Code: A-25                  Subject: PHYSICAL ELECTRONICS AND SOLID STATE DEVICES

Time: 3 Hours                                                                                                     Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1       Choose the correct or best alternative in the following:                                         (2x10)

a. In electronic grade Si the level of impurities present is 

(A) .                           (B) .

(C) .                            (D)  .

b. If GaP has band gap of 2.3 eV then the corresponding wave length lies in the following region of the spectrum:

(A) red.                                              (B)  infrared.

(C) ultraviolet.                                     (D)  green.

             c.   The covalent bonding forces in a lattice arise due to

(A) Coulombic forces.                       

(B) attractive forces.

(C) quantum mechanical/interactions.

(D) none of the above.

             d.   A positive photo resist is more widely used than negative photo resist because  it

(A)  is easily available.                        (B) reacts at a faster rate.

(C)  can achieve better resolution.       (D) is easier to handle.       

             e.   The Hall effect voltage in intrinsic silicon is

(A) positive.                                       (B)  zero.

(C)  negative.                                       (D) positive, zero or negative depending

upon the magnetic field.

             f.    FETs are used in amplifiers to get

(A) high input impedance.                   (B)  high output impedance.

(C)  low input impedance.                    (D)  low output impedance.

             g.   With increase in frequency, power dissipation in CMOS

(A) reduces.                                       (B)  remains unchanged.

(C)  increases.                                     (D)  may increase or decrease depending

on other factors.

             h.   A small increase in collector reverse bias will cause

(A) a large increase in emitter current.      

(B) a large increase in collector current.

(C) very small change in collector reverse saturation current.         

(D) a large decrease in collector current.

             i.    Junction breakdown occurs

(A) with forward bias.                        

(B)  under reverse bias.

(C) under high temperature conditions.

(D) because of manufacturing defect.

             j.    Transistor is a

(A)  current controlled current device.

(B)  current controlled voltage device.

(C)  voltage controlled current device. 

(D) voltage controlled voltage device.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   The electronic and optical properties of semiconductor materials are strongly affected by impurities.  Discuss briefly with examples.                           (8)

             b.   Discuss Hall effect.  How is resistivity measured using this effect?                        (8)

Q.3     a.   Show that transition width of a p-n junction varies as the square root of the potential across the region.                                                                   (10)

             b.   Assume that for Si, E_g = 1.11 eV at .  Find E_g if the temperature reduces to (6)

Q.4     a.   The p-n junction can be operated in reverse breakdown as safely as in the forward bias conditions.  Justify.                                                                                                                        (8)

b.   What are the requirements for good rectifier junctions?                                       (8)

Q.5     a.   Explain why the turn-on transient of a BJT is faster when the device is driven into oversaturation.                                                                (8)

     b.   How can charge transfer efficiency be improved in a basic CCD structure?         (8)

  Q.6     a. Explain the term figure of merit as applied to a solar cell.  Show schematically and discuss how several semiconductor materials might be used together to obtain a more efficient solar cell. (8)

             b.   A Si solar cell with dark saturation current of 5 nA is illuminated such that the short circuit current is 200 mA.  Plot the I-V curve for the cell.              (8)

Q.7     a.   The bias on a Si MOS capacitor is changed from inversion to accumulation mode.  If the substrate doping is  donors, what is the change in the surface band bending at ?    (8)

b.   Briefly discuss the use of LED and LCD as display devices in instrumentation.  Comment on their relative merits and demerits.                             (8)

Q.8     a.   What is the major application of a semiconductor laser?  How do GaAs and InGaAsP devices compare in this regard?                                          (8)

             b.   What is Gunn effect?  Describe different modes of operation of a Gunn device.                   (8)

Q.9     a.   Differentiate between SSI, MSI, LSI and VLSI.  Why do MOS ICs find wide applications in LSI and VLSI chips?                                                  (8)

b.   A Si crystal is to be grown by Czochralski method.  What concentration of phosphorus atoms should the melt contain to give the impurity concentration of .  Assume the distribution co-efficient to be 0.35.                                                                                                             (8)