Class 12 Physics Chapter 14: Semiconductor Electronics: Materials, Devices and Simple Circuits worksheet with answer key
A 20-mark worksheet on Semiconductor Electronics: Materials, Devices and Simple Circuits for about 40 minutes of class or homework: 5 multiple-choice, 3 very short answer and 3 short answer questions, written from the NCERT chapter and checked by a second reviewer. Print the PDF for the class; the answer key carries the marking scheme. Teachers may photocopy it freely.
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Answer: (c) a vacancy in a covalent bond, behaving as a particle of effective charge +q
When an electron breaks away from a bond, the vacancy it leaves has effective charge +q; bound electrons from neighbouring bonds filling it make the hole appear to move like a free positive particle. No atom or nucleus moves.
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Answer: (a) 2.4 × 10¹⁹
In an intrinsic semiconductor every electron freed from a bond leaves one hole, so nₑ = nₕ = nᵢ = 2.4 × 10¹⁹ m⁻³. 5.8 × 10³⁸ is nᵢ², not a concentration.
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Answer: (b) immobile ionised donors on the n-side and immobile ionised acceptors on the p-side
Electrons diffusing out of the n-region leave positive donor ions behind; holes diffusing out of the p-region leave negative acceptor ions. These fixed ions, with no free carriers, form the depletion region.
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Answer: (b) a microammeter, since the reverse current is of the order of microamperes
The reverse current is a drift of minority carriers, only a few μA, so a microammeter is used; a milliammeter suits the forward current, which is in mA.
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Answer: (b) germanium
The threshold (cut-in) voltage is about 0.2 V for germanium and about 0.7 V for silicon. Tin is a metal (Eg = 0) and diamond an insulator, so neither forms such a diode.
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Above the 0.7 V cut-in, the forward current rises steeply (exponentially) with voltage, so with 3.0 V across it and nothing to limit it, the current far exceeds the 50 mA rating. The junction overheats and the diode is destroyed. A series resistor is needed to keep the current below the rated value.
- current far exceeds the rated value, since it rises steeply beyond cut-in with nothing to limit it (1)
- junction is destroyed by overheating (1)
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In the half-cycle when the diode is reverse biased it does not conduct, and the whole ac voltage of that half-cycle, up to its peak, appears across it. If the peak exceeded the breakdown voltage, the reverse current would rise sharply and the diode would conduct in the wrong half-cycle (and could be destroyed), spoiling the rectification.
- in the reverse half-cycle the full peak voltage appears across the diode (1)
- above breakdown it conducts sharply/is damaged, so rectification fails (1)
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Each Si atom has 4 outer electrons, 4N in all, with 8N states available; at the crystal's spacing these split into a valence band of 4N states and a conduction band of 4N states. At 0 K the 4N electrons fill the valence band completely and the conduction band is completely empty.
- 4N states in each band (8N in all) (1)
- at 0 K valence band completely filled by 4N electrons, conduction band empty (1)
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(a) Free electrons move against the field, towards X; holes move towards the negative potential, towards Y. (b) An electron from a neighbouring bond jumps into a hole, leaving a hole at its own site; repeated, the hole appears to move. This is a description of bound electrons' motion; the free electron moves independently. Electrons moving towards X and holes towards Y both carry conventional current from X to Y, so I = Iₑ + Iₕ.
- electrons towards X (higher potential), holes towards Y (1)
- hole moves as bound electrons jump into vacancies, leaving vacancies behind (1)
- both give conventional current in the same direction, so I = Iₑ + Iₕ (1)
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(a) Boron is trivalent: it bonds with three Si neighbours, leaving a vacancy (hole) in the bond with the fourth. When an electron from a neighbouring Si atom fills it, boron has shared a fourth electron and becomes effectively a negative ion, and the hole moves to the neighbour's site: one acceptor, one hole. (b) Slightly above the top of the valence band EV; a very small energy lets a valence electron jump to EA. (c) The extra holes' total positive charge is equal and opposite to the charge of the ionised acceptor cores.
- three bonds, one vacancy; filling it makes B a negative ion and gives one hole (1)
- EA slightly above EV (1)
- extra holes' charge balanced by ionised acceptor cores (1)
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Holes are more concentrated in p and electrons in n, so holes diffuse p → n and electrons n → p: a diffusion current, large at first. Each carrier that leaves exposes an immobile ion, building positive space charge on the n-side and negative on the p-side; this sets up a field from n to p, which drives minority carriers back across: a drift current, opposite to the diffusion current, small at first. As more charge is exposed the field and the drift current grow while diffusion slows, until diffusion current equals drift current, and the junction is in equilibrium with no net current.
- diffusion of holes p → n and electrons n → p due to the concentration gradient; large at first (1)
- exposed ions set up a field n → p, giving a drift current opposite to diffusion that grows with the space charge (1)
- equilibrium when diffusion current = drift current, so net current is zero (1)