[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 ni is the intrinsic carrier concentration then
a. Δn < Δp
b. Δn > Δp
c. Δn = Δp
d. Δn x Δp = ni2
Answer: C
6. The concentration of ionized acceptors and donors in a semiconductor are NA, ND respectively. If NA > ND and ni is the intrinsic concentration, then the position of the Fermi level with respect to the intrinsic level depends on
a. NA – ND
b. NA + ND
c. (NA x ND) / ni2
d. ni
Answer: A
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
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
11. 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 ………..
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:
Jn = 19.2 x 106 A/cm2
υn = 12 x 103 cm/sec
Jn = 19.2 x 106 A/cm2
υn = 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 ND = 1019 cm-3. If the excess electron concentration in the steady state is Δn = 1015 cm-3 and if τp = 10 µsec (minority carrier life time), then the generation rate due to irradiation is ………….
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 µn = 1350 cm2/volt-sec, µp = 450 cm2/volt-sec, and the intrinsic carrier concentration, ni = 1.5 X 1010 cm-3.
Answer: no = ni√µp∕µn and po = ni√µn∕µp
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 > 0oK)
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 :
ni = 1013 cm-3,
µn = 3500 cm2/V-sec,
µp = 1500 cm2/V-sec,
KT/q = 26 mV.
BSNL TTA JTO JE
22. The intrinsic carrier density at 300oK 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
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 ni and pi respectively. When doped with a P-type material, these changes to n and p respectively. Then
a. n + P = ni + Pi
b. n + ni = p + pi
c. npi = nip
d. np = nipi
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
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
BSNL TTA JTO JE
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