Semiconductors and Diodes

Overview

This topic is about materials and devices whose electrical behaviour can be controlled.

In earlier circuit topics, many components were treated as fixed resistors or ideal sources. Semiconductors are different. Their resistance and current can change strongly when conditions change, such as temperature, light intensity, and applied voltage.

That controllability is why semiconductors are useful in:

• diodes
• rectifiers
• light-emitting diodes
• thermistors
• LDRs
• sensing and switching circuits

The main idea is not that semiconductors are simply “halfway between” conductors and insulators. The important point is that the number of mobile charge carriers in a semiconductor can change strongly, so its conductivity can be controlled.

This topic builds on Current Electricity Fundamentals and DC Circuits, and links forward to Alternating Current through rectification.

Big Picture

Semiconductor devices are useful because their electrical behaviour is controllable.

The common reasoning chain is:

$$\text{condition or applied p.d. changes}\Rightarrow \text{carrier availability or ease of conduction changes}\Rightarrow \text{current or resistance changes}\Rightarrow \text{useful circuit function}$$

For example:

• heating a thermistor changes its resistance
• shining light on an LDR changes its resistance
• changing the polarity of the p.d. across a diode changes whether it conducts

Caption: Semiconductor devices are useful because temperature, light, or applied p.d. can change carrier availability or ease of conduction, producing a useful change in resistance or current.

Learning Path

Study the topic in this order:

1. Compare conductors, semiconductors, and insulators.
2. Understand electrons and holes as mobile charge carriers.
3. Explain why semiconductor resistance changes with temperature and light.
4. Treat a diode as a non-ohmic, one-way conducting component.
5. Use the diode I-V characteristic to reason about forward and reverse bias.
6. Apply diode action to rectification and electronic sensing.

Detailed branch notes:

• Thermistors and LDRs
• Rectification

Core Ideas

The topic rests on five linked ideas:

• A semiconductor has conductivity between that of a conductor and an insulator.
• Current in a semiconductor is carried by electrons and holes.
• Increasing the number of mobile charge carriers usually lowers resistance.
• A diode conducts much more easily in one direction than the other.
• A diode is non-ohmic because its current is not proportional to p.d.

Conductors, Semiconductors, and Insulators

Material Type	Conductivity	Typical Behaviour
Conductor	high	current flows easily
Semiconductor	intermediate and controllable	conductivity changes strongly with conditions
Insulator	very low	current flow is negligible under normal conditions

Examples:

Type	Examples
Conductors	copper, aluminium, graphite
Semiconductors	silicon, germanium
Insulators	rubber, plastic, glass

In a metal, there are already many mobile electrons available for conduction. In a semiconductor, there are fewer mobile charge carriers under ordinary conditions, but additional carriers can become available when the material is heated or illuminated.

Charge Carriers in Semiconductors

Electric current requires mobile charge carriers.

In a semiconductor, two carrier types are useful:

• electrons, which are negatively charged
• holes, which behave like positive mobile charge carriers

A hole is the absence of an electron in a bonding structure. At H2 level, it is usually enough to treat a hole as an effective positive carrier that can contribute to conventional current.

Why More Carriers Mean Lower Resistance

For a given p.d., a larger number of mobile charge carriers allows more current to flow.

So the qualitative chain is:

$$\text{more mobile carriers}\Rightarrow \text{larger current for the same p.d.}\Rightarrow \text{lower resistance}$$

This is the key difference between many semiconductor devices and ordinary metallic resistors.

Temperature Behaviour

Metals

For a metallic conductor:

• increasing temperature usually increases resistance
• lattice ions vibrate more strongly
• conduction electrons collide more frequently with the lattice

So, for many metals:

$$T\uparrow \Rightarrow R\uparrow$$

Semiconductors

For a semiconductor:

• increasing temperature can create many more mobile charge carriers
• this carrier increase can dominate over the increased collision effect
• resistance often decreases as temperature increases

So, for a typical semiconductor sensor such as an NTC thermistor:

$$T\uparrow \Rightarrow R\downarrow$$

This is why thermistors are useful in temperature sensing. See Thermistors and LDRs.

