Semiconductors

Learning goals

What students should be able to explain, calculate and investigate after completing Semiconductors.

• Distinguish conductors, insulators and semiconductors in terms of charge carriers.
• Explain doping and distinguish n-type from p-type semiconductors.
• Describe the p-n junction and the depletion layer.
• Explain how a diode conducts in forward bias and blocks in reverse bias.
• Explain how a transistor can be used as a switch.

Key definitions

Precise terms, symbols and SI units used in Semiconductors.

Conductors, Insulators and Semiconductors

Conductors (e.g. metals) have many free charge carriers and conduct well at all normal temperatures.

Insulators have effectively no free charge carriers. Semiconductors sit in between: pure silicon or germanium conducts only weakly at room temperature, but its conductivity increases significantly with temperature (more charge carriers become available as heat energy frees more electrons) and can be dramatically and precisely controlled by doping.

Doping: n-type and p-type

Doping deliberately introduces a small number of impurity atoms into a pure semiconductor crystal.

In n-type doping, atoms with five outer electrons (e.g. phosphorus, in silicon which has four) are added — four of each impurity atom's electrons bond into the crystal lattice, leaving one extra free electron per impurity atom, so electrons become the majority charge carriers. In p-type doping, atoms with three outer electrons (e.g. boron) are added — each leaves one bond incomplete, creating a 'hole' (an absence of an electron) that behaves like a mobile positive charge carrier as neighbouring electrons hop into it, so holes become the majority charge carriers. Both n-type and p-type materials remain electrically neutral overall — doping changes which type of charge carrier is free to move, not the material's overall charge.

The p-n Junction and Depletion Layer

When p-type and n-type regions are joined, electrons and holes recombine near the junction, leaving a depletion layer of fixed ions and an internal potential barrier.

This leaves a narrow depletion layer with no mobile charge carriers, but with fixed, immobile charged ions (positive on the n-side, negative on the p-side) — the p-type and n-type regions must be considered as directly touching at this junction, not separated by any physical gap; the depletion layer is a region within the crystal, not a physical space between two separate pieces. This creates a small internal electric field/potential barrier that must be overcome before current can flow across the junction.

Diodes: Forward and Reverse Bias

A p–n junction allows substantial current in forward bias but only a very small current in reverse bias.

In forward bias, the supply pushes majority carriers towards the junction. The depletion region and barrier narrow, carriers cross the junction, and current rises strongly. In reverse bias, majority carriers are pulled away from the junction. The depletion region widens and the current remains very small.

• Forward bias narrows the depletion region and allows substantial current after the turn-on voltage.
• Reverse bias widens the depletion region and normally allows only a tiny leakage current.
• A diode therefore conducts mainly in one direction.

Transistors as Switches

In an NPN switching circuit, a small base current controls whether a much larger collector current flows through the load and returns through the emitter.

The base (B) is the control terminal. When the base–emitter junction is not sufficiently forward biased, base current is approximately zero and the collector–emitter path is effectively OFF. When the input raises the base voltage sufficiently, a small base current flows through the base resistor. The transistor then allows a much larger current to flow from collector (C) to emitter (E), through the load circuit. The load’s energy comes from the supply, not from the small base current. Conventional currents obey I_E = I_C + I_B.

The complete circuit has two linked parts sharing the same 0 V return. The input circuit supplies a small current to the base through R_B. The output circuit carries the larger current through the load, collector and emitter. The transistor therefore acts as an electronic switch in the collector–emitter path.

• OFF: the input voltage is too low to produce significant I_B; the collector–emitter path does not conduct and the load is off.
• ON: the base–emitter junction is sufficiently forward biased; I_B flows and the collector–emitter path conducts, switching on the load.
• The base resistor R_B limits I_B and protects the transistor.

An LDR or thermistor can form one part of a potential divider. The divider’s midpoint feeds the base through R_B. A change in the sensor resistance changes this midpoint voltage and therefore decides whether base current flows. For a divider with a top resistance R_top and bottom resistance R_bottom, the midpoint voltage rises when R_top falls or when R_bottom rises.

