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Semiconductors & Electronic Devices revision notes

Every idea in the Semiconductors & Electronic Devices chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 22 and download the PDF.

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How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Semiconductors

Energy bands
2× asked, last 2023
Eg(Si)≈1.1 eV,Eg(Ge)≈0.7 eVE_g(\text{Si})\approx1.1\ \text{eV},\qquad E_g(\text{Ge})\approx0.7\ \text{eV}
Conductor: bands overlap. Semiconductor: Eg<3E_g<3 eV. Insulator: Eg>3E_g>3 eV (diamond ≈ 5.4 eV).
conductoroverlapsemiconductorEg≈1.1E_g\approx1.1 eVinsulatorEg≈5.4E_g\approx5.4 eVvalenceconduction
Band gaps to scale: none (metal), about 1.1 eV (Si), above 3 eV (diamond about 5.4 eV)
  • Heating lifts more electrons across EgE_g, so nen_e and nhn_h rise and a semiconductor's resistivity falls with TT (negative temperature coefficient). A metal's resistivity rises.
Photon making an electron–hole pair
7× asked, last 2024
hν≥Eg,λmax⁡=hcEg=1240Eg (eV) nmh\nu\ge E_g,\qquad \lambda_{\max}=\frac{hc}{E_g}=\frac{1240}{E_g\,(\text{eV})}\ \text{nm}
Longer wavelengths make no pairs. The same relation gives an LED's emitted λ and a photodiode's cut-off.
  • Not the shortest wavelength. EgE_g fixes the longest (cut-off) wavelength; every shorter λ also makes pairs.
Extrinsic: n-type and p-type
4× asked, last 2025
n-type: ne≫nh,p-type: nh≫ne\text{n-type: } n_e\gg n_h,\qquad \text{p-type: } n_h\gg n_e
n: pentavalent donor (P, As, Sb). p: trivalent acceptor (B, Al, Ga, In).
  • Majority carriers: electrons in n-type, holes in p-type. Minority carriers come only from thermal pair generation; a donor gives an electron, never a hole.
  • Not negatively charged. Each donor that gives up an electron becomes a fixed ++ ion, so an n-type (or p-type) crystal stays electrically neutral.
Mass action law
2× asked, last 2025
ne nh=ni2n_e\,n_h = n_i^2
Thermal equilibrium at a fixed TT, any doping. Raising one carrier density lowers the other.
  • Intrinsic (pure Si, Ge): ne=nh=nin_e=n_h=n_i, rising steeply with TT. Conductivity σ=e(neμe+nhμh)\sigma=e(n_e\mu_e+n_h\mu_h); electrons are more mobile than holes; in a doped sample keep the majority term.
  • Not nenh≥ni2n_en_h\ge n_i^2 or ≠ni2\ne n_i^2. Doping changes the split, not the product. The minority density is ni2n_i^2 divided by the majority density.

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18 more concepts in 5 topics

The p–n junction, Diode circuits and rectifiers, Zener diode, Optoelectronic devices and Logic gates. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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The p–n junction

  • Depletion layer and barrier5×
  • Forward and reverse bias4×
  • I–V characteristic and dynamic resistance4×

Diode circuits and rectifiers

  • Diode in a circuit13×
  • Clippers: diode and Zener limiters5×
  • Rectifiers3×

Zener diode

  • Zener diode3×
  • Zener voltage regulator11×
  • Power rating and safe series resistor12×

Optoelectronic devices

  • Light-emitting diode (LED)6×
  • Photodiode6×
  • Solar cell2×
  • Also remember1

Logic gates

  • OR, AND and NOT11×
  • NAND and NOR10×
  • Universal gates18×
  • De Morgan and simplification11×
  • Reading a gate circuit or waveform33×
  • Diode gate circuits4×

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