[Solved] MCQs on Semiconductor Diode

Objective Questions and answers on Semiconductor Diode For BSNL TTA JTO GATE

1.Consider two energy levels : E₁, E eV above the Fermi level and E₂, E eV below the Fermi level. P₁ and P₂ are the probabilities of E₁ and E₂ being occupied by the electron respectively. Then

        a.       P₁ > P₂

        b.      P₁ = P₂

        c.       P₁ < P₂

        d.      P₁ and P₂ depend on number of free electrons

Answer:     C

               Solution :  https://www.youtube.com/watch?v=XrZAi2Q0S7Q

2.       In an intrinsic semiconductor, the free electron concentration depends on

        a.       Effective  mass of electrons only

        b.      Effective mass of holes only

        c.       Temperature of the semiconductor

        d.      Width of the forbidden energy band of the semiconductor

Answer:    C

               Solution :  https://www.youtube.com/watch?v=C61o_nRoaBc

3.       According to the Einstein relation, for any semiconductor, the ratio of diffusion constant to mobility of carriers

        a.       Depends upon the temperature of the semiconductor

        b.      Depends upon the type of the semiconductor

        c.       Varies with life time of the semiconductor

        d.      Is a universal constant

Answer:    A

               Solution :  https://www.youtube.com/watch?v=CNsSUnfw85Q

4.       Direct band gap semiconductors

        a.       Exhibit short carrier lifetime and they are used for fabricating BJTs

        b.      Exhibit long carrier lifetime and they are used for fabricating BJTs

        c.       Exhibit short carrier lifetime and they are used for fabricating LASERs

        d.      Exhibit long carrier lifetime and they are used for fabricating LASERs

Answer:    C

               Solution :  https://www.youtube.com/watch?v=fzNBPuE9hEE

5.       Due to illumination by light, the electron and hole concentrations in a heavily doped N-type semiconductor increases by Δn and Δp respectively, if n_i is the intrinsic carrier concentration then

        a.       Δn < Δp

        b.      Δn > Δp

        c.       Δn = Δp

        d.      Δn x Δp = n_i²

Answer:    C

                 Solution : https://www.youtube.com/watch?v=Kdb4wbs7aoQ

6.       The concentration of ionized acceptors and donors in a semiconductor are N_A, N_D respectively. If N_A > N_D and n_i is the intrinsic  concentration, then the position of the Fermi level with respect to the intrinsic level depends on

       a.       N_A – N_D

       b.      N_A + N_D

       c.       (N_A x N_D) / n_i²

       d.      n_i

Answer:   A 

                 Solution :  https://www.youtube.com/watch?v=fncmLfR38V0

8. In a semiconductor, under high electric fields, with increasing electric fields, mobility of charge carriers ............. and velocity of charge carriers ..............
Answer:     Reduces, gets saturated so remains constant

 Solution : http://www.youtube.com/watch?v=8cjTRCdTpbk

2. In a semiconductor at room temperature, the intrinsic carrier concentration and resistivity are 1.5 X 10¹⁶ m^-3 and 2 X 10⁵ Ω-m respectively. It is converted into an extrinsic semiconductor with a doping concentration of 10²⁰ per m³. For the extrinsic semiconductor, calculate the

             a.       Minority carrier concentration

             b.      Resistivity

             c.       Shift in Fermi level due to doping

              d.      Minority carrier concentration when its temperature is increased to a value at which the intrinsic carrier concentration doubles.

9.Assume the mobility of majority and minority carriers to be the same and KT = 26 meV at room temperature.

a) 2.25 x 10¹² /m³
b) 60 Ω-m
c) 0.228 eV
d) 9 x 10¹² /m³
   
Solution :    https://www.youtube.com/watch?v=1E491IgBz-I

10. A silicon sample is uniformly doped with 10¹⁶ phosphorous atoms/cm³ and 2 X 10¹⁶ boron atoms/cm³. If all the dopants are fully ionized, the material is ………..

Answer:     P-type with 10⁶ holes/cm³

Solution :  https://www.youtube.com/watch?v=EiKqkoJSygU

   11.       An n-type silicon sample, having electron mobility µ_n twice the hole mobility µ_p, 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 ………..

Answer:    2

 Solution :  https://www.youtube.com/watch?v=zxbCiNPkQqY

    12.       The current in a forward biased P⁺N junction shown in figure (a) is entirely due to diffusion of holes from x = 0 to x = L. The injected hole concentration distribution in the n-region is linear as shown in figure (b), with P(0) = 10²² per cm³ and L = 10^-3 cm. Determine

              a.       The current density in the diode, assuming that the diffusion coefficient of holes is 12 cm²/sec.

              b.      The velocity of holes in the n-region at x = 0.

        

Answer: 
J_n = 19.2 x 10⁶ A/cm²
υ_n = 12 x 10³ cm/sec

        Solution :  https://www.youtube.com/watch?v=fJJ5YtXBBnA

13.A semiconductor is irradiated with light such that carriers are uniformly generated throughout its volume. The semiconductor is n-type with N_D = 10¹⁹ cm^-3. If the excess electron concentration in the steady state is Δn = 10¹⁵ cm^-3 and if τ_p = 10 µsec (minority carrier life time), then the generation rate due to irradiation is ………….