The circuits shown use an NTC thermistor: its resistance decreases as temperature rises. With the thermistor at the top of the divider, warming lowers R_top, so the base voltage rises and the lamp switches on. Swapping the thermistor and fixed resistor reverses the response: cooling increases the bottom thermistor resistance, raises the base voltage and switches on the lamp.

• 1. The thermistor resistance changes.
• 2. The potential-divider midpoint voltage changes.
• 3. The base–emitter junction becomes sufficiently forward biased.
• 4. A small base current I_B flows through R_B.
• 5. A larger collector current I_C flows through the lamp.
• 6. The lamp switches on.

An LDR has a lower resistance in bright light and a higher resistance in darkness. With the LDR at the bottom of the divider, darkness raises R_bottom, so the base voltage rises and the lamp switches on. Swapping the LDR and fixed resistor makes the circuit respond in the opposite way: bright light lowers the top resistance, raises the base voltage and switches on the lamp.

• 1. The LDR resistance changes.
• 2. The potential-divider midpoint voltage changes.
• 3. The base–emitter junction becomes sufficiently forward biased.
• 4. A small base current I_B flows through R_B.
• 5. A larger collector current I_C flows through the lamp.
• 6. The lamp switches on.

Diagrams and formulae

Key diagrams and equations for reviewing Semiconductors.

• Doping: an n-type impurity (5 outer electrons) donates a free electron; a p-type impurity (3 outer electrons) creates a hole.
• The p-n junction: the p-type and n-type regions meet directly, with the depletion layer straddling the junction itself (not shown as a separate gap between two disconnected blocks).
• A transistor used as a switch: a small current into the base allows a much larger current to flow from collector to emitter, switching on the output device.

Doping: an n-type impurity (5 outer electrons) donates a free electron; a p-type impurity (3 outer electrons) creates a hole.

The p-n junction: the p-type and n-type regions meet directly, with the depletion layer straddling the junction itself (not shown as a separate gap between two disconnected blocks).

A transistor used as a switch: a small current into the base allows a much larger current to flow from collector to emitter, switching on the output device.

This topic is mainly conceptual (particle/band behaviour and circuit function) rather than formula-based. The key quantitative link is back to Ohm's law and the diode I-V characteristic covered in Resistance and Ohm's Law — a diode is non-ohmic, conducting well only above its threshold voltage in forward bias.

• ⚠ Common Mistakes
• Drawing the p-n junction with a physical gap between the p-type and n-type blocks — they meet directly; the depletion layer is a region within the joined crystal, not empty space.
• Thinking doping changes the overall charge of the semiconductor — n-type and p-type materials are both still electrically neutral overall; only the type of majority mobile charge carrier changes.
• Confusing 'holes' with actual particles — a hole is the absence of an electron, which behaves like a mobile positive charge carrier as electrons move to fill it, not a real particle.
• Overusing the term 'knee voltage' for every part of a diode's behaviour — 'threshold voltage' or 'turn-on voltage' is preferred except when specifically describing the sharp bend ('knee') in the I-V graph shape.
• Thinking a transistor amplifies by creating extra energy — it controls a larger current using energy from the collector-circuit supply, not from the small base signal itself.

Worked Example

A step-by-step calculation applying the central relationships in Semiconductors.

• Voltage across resistor = supply voltage − diode voltage = 5 − 0.7 = 4.3 V
• I = V/R = 4.3 ÷ 220 ≈ 0.0195 A ≈ 19.5 mA

Practical / Investigation

Experimental planning, measurement and evaluation for Semiconductors.

Experiment: Investigating the I-V characteristic of a diode/LED in forward and reverse bias

• Apparatus: diode or LED, variable power supply, resistor (to limit current), ammeter, voltmeter.
• Method: connect the diode in forward bias with a protective resistor in series; vary the supply voltage and record current and voltage across the diode; repeat with the diode connections reversed for reverse bias.
• Plot I against V; identify the threshold (turn-on) voltage as the point where current begins to rise sharply in forward bias, and confirm that current stays close to zero throughout reverse bias.
• Sources of error: without a protective resistor, current can rise very sharply and damage the diode/LED once conducting; meter resistance affects readings slightly.
• Safety: always include a suitably sized protective resistor in series with a diode/LED to limit current to a safe value.

