tech) Model Set-III 1. a) The position of Fermi-energy level in a semiconductor is located in (i) conduction band (ii) valence band (iii) Forbidden energy gap. Justify your answer. b) If the intrinsic concentration of a semiconductor is 10 x 1012 cm–3 and the electron concentration is 1.5 x 1015 cm–3. What type of semiconductor is this ? What is the hole concentration ? c) What do you mean by Fermi energy level ? Draw the neat sketch showing the position of Fermi level in P-type and N-type extrinsic semiconductor. d) What is the cause of diffusion current ? Write the expression for diffusion current density due to holes. e) If the ratio of diffusion coefficient to the mobility at const. temp. is const temp., what is about J ? f) The electron concentration in a piece of S i is 105/cm3 at 300K. What is the hole concentration ? i = 1010/cm3. g) What is the probability of electron state being filled if it is located at Fermi level ? h) Compare the mobility of Hole & electron wrt effective mass. i) How many silicon atoms are there in each unit cell ? j) Write the general expression for the unit of the mobility of the change carrier. 2. (a) Derive the relation EFi - Emidgap = . Then discuss how intrinsic Fermi level depends on density of state function. (b) Determine the position of the intrinsic Fermi level w.r.t the centre of the band gap in Gaas at I = 300K given for GaAs m n*/mp* = 0.067 and mp*/mn* = 0.48. 3. (a) Derive the expression for thermal - equilibrium electron concentration in the conduction band using effective density of state function, Fermi energy and other terms. (b) Explain the variation of EF with temperature using suitable expression and plot. 4. (a) Show that , the symbols used in the expression have usual meanings. (b) Derive the expression for total current density due to holes & electrons. Which has greater mobility taking account of electron hole ? 5. (a) Draw the energy band diagram of intrinsic semiconductor and the curves for energy vesus the probability function f(E) at ok and at room temperature. Explain the significance of the curve. (b) Calculate the effective density of the states in the conduction and value band of an intrinsic semiconductor at 300K. (c) Where is EF located in the energy band of Si at 300 K with = 1017/cm3. NC = 2.8 x 1019 / cm3. 6. (a) Describe the concept of energy band diagram of conductor, semiconductor and insulator wrt energy gap. (b) Show that E ~ K graph in parabolic in nature.