Discrete semiconductor devices Revision Notes

Discrete Semiconductor Devices

Overview

• Discrete semiconductor devices form the backbone of modern electronic circuits.
• They provide controlled current flow, voltage regulation, sensing and switching.
• In the AQA A Level Physics specification the main devices are MOSFETs, Zener diodes, photodiodes and Hall effect sensors.

MOSFET (Metal‑Oxide‑Semiconductor Field‑Effect Transistor)

Terminals and Functions

• Gate (G) – small voltage controls a large drain‑source current.
• Drain (D) – main current exits the device.
• Source (S) – main current enters the device.
• Body/Substrate (B) – usually tied to the source.

Switching Behaviour

• Voltage‑controlled device: gate‑to‑source voltage determines ON/OFF state.
• Enhancement‑mode MOSFETs need a positive VGS (N‑channel) to turn ON.
• Depletion‑mode MOSFETs are normally ON; a negative VGS turns them OFF.
• Switching is almost instantaneous, ideal for digital logic and power switching.

Transfer Characteristics

• ID vs VGS curve shows how drain current varies with gate‑to‑source voltage.
• Key points:
• *Threshold voltage (VTH)* – minimum VGS to form a channel.
• *Linear region* – small VDS, ID increases linearly with VDS.
• *Saturation region* – large VDS, ID largely independent of VDS.
• Sketching the curve helps understand MOSFET behaviour as a switch or amplifier.

Simple Control Circuits

• Switching circuit – resistor limits gate current; MOSFET turns ON when VGS > VTH.
• Logic gate – two MOSFETs in series or parallel implement AND, OR, NOT.
• Power regulator – MOSFET acts as a variable resistor in a voltage regulator with feedback.

Zener Diode

Reverse Breakdown Operation

• In reverse bias, a Zener diode reaches a *critical breakdown voltage (Vz)*.
• Beyond Vz, it conducts large reverse current while maintaining a nearly constant voltage.
• Breakdown is due to Zener tunnelling or avalanche mechanisms.

Voltage Reference

• Stable reverse voltage makes a Zener an excellent *voltage reference*.
• Supplies a fixed voltage regardless of supply variations.

Simple Voltage Regulator Circuits

• Series regulator – resistor limits current; Zener clamps voltage across the load.
• Shunt regulator – Zener in parallel with load diverts excess current to keep voltage constant.
• Design points: choose resistor to limit current safely and ensure Zener’s power rating exceeds dissipated power.

Distinguishing from Ordinary Diodes

• Ordinary diodes conduct only in forward bias; Zener diodes conduct in reverse bias once Vz is reached.
• Reverse current of a Zener is much higher than that of a normal diode.
• Voltage across a Zener remains constant over a wide range of reverse currents.

Photodiode

Operation Modes

• Photoconductive mode – reverse biased; light increases conductivity, producing a photocurrent.
• Photovoltaic mode – zero bias; light generates a voltage (photovoltaic effect).

Light Intensity and Photocurrent

• Photocurrent (Iph) is *proportional* to incident light intensity.
• In reverse bias, Iph increases linearly with light until saturation.

I‑V Behaviour

• In reverse bias, the I‑V curve shows a small dark current and a larger photocurrent that rises with light.
• In forward bias, the diode behaves like a normal silicon diode; light has little effect.

Sensing Circuits

• Current‑to‑voltage converter – resistor converts photocurrent to a measurable voltage.
• Transimpedance amplifier – op‑amp with feedback resistor provides low‑noise, high‑gain conversion.
• Applications: light‑level meters, optical communication receivers, safety light switches.

Hall Effect Sensor

Hall Effect in a Current‑Carrying Conductor

• When a conductor carrying current I is placed in a magnetic field B perpendicular to the current, a transverse voltage (Hall voltage, VH) develops.
• VH is directly proportional to B and inversely proportional to charge carrier density and cross‑sectional area.

Dependence on Magnetic Field

• VH is *directly proportional* to magnetic field strength.
• Sign of VH indicates direction of magnetic field relative to current.

Measurement Applications

• Current sensing – measure VH to infer current through the conductor.
• Magnetic field measurement – Hall sensor detects presence and magnitude of a magnetic field.
• Position sensing – used in rotary encoders and linear position sensors.

Key Terms

• MOSFET
• Zener diode
• Photodiode
• Hall effect
• Transfer characteristics
• Reverse breakdown
• Voltage regulator
• Photocurrent
• Hall voltage
• Magnetic field

Exam Tips

1. Sketch transfer characteristics for MOSFETs and Zener diodes; label key points.
2. Check polarity when drawing Zener diode circuits; reverse bias is essential.
3. Identify MOSFET terminals correctly; source, drain, gate, body.
4. Explain the difference between photoconductive and photovoltaic modes.
5. Use the Hall effect equation to relate Hall voltage to magnetic field and current.

Common Mistakes

• Confusing MOSFET source and drain leads.
• Assuming a Zener diode behaves like a normal diode in reverse bias.
• Ignoring the need for reverse bias in photodiode operation.
• Misinterpreting the sign of Hall voltage.
• Forgetting that MOSFETs are voltage‑controlled devices, not current‑controlled.

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This revision note covers the core concepts and practical applications of discrete semiconductor devices required for the AQA A Level Physics specification. By mastering these topics, you’ll be well‑prepared for both the written and practical components of the exam.