EXERCISE SHORT QUESTIONS
CHAPTER # 18: ELECTRONICS
Q # 1: How does the motion of an electron in n-type substance differ from the motion in a p-type substance?
Ans. In n-type material, the electrons are the majority carriers. They move from lower potential to higher potential.
In p-type materials, the holes are the majority carriers. They move from higher potential to lower potential.
Q # 2: What is the net charge on an n-type or p-type substance?
Ans. The penta-valent or trivalent impurity atoms bombard on intrinsic semiconductor, to form it n-type or p-type material, are neutral. Therefore, an n-type or p-type substance is an electrically neutral substance.
Q # 3: The anode of a diode is 0.2 V positive with respect to its cathode. Is it forward biased?
Ans. A junction diode is said to be forward biased if its P-type region is connected to the positive terminal and N-type region is connected to the negative terminal of the battery. Since anode (p-type) is at positive potential 0.2 V with respect to its cathode (n-type), so it satisfied the condition of forward biased.
But the potential barrier of Ge and Si are 0.3 V and 0.7 V respectively, so such small value of voltage can't produce forward current.
Q # 4: Why charge carries are not present in the depletion region?
Ans. When a p-type semiconductor is brought close an n-type to form a PN-junction, and then the free electrons near the junction in the n region begin to diffuse across the junction into the p-type region where they combine with holes near the junction, and neutralize holes in p-type. As a result, a charge less region is formed across the junction in which charge carriers are not present. This region is known as depletion region.
Q # 5: What is the effect of forward and reverse biasing of diode on the width of depletion region?
Ans. When the diode is forward biased, the width of depletion region is decreased. When the diode is reversed biased, the width of depletion region is increased.
Q # 6: Why ordinary silicon diodes don't emit light?
Ans. The potential barrier across the pan-junction of Si is 0.7 V. so in forward biased condition, when electron recombine with the hole, a photon of light having 0.7 eV energy is released. As the energy of emitted photon lies in infrared region of electromagnetic spectrum. That's why we don't observe light emission from Si diode.
Q # 7: Why a photo diode is operated in reverse biased state?
Ans. Photo diode is used for detection of light. It is operated in the reverse biased condition.
- When no light is incident on the junction, the reverse current is almost negligible.
- When a photo diode is exposed to light, the reverse current increases with intensity of light.
Thus the reverse biased condition of a photo diode is useful to detection of light.
Q # 8: Why is the base current in a transistor very small?
Ans. The base of a transistor is kept thin so that a very few charge carriers (electrons or holes)from emitter may combine with electrons or holes of base. This result in larger collector current, hence larger current and power gain to transistor become possible.
Q # 9: What are the biasing requirements of the junction of a transistor for its normal operation? Explain how these requirements are met in a common emitter amplifier?
Ans. For the normal operation of transistor, the EB (emitter-base) junction of transistor is forward biased and CB (collector-base) junction of transistor is reversed.
In npn-transistor in common emitter configuration, the EB-junction is forward biased by \(V_{BB}\)battery and CB-junction is reversed biased by \(V_{CC}\) battery, as shown in the figure.
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