Modern Physics
Semiconductors
5th Year · 6th Year (Leaving Cert)
- ✓By the end of this lesson students will be able to describe the properties of intrinsic, p-type, and n-type semiconductors.
- ✓By the end of this lesson students will be able to explain the formation and operation of a p-n junction diode, including forward and reverse bias.
- ✓By the end of this lesson students will be able to explain the principle of operation of a Light Emitting Diode (LED).
- ✓By the end of this lesson students will be able to explain the principle of operation of a photovoltaic cell (solar cell).
- ✓By the end of this lesson students will be able to distinguish between the functions and energy conversions of an LED and a photovoltaic cell.
Key concepts
A pure semiconductor material (e.g., silicon or germanium) that has equal numbers of free electrons and holes. Its conductivity is low at room temperature but increases significantly with increasing temperature as more electron-hole pairs are generated.
The process of intentionally adding a small amount of impurity atoms (dopants) to an intrinsic semiconductor to increase its electrical conductivity and create either an excess of free electrons or holes.
An extrinsic semiconductor formed by doping an intrinsic semiconductor with pentavalent impurities (e.g., phosphorus, arsenic, antimony). These impurities have five valence electrons; four form covalent bonds with the semiconductor atoms, and the fifth electron is loosely bound and becomes a free electron. In n-type semiconductors, free electrons are the majority charge carriers, and holes are the minority charge carriers.
An extrinsic semiconductor formed by doping an intrinsic semiconductor with trivalent impurities (e.g., boron, aluminium, gallium). These impurities have three valence electrons; they form three covalent bonds with the semiconductor atoms, leaving one bond incomplete, which creates a 'hole'. In p-type semiconductors, holes are the majority charge carriers, and free electrons are the minority charge carriers.
A device formed by joining a p-type semiconductor to an n-type semiconductor. At the junction, electrons from the n-side diffuse into the p-side, and holes from the p-side diffuse into the n-side. This diffusion creates a region around the junction called the depletion layer, which is devoid of free charge carriers and contains fixed positive and negative ions, establishing a potential barrier.
When the positive terminal of a DC power supply is connected to the p-type material and the negative terminal to the n-type material of a p-n junction diode. This connection reduces the width of the depletion layer and lowers the potential barrier, allowing a significant current to flow through the diode.
When the negative terminal of a DC power supply is connected to the p-type material and the positive terminal to the n-type material of a p-n junction diode. This connection increases the width of the depletion layer and raises the potential barrier, effectively preventing current flow (ideally, only a very small leakage current flows).
A p-n junction diode that emits light when it is forward biased. When electrons from the n-side and holes from the p-side recombine at the junction, they release energy in the form of photons (light). The colour of the emitted light depends on the semiconductor material used.
A p-n junction device that converts light energy directly into electrical energy. When photons of light strike the p-n junction, they generate electron-hole pairs. The electric field within the depletion layer separates these charge carriers, driving the electrons to the n-side and holes to the p-side, thereby creating a potential difference (voltage) across the cell.
Key facts to remember
- 1Semiconductors have electrical conductivity between that of conductors and insulators.
- 2Doping is the process of adding impurities to an intrinsic semiconductor to increase its conductivity.
- 3n-type semiconductors have free electrons as majority carriers (from pentavalent dopants).
- 4p-type semiconductors have holes as majority carriers (from trivalent dopants).
- 5A p-n junction forms a depletion layer and a potential barrier due to charge carrier diffusion.
- 6Forward bias reduces the depletion layer and allows current flow in a diode.
- 7Reverse bias increases the depletion layer and blocks current flow in a diode.
- 8LEDs convert electrical energy into light energy, while photovoltaic cells convert light energy into electrical energy.
Worked examples
Example 1
Explain, with the aid of a labelled diagram, how a p-n junction diode can be forward biased.
Answer
A p-n junction diode is forward biased when the positive terminal of a DC power supply is connected to the p-type region and the negative terminal to the n-type region. This external voltage pushes holes towards the junction from the p-side and electrons towards the junction from the n-side. This effectively reduces the width of the depletion layer and lowers the potential barrier, allowing majority charge carriers to cross the junction and a significant current to flow through the diode. [Diagram would show a p-n junction with a battery connected: positive to p-side, negative to n-side. An arrow indicating current flow from p to n. Labels for p-type, n-type, depletion layer, and battery terminals.]
A clear, labelled diagram is essential for full marks in such questions.
Example 2
Describe the difference in operation between a Light Emitting Diode (LED) and a photovoltaic cell.
Answer
A Light Emitting Diode (LED) is a p-n junction device that converts electrical energy into light energy. When forward biased, electrons from the n-type material and holes from the p-type material recombine at the junction, releasing energy in the form of photons, which we perceive as light. In contrast, a photovoltaic cell (solar cell) is also a p-n junction device, but it converts light energy directly into electrical energy. When photons of light strike the junction, they excite electrons, creating electron-hole pairs. The built-in electric field of the p-n junction then separates these charge carriers, driving electrons to the n-side and holes to the p-side, thereby generating a voltage and current when connected to an external circuit. Therefore, the key difference is their function and energy conversion: an LED emits light when supplied with electricity, whereas a photovoltaic cell generates electricity when exposed to light.
Focus on the energy transformations and the direction of energy flow for each device.
Example 3
A pure silicon crystal is doped with a small amount of boron. (i) What type of semiconductor is formed? (ii) Name the majority charge carriers in this semiconductor. (iii) Explain how the boron atoms contribute to the conductivity of the silicon.
Answer
(i) When pure silicon is doped with boron, a p-type semiconductor is formed. (ii) The majority charge carriers in this semiconductor are holes. (iii) Boron is a trivalent impurity, meaning it has three valence electrons. When a boron atom replaces a silicon atom in the crystal lattice, it forms three covalent bonds with neighbouring silicon atoms. However, it needs a fourth electron to complete its outer shell and form a fourth covalent bond. This creates a 'hole' in the crystal structure. These holes can readily accept electrons from adjacent silicon atoms, effectively allowing the holes to move through the material, thereby increasing the overall electrical conductivity of the silicon.
Remember that trivalent impurities create holes, leading to p-type semiconductors, while pentavalent impurities create free electrons, leading to n-type semiconductors.
Common mistakes
- ✗Confusing the majority and minority charge carriers in p-type and n-type semiconductors.
- ✗Incorrectly identifying the type of dopant (trivalent vs. pentavalent) for p-type and n-type semiconductors.
- ✗Mixing up the connections for forward bias and reverse bias in a p-n junction diode.
- ✗Failing to understand that the depletion layer is a region devoid of *free* charge carriers, not all charge carriers.
- ✗Interchanging the functions or energy conversions of Light Emitting Diodes (LEDs) and photovoltaic cells.
Exam tips
- ★Practice drawing and labelling diagrams for p-n junctions, forward bias, and reverse bias, as these are frequently examined.
- ★Clearly understand the role of doping and how it creates either an excess of electrons (n-type) or holes (p-type).
- ★Memorise the energy conversions for LEDs (electrical to light) and photovoltaic cells (light to electrical) and be able to explain the underlying physics.
- ★Use precise terminology, such as 'majority carriers', 'minority carriers', 'depletion layer', and 'potential barrier', in your explanations.
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