Physics · Lesson 25

Electronics: semiconductors, diodes & transistors

JAMBWAECNECO

What the syllabus asks

JAMB: Elementary modern physics and electronics: semiconductors, doping, p–n junction, diode as rectifier, transistor action, uses of semiconductors

WAEC: Conductors, semiconductors and insulators, energy bands, intrinsic and extrinsic (n-type and p-type) semiconductors, doping, the p–n junction diode, forward and reverse bias, diode characteristics, rectification (half-wave and full-wave), smoothing, special diodes, the junction transistor and its uses as a switch and an amplifier, simple logic gates

NECO: Conductors, semiconductors and insulators, energy bands, intrinsic and extrinsic (n-type and p-type) semiconductors, doping, the p–n junction diode, forward and reverse bias, diode characteristics, rectification (half-wave and full-wave), smoothing, special diodes, the junction transistor and its uses as a switch and an amplifier, simple logic gates

What you will learn

Distinguish conductors, semiconductors and insulators using energy bands.
Explain intrinsic conduction and how doping produces n-type and p-type materials.
Describe the p–n junction, its depletion layer, and forward and reverse bias.
Sketch and interpret the I–V characteristic of a diode.
Draw and explain half-wave, full-wave (bridge) rectifier and smoothing circuits.
Describe the structure of npn and pnp transistors and use I_E = I_B + I_C and β = I_C/I_B.
Explain the transistor as a switch and as an amplifier, and give the truth tables of basic logic gates.

Key points

Semiconductors: small band gap; resistance falls as temperature rises.
Intrinsic: electrons = holes; holes act as positive carriers.
n-type: group V donor, electrons majority; p-type: group III acceptor, holes majority; both neutral.
p–n junction: depletion layer and barrier of about 0.7 V (Si), 0.3 V (Ge).
Forward bias conducts; reverse bias blocks until breakdown.
Half-wave: one diode, 50 Hz pulses; bridge: four diodes, full-wave, 100 Hz ripple.
Capacitor smooths; LED emits light; Zener regulates; photodiode and solar cell respond to light.
Transistor: I_E = I_B + I_C, β = I_C/I_B.
Switch: off or saturated; LDR/thermistor control circuits.
Amplifier: active region, gain = V_out/V_in; logic gates AND, OR, NOT, NAND, NOR.

Start here: Conductors, insulators and semiconductors

Your phone, the inverter that keeps lights on during a power cut, the solar panels on rooftops in Kano and the traffic lights in Lagos all depend on a family of materials called semiconductors. To understand them, we compare three classes of material.

Conductors (copper, aluminium, silver) have many free electrons, so they conduct easily. Their resistance rises slightly with temperature, because hotter atoms vibrate more and get in the way of electrons.
Insulators (rubber, glass, dry wood, plastic) have almost no free electrons; their electrons are tightly bound.
Semiconductors (silicon, germanium) have few free electrons at room temperature, so they conduct a little. Heating or light frees more electrons, so their resistance falls as temperature rises.

Energy-band picture. In a solid, electron energies group into bands. The valence band holds electrons used in bonding. The conduction band holds electrons free to move. Between them is the forbidden gap.

In conductors the bands overlap. In insulators the gap is wide (several eV), so electrons cannot jump it. In semiconductors the gap is small (about 1.1 eV for silicon), so a few electrons gain enough heat energy to jump across.

⚠️ Common mistake: saying semiconductor resistance rises with temperature like a metal. It falls; this is used in thermistors.

Try it yourself. Why does a semiconductor conduct better when heated?

Show solution

Heat gives more electrons enough energy to jump the small forbidden gap into the conduction band, creating more free electrons and holes.

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Practice questions

1. Name two elements that make p-type silicon.

Show worked answer

Any two of boron, aluminium, gallium, indium.

2. State one difference between the resistance–temperature behaviour of a metal and a semiconductor.

Show worked answer

Metal: resistance rises with temperature; semiconductor: resistance falls.

3. A silicon diode in forward bias is in series with a 2.2 kΩ resistor across 9.5 V. Find the current.

Show worked answer

(9.5 − 0.7)/2200 = 4.0 mA.

4. A bridge rectifier uses 50 Hz mains. What is the ripple frequency?

Show worked answer

100 Hz.

5. A transistor has I_B = 30 μA and I_C = 4.5 mA. Find β and I_E.

Show worked answer

β = 4.5/0.03 = 150; I_E = 4.53 mA.

Key words

Semiconductor: A material whose conductivity lies between conductors and insulators and rises with temperature.
Energy band: A range of allowed electron energies in a solid.
Forbidden gap: The energy gap between the valence and conduction bands.
Intrinsic semiconductor: A pure semiconductor, with equal numbers of free electrons and holes.
Doping: Adding tiny amounts of impurity to change a semiconductor’s conductivity.
n-type: A semiconductor doped with a pentavalent (group V) impurity; majority carriers are electrons.
p-type: A semiconductor doped with a trivalent (group III) impurity; majority carriers are holes.
Hole: A missing electron in a bond; it behaves like a positive charge carrier.
Depletion layer: A region at a p–n junction with no free charge carriers.
Forward bias: p-side connected to positive terminal; the diode conducts.
Reverse bias: p-side connected to negative terminal; the diode blocks current.
Rectification: Converting alternating current to direct current.

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