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

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. Draw the Output Waveform Across the Resistor 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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