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Semiconductor Electronics closes the JEE Main syllabus and it is almost pure recall. Doping, the p-n junction, rectifiers, the Zener diode, the LED and photodiode, and logic gates — each has a handful of facts and one standard question. This is a chapter you can secure in two sittings, so do not leave it for the last week. Ten questions below. Give them ten honest minutes.
What this chapter carries in JEE Main
- Usually 2 questions every year, which is 8 marks.
- Logic gates, rectifiers and the p-n junction are asked every year.
- Doping, the Zener diode and the optoelectronic diodes fill the rest.
What the syllabus says
In the syllabus (Unit 19, Electronic Devices): Semiconductors, the semiconductor diode and its I–V characteristics in forward and reverse bias, the diode as a rectifier, I–V characteristics of the LED, the photodiode, the solar cell and the Zener diode, the Zener diode as a voltage regulator, and logic gates — OR, AND, NOT, NAND and NOR.
Taken out in 2024: the junction transistor, transistor action and transistor characteristics; the transistor as an amplifier in common emitter configuration and as an oscillator; the transistor as a switch.
Never in the JEE Main syllabus: energy bands in solids. They are in NCERT, so they still turn up in old question banks.
Taken from the NTA syllabus for JEE Main 2027. See the full syllabus, unit by unit.
Formulas you must know by heart
| Intrinsic semiconductor | ne = nh = ni |
| n-type | doped with a pentavalent atom, electrons are the majority carriers |
| p-type | doped with a trivalent atom, holes are the majority carriers |
| Mass action law | nenh = ni2 |
| Forward bias | the depletion layer narrows and current flows |
| Reverse bias | the depletion layer widens, only a tiny leakage current flows |
| Half wave rectifier | ripple frequency equals the supply frequency |
| Full wave rectifier | ripple frequency is twice the supply frequency |
| Zener diode | works in reverse breakdown, used as a voltage regulator |
| LED and photon energy | Eg = hc/λ, with hc = 1240 eV·nm |
| Band gaps | conductor zero or overlapping, semiconductor about 1 eV, insulator above 3 eV |
Note that the resistance of a semiconductor falls as it gets hotter, which is the opposite of a metal. Heat frees more charge carriers, and that gain beats the extra scattering by a wide margin.
Ten questions. Give yourself ten minutes.
Write your answers down before you scroll. A question you looked up is a question you have not answered.
Q1. In a pure intrinsic semiconductor, the number of free electrons is
- (a) greater than the number of holes
- (b) equal to the number of holes
- (c) smaller than the number of holes
- (d) zero
Q2. Silicon doped with phosphorus becomes
- (a) p-type
- (b) n-type
- (c) intrinsic
- (d) an insulator
Q3. When a p-n junction is forward biased, the depletion layer
- (a) becomes wider
- (b) becomes narrower
- (c) stays the same
- (d) vanishes completely
Q4. A full wave rectifier is fed from a 50 Hz supply. The ripple frequency of its output is
- (a) 25 Hz
- (b) 50 Hz
- (c) 100 Hz
- (d) 200 Hz
Q5. A Zener diode is normally used as
- (a) an amplifier
- (b) a voltage regulator
- (c) an oscillator
- (d) a forward biased rectifier
Q6. An LED glows red with light of wavelength about 650 nm. The band gap of the semiconductor used in it is closest to
- (a) 0.7 eV
- (b) 1.1 eV
- (c) 1.9 eV
- (d) 3.4 eV
Q7. The output of a NAND gate when both its inputs are 1 is
- (a) 0
- (b) 1
- (c) undefined
- (d) the same as the input
Q8. The forbidden energy gap of a conductor is
- (a) very large
- (b) about 1 eV
- (c) zero, with the bands overlapping
- (d) about 6 eV
Q9. As the temperature of a semiconductor rises, its resistance
- (a) increases
- (b) decreases
- (c) stays the same
- (d) becomes infinite
Q10. A photodiode is always operated in reverse bias rather than forward bias because
- (a) it does not conduct at all in forward bias
- (b) the fractional change in current caused by the light is far larger in reverse bias
- (c) the reverse current does not depend on the light falling on it
- (d) reverse bias makes the band gap smaller
Answer key, with a hint for each
| Q1 | (b) | Every electron that leaves the valence band leaves a hole behind, so the two numbers must match. |
| Q2 | (b) | Phosphorus has five valence electrons and donates the fifth one, so the carriers are electrons. |
| Q3 | (b) | The applied voltage opposes the built-in field, so the barrier shrinks and carriers cross easily. |
| Q4 | (c) | Both halves of the cycle are used, so the output pulses come twice as often as the input cycles. |
| Q5 | (b) | It holds a steady voltage across itself once breakdown begins, however the current changes. |
| Q6 | (c) | Use Eg = hc/λ with hc = 1240 eV·nm, so Eg = 1240/650 ≈ 1.9 eV. Silicon, at 1.1 eV, would put the photon in the infrared — which is why a silicon diode never glows. |
| Q7 | (a) | NAND is AND followed by NOT. AND gives 1, and NOT turns it into 0. |
| Q8 | (c) | With the valence and conduction bands overlapping, electrons are free to move with no energy cost at all. |
| Q9 | (b) | Heating frees many more carriers, and that outweighs the extra scattering. A metal behaves the other way. |
| Q10 | (b) | In forward bias the current is already large, so the small light‑generated current hardly shows. In reverse bias the dark current is tiny, so the same light‑generated current is a big fractional change — that is what makes the reading sensitive. |
Now mark yourself honestly
JEE Main marks the way it marks. Use the same rule here:
- +4 for a right answer
- −1 for a wrong one
- 0 for a blank
So ten questions here are out of 40. In the real paper, 25 Physics questions are out of 100.
If you scored 32 or more, the basics of this chapter are secure. Move to harder questions, not more of these.
If you scored below 24, do not feel bad about it. You have just found out something useful before the exam did. Go back to the formula table above, learn it properly, and sit these ten again tomorrow.
And if you guessed a few and they landed, count those as wrong. A guess that works once will not work in January.
The full chapter, on a clock
Ten questions tell you a little. Twenty-five tell you the truth.
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