Semiconductor Electronics — Class 12 Physics Worksheet | AAI ATC

Before you begin

Formulas you will need

Key formulas Silicon: 4 valence electrons, gap ≈ 1.1 eV · Germanium: gap ≈ 0.7 eV
Pentavalent dopant → n-type (electrons majority) · Trivalent dopant → p-type (holes majority)
Barrier potential: Si ≈ 0.7 V, Ge ≈ 0.3 V · Diode resistance R = V ∕ I
Full-wave rectifier ripple frequency = 2 × supply frequency

How to use this worksheet. Pentavalent dopants give n-type material with electrons as majority carriers; trivalent dopants give p-type with holes. Get that pairing fixed first and half this chapter answers itself. Remember also that doping raises conductivity — the opposite claim is the answer to one of the previous year questions.

Section A · Level 1

Bands, doping and the junction

Ten questions. None should take more than twenty seconds.

Q1

The number of valence electrons in a silicon atom is:

Silicon sits in group 14, so it has four valence electrons. Each one forms a covalent bond with a neighbour in the crystal lattice.

Q2

At absolute zero temperature, a pure semiconductor behaves as:

At absolute zero no electron has enough energy to cross the forbidden gap, so there are no free carriers and the material behaves as a perfect insulator.

Q3

The majority charge carriers in a p-type semiconductor are:

A trivalent dopant leaves a vacancy in the bonding structure, creating holes. In p-type material holes are the majority carriers and electrons the minority carriers.

Q4

Which of the following is a trivalent impurity?

Boron has three valence electrons and produces p-type material. Phosphorus, arsenic and antimony are all pentavalent and produce n-type material.

Q5

The forbidden energy gap in a good conductor is:

In conductors the valence and conduction bands overlap, so there is effectively no gap and electrons move freely. Insulators have a large gap and semiconductors an intermediate one.

Q6

Doping a pure semiconductor:

Adding a controlled impurity introduces extra carriers, so conductivity rises sharply. This is the entire purpose of doping.

Q7

The depletion region of an unbiased p-n junction contains:

Diffusion across the junction leaves a narrow region depleted of mobile carriers, containing only the fixed immobile ions that set up the barrier potential.

Q8

Under forward bias, the width of the depletion region:

Forward bias opposes the barrier potential, narrowing the depletion region and allowing current to flow. Reverse bias does the opposite.

Q9

A p-n junction diode conducts appreciably when it is:

Only forward bias lowers the barrier enough for majority carriers to cross in large numbers. In reverse bias just a tiny leakage current flows.

Q10

Comparing the energy gaps of silicon and germanium:

Silicon has a gap of about 1.1 eV while germanium has about 0.7 eV, which is why silicon devices tolerate higher temperatures before leakage becomes a problem.

Section B · Level 2

Diodes, rectifiers and carriers

Nine questions. Know which carrier is which before you answer.

Q11

Adding a pentavalent impurity to a pure semiconductor produces:

A pentavalent atom contributes a fifth electron that is not needed for bonding, so it becomes a free carrier. The result is n-type material.

Q12

In an n-type semiconductor, the minority carriers are:

Electrons are the majority carriers in n-type material, so the thermally generated holes are the minority carriers.

Q13

A half-wave rectifier uses how many diodes?

A half-wave rectifier needs a single diode, which conducts on one half of each cycle. A centre-tap full-wave rectifier uses two and a bridge uses four.

Q14

A full-wave bridge rectifier uses how many diodes?

The bridge configuration uses four diodes arranged so that current flows through the load in the same direction on both halves of the cycle.

Q15

The resistance of a p-n junction diode in reverse bias is:

Reverse bias widens the depletion region, so only a tiny leakage current flows and the effective resistance is enormous, typically in the megohm range.

Q16

A Zener diode is normally used as a:

A Zener diode is operated in reverse breakdown, where the voltage across it stays almost constant over a wide range of current. That property makes it a voltage regulator.

