Video Solutions for GATE 2004 EC : Two Mark Questions (Electron Devices)

1. In abrupt PN junction, the doping concentrations on the p-side 

    and n-side are N_A = 9 x 10¹⁶ /cm³ and N_D = 1 x 10¹⁶ /cm³

  

    respectively. The PN junction is reverse biased and the total 

    depletion width is 3 µm. The depletion width on the p-side is

           (A)     2.7 µm                          (B)   0.3 µm        

           (C)     2.25 µm                        (D)   0.75 µm

2. The resistivity of a uniformly doped n-type silicon sample is 

    0.5 Ω-cm. If the electron mobility (µ_n) is 1250 cm²/V-sec and 

    the charge of an electron is 1.6 x 10^-19 coulomb, the donor 

    impurity concentration (N_D) in the sample is 

  

3. Consider an abrupt PN junction. Let V_bi be the built in potential 

    of this junction and V_R be the applied reverse bias. If the 

    junction capacitance (C_j) is 1 pF for V_bi + V_R = 1 volt, then for 

    V_bi + V_R = 4 volts, C_j will be

   (A)  4 pF         (B)  2 pF          (C)  0.25 pF         (D)  0.25 pF

4. Consider the following statements S1 and S2.

                S1 : The threshold voltage (V_T) of a MOS capacitor 

                       decreases with increase in gate oxide thickness

                S2 : The threshold voltage (V_T) of a MOS capacitor 

                       decreases with increase in substrate doping 

                       concentration.

  Which one of the following is correct ?

 

5. The drain of an n-channel MOSFET is shorted to the gate so that 

    V_GS = V_DS. The threshold voltage (V_T) of MOSFET is 1 volt.

    If the drain current (I_D) is 1 mA for V_GS = 2 volts, then 

    for V_GS = 3 volts, I_D is 

           (A)   2 mA                           (B)   3 mA    

           (C)   9 mA                           (D)   4 mA

6. The longest wavelength that can be absorbed by silicon, which 

     has the bandgap of 1.12 eV, is 1.1 µm. If the longest wavelength 

     that can be absorbed by another material is 0.87 µm, then the 

     bandgap of this material is 

              (A)  1.416 eV              (B)  0.886 eV                                   

              (C)  0.854 eV              (D)  0.706 eV

 

7. The neutral base width of a bipolar transistor, biased in the  

    active region, is 0.5 µm. The maximum electron concentration 

    and the diffusion constant in the base are 10¹⁴ /cm³ and 

    D_n = 25 cm²/sec respectively. Assuming negligible 

    recombination in the base, the collector current density is 

8. Assuming that the β of the transistor is extremely large and 

    V_BE = 0.7 volts, I_C and V_CE in the circuit shown are,

  

  

9. In the voltage regulator shown , the load current can vary from 

   100 mA to 500 mA. Assuming that the zener diode is ideal

   (i.e. the zener knee current is negligibly small and zener 

    resistance is zero in the breakdown region), the value of R is