NEET UG (Medical Entrance) · Physics (JEE & NEET)
Electronic Devices (Semiconductors)
Semiconductor basics, p-n junction diodes, rectifiers, optoelectronic devices, Zener regulation and logic gates for JEE Main.
A JEE Main device topic: carrier physics first, diode behaviour second, then rectifiers, optoelectronic devices, Zener regulation and gates.
- NEET UG (Medical Entrance)
- Medium level
- 5 concepts
- 5 practice questions
1Intrinsic and extrinsic semiconductors
Pure silicon at room temperature has thermally generated electrons and holes in equal numbers. Doping breaks that equality on purpose. A donor impurity supplies electrons and makes n-type material; an acceptor creates holes and makes p-type material. Majority carrier does not mean net charge: the doped crystal remains electrically neutral.
Figure. Pure silicon has n = p = nᵢ. Donor doping lifts the electron bar; acceptor doping lifts the hole bar. Charge neutrality still holds — the majority carrier just changes.
How it works
- IntrinsicElectrons and holes are generated in pairs, so n_e=n_h.
- n-typePentavalent dopants add donor levels and electrons dominate conduction.
- p-typeTrivalent dopants create acceptor levels and holes dominate conduction.
| Dopant | Type | Majority carrier |
|---|---|---|
| Pentavalent donor (P, As, Sb) | n-type | Electron |
| Trivalent acceptor (B, Al, Ga) | p-type | Hole |
Silicon doped with boron is
- n-type with electrons as majority carriers
- p-type with holes as majority carriers
- intrinsic because boron is neutral
Boron is trivalent, so it creates acceptor levels and holes are the majority carriers.
2The p-n junction diode
When p-type and n-type regions touch, electrons and holes diffuse across the boundary and recombine, leaving fixed ion cores behind. That uncovered charge forms the depletion layer and a built-in electric field. Forward bias lowers this barrier and current rises sharply; reverse bias raises it and only a tiny saturation current flows until breakdown.
Figure. Diffusion leaves a depleted strip of uncovered ion charge and a built-in field from n toward p. Forward bias fights that field and thins the strip; reverse bias widens it.
How it works
- Diffusion firstMajority carriers cross the junction because their concentration is high on one side.
- Depletion formsRecombination leaves fixed donor and acceptor ions, creating a barrier field.
- Bias controls barrierForward bias lowers it; reverse bias widens it.
Forward or reverse?
The p-side of a diode is connected to the positive terminal of a battery and the n-side to the negative terminal.
- p-side at +, n-side at -forward bias
- Barrier potentialreduced
- Current after kneelarge
Pro tip. Forward bias means the external field helps majority carriers cross the junction.
In reverse bias, the depletion layer generally
- Widens
- Vanishes
- Becomes a metal conductor
Reverse bias pulls majority carriers away from the junction, exposing more fixed ions and widening the depletion region.
3Rectifiers turn AC into one-direction current
A rectifier uses diode one-way conduction. A half-wave rectifier keeps only one half-cycle, so the output is pulsating and inefficient. A full-wave rectifier flips the negative half-cycle into the positive direction, doubling the ripple frequency and increasing average output. A capacitor filter charges near the peak and discharges through the load between peaks.
Figure. Half-wave conduction keeps only the positive half-cycles. Full-wave flips the negative half up — same diode idea, twice the pulses per period. A capacitor filter then holds the peaks.
How it works
- One-way valveA forward-biased diode conducts during the allowed half-cycle.
- Full-wave useA centre-tap pair or bridge arrangement uses both halves of the input.
- FilterThe capacitor fills valleys by discharging through the load.
Compared with a half-wave rectifier using the same AC frequency, a full-wave rectifier has ripple frequency
- The same
- Twice as large
- Half as large
Both positive and negative half-cycles produce output pulses, so pulses arrive twice per input cycle.
4LED, photodiode and solar cell
The same p-n junction can emit, detect or generate electrical energy depending on how it is used. An LED is forward biased and emits photons when electrons recombine with holes. A photodiode is usually reverse biased; incoming light creates electron-hole pairs that the junction field sweeps into a photocurrent. A solar cell works in photovoltaic mode, delivering power to a load without an external reverse-bias supply.
Figure. Same junction, three jobs: LED spends electrical energy to emit photons; photodiode and solar cell absorb photons — one as a reverse-biased detector current, one as a power source.
How it works
- LEDForward recombination releases light; colour depends on band gap.
- PhotodiodeReverse bias makes light-generated current easy to measure.
