# Chapter 14: Semiconductor

14.1 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.

14.2 Which of the statements given in Exercise 14.1 is true for p-type

semiconductos.

14.3 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. Whichof the following statements is true?

(a) (Eg)Si < (Eg)Ge < (Eg)C

(b) (Eg)C< (Eg )Ge > (Eg)Si

(c) (Eg)C(Eg)Si > (Eg)Ge

(d) (Eg)C= (Eg)Si = (Eg)Ge

Answer:

(c) (Eg)C(Eg)Si > (Eg)Ge

14.4 In an unbiased p-n junction, holes diffuse from the p-region to

n-region because

(a) free electrons in the n-region attract them.

(b) they move across the junction by the potential difference.

(c) hole concentration in p-region is more as compared to n-region.

(d) All the above.

Answer:

(c) hole concentration in p-region is more as compared to n-region.

14.5 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.

Answer

(c) hole concentration in p-region is more as compared to n-region.

14.6 In half-wave rectification, what is the output frequency if the input

frequency is 50 Hz. What is the output frequency of a full-wave rectifier

for the same input frequency.

14.7 A p-n photodiode is fabricated from a semiconductor with band gap

of 2.8 eV. Can it detect a wavelength of 6000 nm?

the ratio of Ga:As is 1:1 but in Ga-rich or As-rich GaAs it could

respectively be Ga1.1 As0.9 or Ga0.9 As1.1. In general, the presence of

defects control the properties of semiconductors in many ways.

14.8 The number of silicon atoms per m3is 5 × 1028. This is dopedsimultaneously with 5 × 1022 atoms per m3of Arsenic and 5 × 1020per m3atoms of Indium. Calculate the number of electrons and holes.

Given that ni= 1.5 × 1016 m–3. Is the material n-type or p-type?

14.9 In an intrinsic semiconductor the energy gap Egis 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 where n0 is a constant.

14.10 In a p-n junction diode, the current I 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 I is the current through the diode, kB

is theBoltzmann constant (8.6×10–5 eV/K) and T is the absolute

temperature. If for a given diode I0= 5 × 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?

14.11 You are given the two circuits as shown in Fig. 14.36. Show that

circuit (a) acts as OR gate while the circuit (b) acts as AND gate.

Answer:

14.12 Write the truth table for a NAND gate connected as given in

Fig. 14.37.

14.13 You are given two circuits as shown in Fig. 14.38, which consist

of NAND gates. Identify the logic operation carried out by the two

circuits.

14.14 Write the truth table for circuit given in Fig. 14.39 below consisting

of NOR gates and identify the logic operation (OR, AND, NOT) which

this circuit is performing.

FIGURE 14.39

(Hint: A = 0, B = 1 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.)

14.15 Write the truth table for the circuits given in Fig. 14.40 consisting of

NOR gates only. Identify the logic operations (OR, AND, NOT) performed

by the two circuits.

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