Answer:    A
Solution :  https://www.youtube.com/watch?v=1Y5PZaVLBPg

14.       A small concentration of minority carriers is injected into a homogeneous semiconductor crystal at one point. An electric field of 10 V/cm is applied across the crystal and this moves the minority carriers a distance of 1 cm in 20 µsec. The mobility in cm²/volt-sec will be

       a.       1000

       b.      2000

       c.       5000

       d.      50000

Answer:    C

                Solution :  https://www.youtube.com/watch?v=Pe9nM75oo1U

15.       A p-type silicon sample has a higher conductivity compared to an n-type sample having the same dopant concentration.  (TRUE / FALSE)

Answer:    FALSE

                Solution :  https://www.youtube.com/watch?v=WUKXrsePs-A

16.       Show that the minimum conductivity of an extrinsic silicon sample occurs when it is slightly p-type. Calculate the electron and hole concentrations when the conductivity is minimum. Given that µ_n = 1350 cm²/volt-sec, µ_p = 450 cm²/volt-sec, and the intrinsic carrier concentration, n_i = 1.5 X 10¹⁰ cm^-3.

Answer:     n_o = n_i√µ_p∕µ_n and p_o = n_i√µ_n∕µ_p

            Solution :  https://www.youtube.com/watch?v=_O5iozGDyEM

17.       The drift velocity of electrons, in silicon

       a.       Is proportional to the electric field for all values of electric field

       b.      Is independent of the electric field

       c.       Increases at low values of electric field and decreases at high values of electric field exhibiting negative differential resistance.

       d.      Increases linearly with electric field at low values of electric field and gradually saturates at higher values of electric field.

Answer:    D

                 Solution :  https://www.youtube.com/watch?v=RJPyzF2-gcc

18.       In a p-type silicon sample, the hole concentration is 2.25 X 10¹⁵ cm^-3. If the intrinsic carrier concentration is 1.5 X 10¹⁰ cm^-3, the electron concentration is

       a.       Zero

       b.      10¹⁰ cm^-3

       c.       10⁵ cm^-3

       d.      1.5 X 10¹⁰ cm^-3

Answer:    C

                 Solution :  https://www.youtube.com/watch?v=j2CvMPiwMW8

19.       The probability that an electron in a metal occupies the Fermi level at any temperature T is (T > 0^oK)

Answer:    1

                Solution :   https://www.youtube.com/watch?v=FgCetVfygC0

20.   In an extrinsic semiconductor, if

Answer:     a-5, b-5, c-3

                Solution :  https://www.youtube.com/watch?v=Zk5QaYZDYbs

21. The Fermi level of an n-type germanium film is 0.2 eV above the intrinsic Fermi level towards the

        conduction band. The thickness of the film is 0.5 µm. Calculate the sheet resistance of the film.

Assume :

         n_i = 10¹³ cm^-3,  

         µ_n = 3500 cm²/V-sec,

         µ_p = 1500 cm²/V-sec,

         KT/q = 26 mV.

Answer:     1.63 kΩ
Solution :   https://www.youtube.com/watch?v=7q9ggl2_4rk

BSNL TTA JTO JE

22.       The intrinsic carrier density at 300^oK is 1.5 X 10¹⁰ per cm³ for silicon. For n-type silicon doped to 2.25 X 10¹⁵ atoms/cm³, the equilibrium electron and hole densities are

Answer:    C

                  Solution :  https://www.youtube.com/watch?v=PCnFSBDZldQ

23.   An n-type silicon bar is doped uniformly by phosphorous atoms to a concentration 4.5 x 10¹³ cm^-3. The bar has cross section of 1 mm² and length of 10 cm. It is illuminated uniformly for region x < 0 as shown.

Assume optical generation rate as 10²¹ electron-hole pairs per cm³ per second, the hole lifetime and electron lifetime are equal to 1 µsec.

Evaluate the hole and electron diffusion currents at x = 36.4 µm.

24.       The electron and hole concentrations in a intrinsic semiconductor are n_i and p_i respectively. When doped with a P-type material, these changes to n and p respectively. Then

       a.       n + P = n_i + P_i

       b.      n + n_i = p + p_i

       c.       np_i = n_ip

       d.      np = n_ip_i

Answer:    D

                Solution : https://www.youtube.com/watch?v=Zm31Ob0KekU

25.       A long specimen of p-type semiconductor material

       a.       Is positively charged

       b.      Is electrically neutral

       c.       Has an electric field directed along its length

       d.      Acts as a dipole

Answer:    B

                Solution :  https://www.youtube.com/watch?v=CMnmcekH-3g

26.       The units of (q/KT) are

       a.       V

       b.      V^-1

       c.       J

       d.      J/K

Answer:    B

                Solution : https://www.youtube.com/watch?v=DQYndoxV0ps

 27.       N – Type silicon is obtained by doping silicon with

       a.       Germanium

       b.      Aluminum

       c.       Boron

       d.      Phosphorous

Answer:    D

Solutoin : https://www.youtube.com/watch?v=VlGHV08XPHE

28.       The band gap of silicon at 300⁰K is

       a.       1.36 eV

       b.      1.10 eV

       c.       0.80 eV

       d.      0.67 eV

Answer:    B

               Solutoin :  https://www.youtube.com/watch?v=_NjNJR8cCEU

BSNL TTA JTO JE

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