Effect of Light

Light can also increase conductivity in some semiconductor devices.

When light is absorbed, it can provide energy that increases the number of mobile charge carriers.

For an LDR:

$$\text{light intensity}\uparrow \Rightarrow R\downarrow$$

This makes LDRs useful in automatic lighting and light-sensing circuits. The circuit behaviour still depends on where the output voltage is measured in the potential divider.

Diodes

Scope Note

At this level, the internal construction of the diode is less important than its observable circuit behaviour: how current changes with applied p.d. and how the diode affects current direction in a circuit.

Do not expand this topic into detailed internal microscopic physics unless a question explicitly provides that model.

Definition

A diode is a semiconductor component that allows current to pass mainly in one direction.

It is used for:

• rectification
• signal control
• switching
• circuit protection
• light emission in LEDs

The diode is not just a resistor with a large or small resistance. Its resistance depends strongly on the direction and size of the applied p.d.

What Makes a Diode Different?

A diode is different from an ordinary resistor because its current depends strongly on the direction of the applied p.d.

In the simple H2 circuit model:

• in forward bias, the diode can conduct after a threshold region
• in reverse bias, the current is negligible
• the I-V graph is asymmetric and non-linear

Therefore, a diode is not an ohmic resistor. Its resistance is not constant.

Forward Bias and Reverse Bias

Forward Bias

A diode is forward biased when it is connected in the conducting direction.

In forward bias:

• current is very small at first
• after a turn-on or threshold region, current rises rapidly
• the diode conducts strongly

For a silicon diode, a typical turn-on p.d. is often treated as about $0.6\text{ V}$ to $0.7\text{ V}$ in simple circuit contexts, but use the value given in the question.

Reverse Bias

A diode is reverse biased when it is connected in the blocking direction.

In reverse bias:

• current is very small in the simple H2 model
• the diode is often treated as not conducting

Unless a question explicitly states otherwise, do not assume a large reverse current.

Diode I-V Characteristic

The diode I-V characteristic is asymmetric.

For the usual axes:

• horizontal axis: p.d. $V$
• vertical axis: current $I$

Caption: A diode has an asymmetric, non-linear I-V characteristic. In reverse bias the current is negligible in the simple model; in forward bias the current rises rapidly after the threshold region.

Forward Region

At small forward p.d.:

• current is small
• the diode is not yet conducting strongly

Beyond the threshold region:

• current increases rapidly for a small further increase in p.d.

Reverse Region

For reverse p.d. in the simple operating range:

• current is negligible
• the diode behaves approximately like an open circuit

Non-Ohmic Behaviour

An ohmic conductor has:

$$I\propto V$$

provided temperature is constant.

A diode does not satisfy this condition because the I-V graph is not a straight line through the origin.

Do not write:

$$V=IR$$

as if $R$ were a constant diode resistance. If resistance is discussed at all, it is changing with operating point.

Rectification Overview

Rectification means converting alternating current into current that flows in one direction only.

A diode can do this because:

• during one half-cycle, it is forward biased and conducts
• during the opposite half-cycle, it is reverse biased and blocks

This produces pulsating direct current, not perfectly steady DC.

For the detailed half-wave and full-wave discussion, see Rectification.

Applications

Rectifiers

Diodes are used in power supplies to convert AC into pulsating DC.

LEDs

A light-emitting diode emits light when forward biased. Energy is transferred when charge carriers recombine inside the semiconductor.

Sensors

Thermistors and LDRs are semiconductor devices whose resistance changes with temperature or light intensity.

They are usually used with potential dividers so that a resistance change becomes a measurable voltage change.

Protection and Switching

Diodes can be used to prevent current flowing in an unwanted direction or to choose which parts of a circuit conduct.

Concept Checkpoints

After studying this hub, you should be able to answer:

• Why can a semiconductor’s resistance decrease when temperature increases?
• Why does an LDR have lower resistance in brighter light?
• What is meant by a hole in a semiconductor?
• Why is a diode non-ohmic?
• What does forward bias mean?
• What does reverse bias mean?
• Why does a diode allow half-wave rectification?
• Why is rectified output still not the same as perfectly steady battery DC?

