Friday, 20 September 2013

Previous 27 Years GATE Questions on Extrinsic Semiconductors (1987 - 2013)

   1.       An  N – Type silicon sample, having electron mobility µn twice the hole mobility µp, is subjected to a steady illumination such that the electron concentration doubles from its thermal equilibrium value. As a result, the conductivity of the sample increases

 by a factor of ……….                                                                                                [GATE’91]

   2.       A P – Type silicon sample has a higher conductivity compared to an N – Type sample having the same dopant concentration.      (TRUE / FALSE)                                                      [GATE’94]

   3.       An  N – Type silicon bar 0.1 cm long and 100 µm2 in cross sectional area has a majority carrier concentration of 5 x 1020 /m3 and the carrier mobility is 0.13 m2/V-sec  at  300oK. The resistance of the bar is                                                                                                                 [GATE’03]
a.       106 ohm
b.      104 ohm
c.       10-1 ohm
d.      10-4 ohm

   4.       The resistivity of a uniformly doped N –Type silicon sample is 0.5 -cm.  if the electron mobility is 1250 cm2/V-sec then the donor impurity concentration (ND) in the sample is      [GATE’04]                       
a.       2 x 1016 /cm3
b.      1 x 1016 /cm3
c.       2.5 x 1015  /cm3
d.      2 x 1015  /cm3

   5.       A silicon sample is doped with 1018 atoms /cm3 of boron. Another sample B of identical dimensions is doped with 1018 atoms/cm3 of phosphorous. The ration of electron to hole mobility is 3. The ratio of conductivity of the sample A to B is                                                [GATE’05]
a.       3
b.      1/3
c.       2/3
d.      3/2

   6.       A heavily doped N – Type semiconductor has the following data:                   [GATE’06]                                            
Hole – electron mobility ratio : 0.4

Doping concentration              :  4.2 x 108 /m3

Intrinsic concentration             :  1.5 x 104 /m3

The ratio of conductance of the N – Type semiconductor to that of the intrinsic semiconductor of same material and at the same temperature is given by 
a.       0.00005
b.      2,000
c.       10,000
d.      20,000

   7.       Silicon is doped with boron to a concentration of 4 x 1017 atoms /cm3. Assuming the intrinsic carrier concentration of silicon to be 1.5 x 1010 / cm3 and the value of KT/q to be 25mV at 300oK. compared to undoped silicon, the Fermi level of doped silicon                     [GATE’08]
a.       Goes down by 0.13 eV
b.      Goes up by 0.13eV
c.       Goes down by 0.427 eV
d.      Goes up by 0.427 eV

   8.       N – Type silicon is obtained by doping silicon with             [GATE’03]
a.       Germanium
b.      Aluminium
c.       Boron
d.      Phosphorous

   9.       A long specimen of P – Type semiconductor is                    [GATE’98]
a.       Is positively charged
b.      Is electrically neutral
c.       Has an electric field directed along its length
d.      Acts as a dipole

   10.       In an N – Type silicon crystal at room temperature, which of the following can have a concentration of  4 x 1019 per cm3?                                            [GATE’09]
a.       Silicon atoms
b.      Holes
c.       Dopant  atoms
d.      Valence electrons

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