# NCERT Solutions for class 12th Physics Chapter 14 Semiconductor Electronics: Materials, Device and Simple Circuits

#### Exercise

Question 1. In an n-type silicon, which of the following statement is true:
(a) Electrons are majority carriers and trivalent atoms are the dopants.
(b) Electrons are minority carriers and pentavalent atoms are the dopants.
(c) Holes are minority carriers and pentavalent atoms are the dopants.
(d) Holes are majority carriers and trivalent atoms are the dopants.

Sol. (c) In an n-type silicon, holes-minority carriers and dopants – pentavalent atoms

Note: It should be noted that under thermal equilibrium the product of concentraction of majority carrriers and minority carriers is constant.

Question 2. Which of the statements given in Qs.14.1 is true for p-type semiconductors?

Sol. (d)

Question 3. Carbon, silicon and germanium have four valence electrons each. These are characterised by valence and conduction bands separated by energy band gap respectively equal to(Eg)c, (Eg)si and (Eg)Ge. Which of the following statements is true?

Sol. (c) The energy band gap is maximum for carbon and minimum for germanium.

Question 4. In an unbiased p-n junction, holes diffuse from the p-region ton-region because
(a) free electrons in then-region attract them.
(b) they move across the junction by the potential difference.
(c) hole concentration in p-region is more as compared ton-region.
(d) all the above.

Sol. (c)

Question 5. When a forward bias is applied to a p-n junction, it
(a) raises the potential barrier.
(b) reduces the majority carrier current to zero.
(c) lowers the potential barrier.
(d) none of the above.

Sol. (c) Width of depletion layer reduces, hence lower the potential barrier.

Question 6. For transistor action, which of the following statements are correct:
(a) Base, emitter and collector regions should have similar size and doping concentrations.
(b) The base region must be very thin and lightly doped.
(c) The emitter junction is forward biased and collector junction is reverse biased.
(d) Both the emitter junction as well as the collector junction are forward biased.

Sol. (b) and (c)

Question 7. For a transistor amplifier, the voltage gain
(a) remains constant for all frequencies.
(b) is high at high and low frequencies and constant in the middle frequency range.
(c) is low at high and low frequencies and constant at mid frequencies.
(d) none of the above.

Sol. (c)

Question 8. In half-wave rectification,what is the output frequency ifthe input frequency is 50 Hz? What is the output frequency of a full-wave rectifier for the same input frequency?

Sol. In half wave rectification, output frequency remains same as input i.e., 50Hz. In full wave rectification, output shows two positive pulses for each input pulse hence frequency doubles i.e., 100 Hz.

Question 9. For a CE-transistor amplifier, the audio signal voltage across the collected resistance of 2 kOhm is 2V. Suppose the current amplification factor of the transistor is 100, find the input signal voltage and base current, if the base resistance is 1 kOhm.

Question 10. Two amplifiers are connected one after the other in series (cascaded). The first amplifier has a voltage gain of l0 and the second has a voltage gain of 20. If the input signal is 0.01 volt, calculate the output ac signal.
[Not in syllabus of academic session 2019-2020)

Question 11. A p-n photodiode is fabricated from a semiconductor with band gap of 2.8 eV. Can it detect a wavelength of 6000 nm?

Question 12. The number of silicon atoms per m3 is 5 x 1028â€¢ This is doped simultaneously with 5 x 1022 atoms per m3 of Arsenic and 5 x 1020 per m3 atoms of lndium. Calculate the number of electrons and holes. Given that ni = 1.5 x 1016 m-3. Is the material n-type or p-type?

Question 13. In an intrinsic semiconductor the energy gap E_g is 1.2eV. Its hole mobility is much smaller than electron mobility and independent of temperature. What is the ratio between conductivity at 600K and that at 300K? Assume that the temperature dependence of intrinsic carrier concentration ni is given by

Question 14. In a p-n junction diode, the current J can be expressed as

where I0 is called the reverse saturation current, V is the voltage across the diode and is positive for forward bias and negative for reverse bias, and J is the current through the diode, k8 is the Boltzmann constant (8.6 x 10-5 eVIK) and T is the absolute temperature. If for a given diode J0 =5 x 10-12 A and T= 300 K, then

• (a) What will be the forward current at a forward voltage of 0.6 V?
• (b) What will be the increase in the current if the voltage across the diode is increased to 0.7 V?
• (c) What is the dynamic resistance?
• (d) What will be the current if reverse bias voltage changes from 1 V to 2 V?

Note: Dynamic resistance varries with potential difference. It is reciprocal of slope of i-v curve.
[Question number 14.15 to 14.19 based on logic gates are not in CBSE syllabus.

Question 15. You are given the two circuits as shown in Fig. Show that circuit (a) acts as OR gate while the circuit (b) acts as AND gate.

Question 16. Write the truth table for a NAND gate connected as given in Fig.

Hence identify the exact logic operation carried out by this circuit.

Question 17. You are given two circuits as shown in Fig. which consist of NAND gates. Identify the logic operation carried out by the two circuits.

Question 18. Write the truth table for circuit given in Fig. below consisting of NOR gates and identify the logic operation (OR, AND, NOT) which this circuit is performing.

(Hint: A=0, B=l then A and B inputs of second NOR gate will be 0 and hence Y = 1. Similarly work out the values of Y for other combinations of A and B. Compare with the truth table of OR, AND, NOT gates and find the correct one.).

Question 19. Write the truth table for the circuits given in Fig. consisting of NOR gates only. Identify the logic operations (OR, AND, NOT) performed by the two circuits.

Note: Some useful Identities

A + O = A
A + 1 = 1
A + A = A

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