Exam Relevance

Exam questions in this topic usually test qualitative reasoning rather than long calculations.

Common task types include:

• comparing semiconductor and metal temperature behaviour
• explaining why thermistor or LDR resistance changes
• identifying whether a diode is forward biased or reverse biased
• sketching or interpreting a diode I-V graph
• explaining why a diode is non-ohmic
• predicting the output of a rectifier circuit
• tracking how a sensor resistance change affects a potential-divider output

For circuit questions, always combine the semiconductor device behaviour with the circuit rule being used. For example, knowing that an LDR resistance decreases in brighter light is only the first step; the output voltage also depends on where the output is measured.

Diode Question Strategy

For diode circuit questions, identify:

• the direction in which conventional current would try to flow
• whether the diode is forward biased or reverse biased
• whether the question uses an ideal diode model or a threshold-voltage model
• where the output voltage is measured

In an ideal diode model, a forward-biased diode is treated as conducting with negligible p.d. across it. In a threshold-voltage model, the diode only conducts significantly after the turn-on p.d. is reached.

Common Exam Traps

Treating a Semiconductor Like a Metal

For metals, higher temperature usually increases resistance.

For many semiconductor devices, higher temperature or illumination can increase carrier density and reduce resistance.

Saying Holes Are Protons Moving Through the Circuit

Holes are not protons moving through the material. They are effective positive mobile carriers caused by missing electrons in the semiconductor structure.

Assuming a Diode Obeys Ohm’s Law

A diode is non-ohmic. Its current is not proportional to p.d.

Reversing Forward and Reverse Bias

Forward bias means significant conduction after the threshold region.

Reverse bias means negligible current in the simple model.

Calling Rectified Output Constant DC

Rectified output is unidirectional but usually still varies with time. It is pulsating DC unless smoothed.

Formula and Graph Summary

Idea	What to Remember
Ohmic conductor	$I\propto V$ only if temperature and resistance are constant
Diode	non-ohmic, asymmetric I-V characteristic
Forward bias	current rises rapidly after threshold region
Reverse bias	current is negligible in the simple operating range
NTC thermistor	$T\uparrow \Rightarrow R\downarrow$
LDR	light intensity $\uparrow \Rightarrow R\downarrow$

Worked Reasoning Example

Example 1: Interpreting a Diode I-V Graph

A diode I-V graph shows almost no current for negative p.d., but a rapidly increasing current after a positive threshold region.

This means:

• the negative-p.d. side represents reverse bias
• the positive-p.d. side represents forward bias
• the diode is non-ohmic because the graph is not a straight line through the origin

The steep forward region does not mean the diode has a fixed small resistance. It means the operating resistance changes with p.d. and current.

Example 2: Sensor Divider Reasoning

An LDR is placed in a potential divider and the light intensity increases.

Step 1: Identify the component trend.

$$\text{light intensity}\uparrow \Rightarrow R_{\text{LDR}}\downarrow$$

Step 2: Do not immediately guess the output voltage.

The output depends on whether it is measured across the LDR or across the other resistor.

Step 3: Use voltage sharing.

The component with smaller resistance takes a smaller share of the supply p.d. in a series potential divider.

So, if the output is across the LDR, the output voltage decreases. If the output is across the fixed resistor, the output voltage increases.

Links

• Thermistors and LDRs
• Rectification
• Current Electricity Fundamentals
• DC Circuits
• Alternating Current
• Potential Divider

Summary

Semiconductors are useful because their carrier density, resistance, and current can be controlled. Temperature and light can increase the number of mobile carriers, reducing resistance in devices such as thermistors and LDRs.

A diode is a semiconductor device with asymmetric conduction. In forward bias it conducts strongly after a threshold region; in reverse bias it conducts negligibly in the simple model. This non-ohmic, one-way behaviour makes diodes useful for rectification, switching, sensing, and electronic control.