• The current-limiting resistor is compulsory. Measure diode voltage with the voltmeter directly across the diode and current with the ammeter in series.
• Take closely spaced forward-bias readings near turn-on; reverse the diode for reverse-bias readings without exceeding its rating.
• Plot signed I against signed V and identify the rapid forward-current rise. The turn-on is a region, not an exact universal 0.700 V switch point.

Quick-Check Questions

Short retrieval questions covering the essential ideas in Semiconductors.

1. What is the majority charge carrier in a p-type semiconductor?

2. Why does doping not change the overall charge of a semiconductor?

3. What forms the depletion layer at a p-n junction?

4. In which bias direction does a diode conduct easily?

5. Describe the role of the base current in a transistor used as a switch.

Common mistakes and exam tips

Common misconceptions and exam-technique checks for Semiconductors.

• Avoid: Calling n-type material negatively charged and p-type material positively charged. Correct idea: Both are electrically neutral overall; n-type has electrons and p-type has holes as majority carriers.
• Avoid: Describing the depletion region as empty of all charge. Correct idea: It lacks mobile charge carriers but contains fixed ionised donor and acceptor atoms.
• Avoid: Reversing the bias needed for easy diode conduction. Correct idea: A diode conducts readily in forward bias after its characteristic turn-on region; reverse current is normally very small.

• Name the majority carrier when describing n-type or p-type material.
• State both the absence of mobile carriers and the presence of fixed ions in the depletion layer.
• Identify forward or reverse bias before describing diode behaviour.

Exam-style questions

Five exam-style questions with worked answers on Semiconductors.

End-of-unit revision

Formulae, definitions, experiments and mixed revision questions for Semiconductors.

• No new formulas — apply Ohm's law (V = IR) to the non-junction parts of a diode circuit
• Diode: negligible current below threshold voltage; current rises sharply above it (forward bias)

• Doping
• n-type vs p-type
• p-n junction and depletion layer
• Threshold (turn-on) voltage

• n-type and p-type doping (extra electron vs hole)
• p-n junction with the depletion layer touching both regions correctly
• Transistor as a switch circuit

• Investigating the I-V characteristic of a diode/LED in forward and reverse bias

• Drawing a gap in the p-n junction diagram
• Thinking n-type/p-type materials are charged overall
• Overusing 'knee voltage' instead of 'threshold voltage'
• Thinking a transistor creates energy rather than controlling it

1. Explain, in terms of charge carriers, why a p-type semiconductor conducts electricity.

2. A student says the depletion layer contains free charge carriers that have not yet crossed the junction. Correct this statement.

3. Explain why an LED only lights up when connected the correct way round in a circuit.

4. Describe one advantage of using a transistor rather than a simple mechanical switch to control a high-power device from a low-power sensor.

1. A p-type semiconductor has 'holes' (missing electrons) as its majority charge carrier; neighbouring electrons can hop into a hole, which effectively moves the hole in the opposite direction, allowing current to flow.

2. The depletion layer has no free (mobile) charge carriers at all — they have already diffused across and recombined, leaving only fixed, immobile charged ions behind.

3. An LED is a diode and only conducts (and therefore only emits light) in forward bias; connected in reverse bias, negligible current flows, so it does not light up.

4. A transistor has no moving parts to wear out, can switch much faster than a mechanical switch, and allows a very small sensor current to safely control a much larger current without the sensor circuit itself carrying that current.

The table below shows which content from this unit appears in the official State Examinations Commission Formulae and Tables book, and where.

Note: the new Leaving Certificate Physics specification confirms doping, n-type/p-type, p-n junction, depletion layer, and transistors as switches as the core content for this topic — all conceptual, with the diode I-V relationship as the main quantitative link.