Semiconductors and Diodes
Overview
Semiconductors are materials whose electrical conductivity lies between that of conductors and insulators. Their resistance can change significantly with:
- temperature
- light intensity
- impurities added to the material
This makes semiconductors useful in sensing, control, and electronic devices.
This topic builds on ideas from Current Electricity Fundamentals and supports later applications in DC Circuits and Alternating Current.
Core Ideas
- semiconductor conductivity lies between that of conductors and insulators
- semiconductor resistance can change strongly with temperature, light, and carrier density
- a diode conducts mainly in one direction
- a diode is non-ohmic and has a threshold region in forward bias
- one-way conduction allows rectification
Conductors vs Semiconductors vs Insulators
| Material Type | Conductivity | Typical Behaviour |
|---|---|---|
| Conductor | high | current flows easily |
| Semiconductor | intermediate | conductivity changes with conditions |
| Insulator | very low | current flow is negligible |
Examples
- conductors: copper, aluminium
- semiconductors: silicon, germanium
- insulators: rubber, plastic, glass
Why Semiconductors Are Useful
Unlike metals, semiconductor resistance can be controlled more strongly by the surroundings and by the material structure itself.
Examples:
- temperature sensors
- light sensors
- diodes
- switching circuits
- rectifiers
Charge Carriers
Electric current in semiconductors is carried by mobile charge carriers.
Electrons
Negatively charged particles that move through the material.
Holes
A hole is the absence of an electron in the lattice and behaves like an effective positive charge carrier.
At this level, holes are best treated simply as another mobile carrier contributing to current.
Increased Carrier Density
When temperature rises or light falls on a semiconductor:
- more charge carriers become available
- conductivity increases
- resistance decreases
Temperature Behaviour
Metals
For metallic conductors:
- resistance generally increases as temperature rises
Reason:
- stronger lattice vibration causes more collisions with electrons
Semiconductors
For semiconductors:
- resistance generally decreases as temperature rises
Reason:
- more mobile charge carriers become available
Comparison
| Material | Effect of Increasing Temperature |
|---|---|
| Metal | resistance increases |
| Semiconductor | resistance decreases |
Diodes
Definition
A diode is a semiconductor device that allows current to pass mainly in one direction.
It is commonly used for:
- rectification
- protection circuits
- switching
- signal control
One-Way Conduction
A diode conducts well in one direction but poorly in the opposite direction.
This makes it a non-ohmic component.
Forward Bias
A diode is forward biased when connected in its conducting direction.
Effects:
- current is small at first
- after sufficient voltage, current rises rapidly
Reverse Bias
A diode is reverse biased when connected opposite to its conducting direction.
Effects:
- current is very small in the simple model
Threshold / Turn-On Voltage
In forward bias, the diode usually requires a minimum voltage before current rises significantly.
This is called:
- threshold voltage
- turn-on voltage
Diode I-V Characteristics
Axes Convention
- horizontal axis: voltage
- vertical axis: current
Forward Region
- little current at low forward voltage
- after threshold, current increases rapidly
Reverse Region
- very small current in reverse bias
Non-Ohmic Behaviour
The diode does not obey:
So its resistance is not constant.
Comparison with Resistor Graph
| Component | I-V Graph |
|---|---|
| Resistor | straight line through origin |
| Diode | asymmetric curved graph |
Rectification Overview
A diode can convert alternating current into pulsating direct current because it allows current in only one direction.
Half-Wave Idea
One half of the AC cycle passes while the opposite half is blocked.
See Rectification.
Applications
Rectifiers
Used in power supplies to convert AC to DC.
LEDs
Light-emitting diodes emit light when forward biased.
Protection Circuits
Prevent current flowing in the wrong direction.
Sensors
Used together with thermistors or LDR circuits.
Switching
Used where one-way current control is needed.
Common Exam Traps
1. Reversing Diode Direction
Check symbol orientation carefully.
2. Confusing Forward and Reverse Bias
Forward bias allows significant current.
Reverse bias gives very small current.
3. Assuming Diode Obeys Ohm’s Law
A diode is non-ohmic.
4. Misreading I-V Graph
Remember current rises sharply only after threshold in forward bias.
5. Treating Reverse Current as Large
In normal operation reverse current is tiny.
Summary Comparison Table
| Feature | Metal Resistor | Semiconductor Diode |
|---|---|---|
| Ohmic? | usually yes at constant temperature | no |
| Current Direction | both directions equally | mainly one direction |
| Resistance Change | usually small | strongly depends on bias |
| Main Use | control current / heating | rectification / switching |
Quick Revision Points
- Semiconductors have conductivity between conductors and insulators.
- Their resistance changes strongly with temperature and light.
- Diodes conduct mainly in one direction.
- Forward bias allows current after threshold.
- Reverse bias gives very small current.
- Diodes are non-ohmic.
- Diodes are used for rectification and switching.
Exam Relevance
Students should be able to:
- interpret the qualitative I-V characteristic of a diode
- distinguish forward bias from reverse bias
- explain why a diode is non-ohmic
- relate one-way diode conduction to half-wave rectification
- compare semiconductor temperature behaviour with metallic conductors