# NCERT Physics Class 12 Exemplar Ch 14 Semiconductor Electronics Materials Devices and Simple Circuits Part 1

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17 Why are elemental do pants for Silicon or Germanium usually chosen from group XIII or group XV?

Ans:

• The size of dopant atoms should be such as not to distort the pure semiconductor lattice structure and yet easily contribute a charge carrier on forming co-valent bonds with Si or Ge.

18 Sn, C, and Si, Ge are all group XIV elements. Yet, Sn is a conductor; C is an insulator while Si and Ge are semiconductors. Why?

Ans:

• The energy gap for Sn is , for C is , for Si is and for Ge is , related to their atomic size.

19. Can the potential barrier across a p-n junction be measured by simply connecting a voltmeter across the junction?

Ans:

• No, because the voltmeter must have a resistance very high compared to the junction resistance, the latter being nearly infinite.

20 Draw the output waveform across the resistor.

Ans:

21 The amplifiers X, Y and Z are connected in series. If the voltage gains of X, Y and Z are and, respectively and the input signal is 1 mV peak value, then what is the output signal voltage (peak value?)

(i) If dc supply voltage is?

(ii) If dc supply voltage is?

Ans:

(i)

(ii) If dc supply voltage is , the output peak will not exceed. Hence, .

22 In a CE transistor amplifier there is a current and voltage gain associated with the circuit. In other words there is a power gain. Considering power a measure of energy, does the circuit violate conservation of energy?

Ans:

• No, the extra power required for amplified output is obtained from the DC source.

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