Q17

A silicon diode drops 0.7 V while carrying 10 mA. Its effective resistance is:

R = V ∕ I = 0.7 ∕ 0.01 = 70 Ω. Converting the milliamps to amps before dividing is the only step that goes wrong here.

Q18

The small reverse saturation current in a diode is caused by:

Reverse bias sweeps the thermally generated minority carriers across the junction. Their number depends on temperature but not on the applied voltage, which is why the current saturates.

Q19

As the temperature of a semiconductor rises, its resistance:

Higher temperature frees more carriers across the gap, so conductivity rises and resistance falls. Metals behave the opposite way.

Section C · Level 3

Devices and characteristics

Seven questions. Each needs a device fact plus a piece of reasoning.

Q20

Compared with an intrinsic semiconductor at the same temperature, the conductivity of an extrinsic semiconductor is:

Doping adds carriers that would not otherwise exist, so extrinsic material conducts far better than intrinsic material. This is precisely the statement tested in one of the previous year questions.

Q21

A photodiode is normally operated in:

In reverse bias the dark current is tiny, so the extra current generated by incident light stands out clearly against it. That makes the device a sensitive detector.

Q22

Light is emitted by an LED because of:

Under forward bias, electrons and holes recombine across the junction and release the energy difference as photons. The colour depends on the band gap of the material.

Q23

The approximate barrier potential of a silicon p-n junction is:

Silicon junctions have a barrier of roughly 0.7 V while germanium junctions have about 0.3 V. This is the knee voltage on the forward characteristic curve.

Q24

A full-wave rectifier is supplied from a 50 Hz mains. The ripple frequency of its output is:

A full-wave rectifier produces one output pulse for each half cycle, so the ripple frequency is twice the supply frequency, that is 100 Hz. A half-wave rectifier would give 50 Hz.

Q25

The number of p-n junctions in a bipolar junction transistor is:

A transistor has three doped regions — emitter, base and collector — which form two junctions between them.

Q26

A solar cell converts light into electricity using the:

Photons absorbed near the junction create electron-hole pairs, which the built-in field separates to produce a voltage. That is the photovoltaic effect.

Section D · Previous year questions

Straight from the AAI ATC papers

Four questions, every one exactly as it appeared in the 2025 papers.

Q27PYQ 2025 · Shift 1

A reverse bias voltage of 10 V is applied across a p-n junction diode such that the current flowing through the diode in the reverse bias configuration is 2 µA. What will be the resistance of the diode in this configuration?

R = V ∕ I = 10 ∕ (2 × 10⁻⁶) = 5 × 10⁶ Ω, which is 5000 kΩ or 5 MΩ. The enormous value is the physics of the question — a reverse biased diode barely conducts. All three wrong options are off by a power of ten.

Q28PYQ 2025 · Shift 1

Which of the following statements about extrinsic semiconductors is incorrect?

The first three statements are all true. The fourth is false and is therefore the answer: doping raises conductivity, so an extrinsic semiconductor always conducts better than an intrinsic one at the same temperature. Note that doping does not make the material charged — the added atoms are themselves neutral, which is why the second statement holds.

Q29PYQ 2025 · Shift 2

Which of the following materials is commonly used as a pentavalent impurity in n-type semiconductors?

Phosphorus has five valence electrons and donates the spare one as a free carrier, producing n-type material. Gallium, boron and indium are all trivalent and would produce p-type material instead.

Q30PYQ 2025 · Shift 2

Which of the following materials exhibits behaviour that violates Ohm's law under specific conditions?

Ohm's law requires current to be proportional to voltage. Three of the options explicitly describe that linear behaviour, so they obey the law. Only the semiconductor, whose characteristic curve bends, violates it. Read the descriptions rather than the material names.

↓ Free download — Semiconductor Electronics PDF

All thirty questions with the formula sheet printed on the front page and ruled working space for your circuit reductions. Print it, solve it, keep it.

Download the free PDF Join our live classes — extremely affordable