- Solar cellLight separates carriers and the junction drives current through an external load.
| Device | Bias/use | Output |
|---|---|---|
| LED | Forward bias | Light |
| Photodiode | Reverse bias | Photocurrent |
| Solar cell | Photovoltaic mode | Power to load |
A photodiode used as a detector is usually operated in
- Forward bias
- Reverse bias
- No junction at all
Reverse bias widens the depletion region and sweeps light-generated carriers quickly, giving a measurable photocurrent.
5Zener regulation and logic gates
A Zener diode is made to operate safely in reverse breakdown. Once the supply is high enough, the Zener holds nearly constant voltage V_Z across the load while the series resistor absorbs the changing input. Logic gates are the digital end of the topic: OR gives 1 if any input is 1, AND gives 1 only if all inputs are 1, NOT inverts, and NAND/NOR are inverted AND/OR gates.
Figure. Logic levels are the visual for gate behaviour; the Zener's job beside them is to hold a fixed reverse voltage across the load once breakdown is reached. Truth tables beat fake gate glyphs.
How it works
- Series resistorIt limits current and drops V_{in}-V_Z while regulation holds.
- Zener branchThe diode takes excess current so the load voltage stays near V_Z.
- Gate truthRead the Boolean operation before trying to memorise the whole table.
| Inputs | AND | OR | NAND | NOR |
|---|---|---|---|---|
| 0, 0 | 0 | 0 | 1 | 1 |
| 0, 1 | 0 | 1 | 1 | 0 |
| 1, 0 | 0 | 1 | 1 | 0 |
| 1, 1 | 1 | 1 | 0 | 0 |
Zener current
A 9\text{ V} source feeds a 5.1\text{ V} Zener through 390\ \Omega. Find the series current.
- I_S=(V_{in}-V_Z)/R_S(9-5.1)/390
- I_S0.010\text{ A}=10\text{ mA}
Pro tip. That 10 mA is shared by load and Zener; regulation fails if the load demands more than the series path can supply.
The NAND output for inputs 1 and 1 is
- 1
- 0
- Undefined
AND would be 1, and NAND is NOT-AND, so it outputs 0.
Notes
- Intrinsic semiconductors have electron and hole concentrations equal: n_e=n_h=n_i. Extrinsic semiconductors are doped: pentavalent donors make n-type material with electrons as majority carriers; trivalent acceptors make p-type material with holes as majority carriers.
- A p-n junction forms a depletion region and built-in barrier potential. Forward bias lowers the barrier and allows large current after the knee voltage; reverse bias widens the depletion layer and allows only a small saturation current until breakdown.
- A diode conducts mainly in one direction. A half-wave rectifier passes one half-cycle of AC; a full-wave rectifier uses both half-cycles and gives a higher average DC output, usually smoothed by a capacitor filter.
- LEDs emit light in forward bias by electron-hole recombination; photodiodes are used in reverse bias so light-generated carriers give a measurable photocurrent; solar cells operate photovoltaicly without an external reverse bias.
- A Zener diode is designed to operate in reverse breakdown at nearly constant voltage V_Z, so with a series resistor it can regulate load voltage. Logic gates implement Boolean operations: OR, AND, NOT, NAND and NOR.
Formulas
- Intrinsic semiconductor: n_e=n_h=n_i
- Mass action law: n_e n_h=n_i^2 at fixed temperature
- Diode current idea: forward current rises rapidly after barrier reduction; reverse saturation current is small before breakdown
- Zener regulator series current: I_S=\dfrac{V_{in}-V_Z}{R_S}, with I_S=I_L+I_Z
- Logic: NAND output =\overline{AB}, NOR output =\overline{A+B}
Exam traps & shortcuts
- n-type means negative majority carriers but the crystal is electrically neutral; do not call the material negatively charged.
- A photodiode is normally reverse biased for detection; an LED is forward biased for emission.
- NAND and NOR are universal gates; any Boolean expression can be built from only NANDs or only NORs.
Reference tables
| Device | Usual bias | Exam cue |
|---|---|---|
| Ordinary diode | Forward for conduction | Rectification |
| Zener diode | Reverse breakdown | Voltage regulation |
| LED | Forward | Light emission |
| Photodiode | Reverse | Light detection |
| Solar cell | Photovoltaic | Power generation |
Recap
Read only this before a semiconductor question.
- Doping
- Donor makes n-type with electrons; acceptor makes p-type with holes. The crystal remains neutral.
- Diode
- Forward bias lowers the barrier; reverse bias widens it until breakdown.
- Opto
- LED forward emits, photodiode reverse detects, solar cell generates power.
- Zener
- Reverse breakdown at nearly constant V_Z regulates voltage with a series resistor.
- Logic
- NAND is inverted AND; NOR is inverted OR; both are universal.
Practise Electronic Devices (Semiconductors)
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- 5 exam-style questions on this topic, with explanations
- A 5-question practice set that ends the chapter
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