Welcome to Topic 5.8: Semiconductors
Have you ever wondered how a modern hospital pulse oximeter reads your oxygen levels using light, or how a digital thermometer takes an accurate body temperature in seconds? The secret behind these lifesaving clinical instruments is the semiconductor.
In this chapter of AS 5: Material Science, we will explore how semiconductors work at an atomic level, how we can modify them to carry electrical current, and how they form the backbone of diagnostic medical technology. Don't worry if physics and chemistry concepts feel daunting—we will break down every idea step by step!
1. What is a Semiconductor?
Materials around us are generally classified by how easily they conduct electricity:
• Conductors (e.g. copper, aluminium): Allow electric current to pass through freely.
• Insulators (e.g. rubber, glass): Prevent the flow of electric current.
• Semiconductors (e.g. silicon, germanium): Materials whose electrical conductivity lies between that of a good conductor and an insulator.
A pure semiconductor (such as pure silicon or germanium, which belong to Group IV of the periodic table) is known as an intrinsic semiconductor.
Band Theory Made Simple
To understand why semiconductors behave the way they do, we look at energy bands in solid materials:
1. The Valence Band: This is the highest range of electron energy levels that are completely filled with electrons at absolute zero (\(0\text{ K}\)). Think of this as the "ground floor" where electrons normally reside.
2. The Conduction Band: This is the higher energy band where electrons are free to move throughout the crystal lattice. When electrons enter this band, they can carry an electric current.
3. The Forbidden Energy Gap (Bandgap): This is the energy separation between the valence band and the conduction band. Electrons cannot exist inside this gap; they must gain enough energy to jump completely across it.
Comparing Band Structures
• Conductors: The conduction band and valence band overlap, or there is zero bandgap. Electrons move freely into the conduction band without needing external energy.
• Insulators: The forbidden energy gap is very large. Under normal conditions, electrons cannot gain enough energy to jump across, so no current flows.
• Semiconductors: The forbidden energy gap is relatively narrow. At absolute zero, all electrons stay in the valence band (acting as an insulator). However, even at room temperature, some electrons gain enough thermal energy to jump across the narrow gap into the conduction band.
The Effect of Temperature on Intrinsic Semiconductors
How does temperature affect electrical flow? Let's compare metals to semiconductors:
• In a metallic conductor, increasing temperature causes metal ions to vibrate more vigorously, colliding with flowing electrons and increasing electrical resistance.
• In an intrinsic semiconductor, increasing temperature provides thermal energy that excites more valence electrons across the narrow forbidden gap into the conduction band. When an electron jumps up, it leaves behind a vacant space called a hole in the valence band (which behaves like a positive charge carrier). Because more charge carriers are created, electrical conductivity increases and electrical resistance decreases.
Key Rule: Semiconductors have a negative temperature coefficient of resistance (as temperature goes up, resistance goes down).
Section Takeaway: Pure semiconductors have a narrow bandgap. Heating them excites electrons across the gap into the conduction band, creating electron-hole pairs and increasing electrical conductivity.
2. Doping: Creating Extrinsic Semiconductors
Pure (intrinsic) semiconductors do not conduct very much electricity on their own at room temperature. To boost their conductivity and control their behaviour, scientists use a process called doping.
Definition: Doping is the deliberate introduction of specific impurity atoms into an intrinsic semiconductor lattice to dramatically alter its electrical properties. A doped semiconductor is called an extrinsic semiconductor.
N-Type Semiconductors (Negative-Type)
• Dopant Used: Group V elements (pentavalent atoms with 5 valence electrons, e.g. Phosphorus, Arsenic, Antimony).
• How it works: Silicon atoms have 4 valence electrons and form covalent bonds with 4 neighbours. When a Group V atom replaces a silicon atom in the lattice, 4 of its valence electrons bond with adjacent silicon atoms, leaving one extra electron free.
• Donor Impurities: Because Group V atoms donate extra electrons that are easily promoted into the conduction band, they are called donor impurities.
• Charge Carriers:
– Majority charge carriers: Electrons (negative)
– Minority charge carriers: Holes (positive)
P-Type Semiconductors (Positive-Type)
• Dopant Used: Group III elements (trivalent atoms with 3 valence electrons, e.g. Boron, Aluminium, Gallium, Indium).
• How it works: When a Group III atom replaces a silicon atom, it only has 3 valence electrons. It cannot complete the fourth covalent bond, leaving an empty spot or hole in the valence band.
• Acceptor Impurities: This hole readily accepts an electron from an adjacent bond, so Group III atoms are called acceptor impurities.
• Charge Carriers:
– Majority charge carriers: Holes (positive)
– Minority charge carriers: Electrons (negative)
Crucial Concept: Overall Charge Neutrality
Never lose marks on this classic exam trap!
Even though an n-type material has free electrons and a p-type material has free holes, both n-type and p-type semiconductors remain electrically neutral overall. Every dopant atom added to the lattice has an equal number of protons in its nucleus and electrons surrounding it. No net electric charge is added to the material.
Memory Trick:
• N-type = Group 5 (V) = Negative extra electron.
• P-type = Group 3 (III) = Positive Prominent hole.
Section Takeaway: Doping silicon (Group IV) with Group V elements creates n-type semiconductors (majority carriers: electrons). Doping with Group III elements creates p-type semiconductors (majority carriers: holes). Both types remain electrically neutral overall.
3. The p–n Junction and Diodes
When a p-type semiconductor and an n-type semiconductor are joined together within the same crystal lattice, they form a p–n junction. This junction is the foundational building block of diodes, LEDs, and medical sensors.
Formation of the Depletion Layer & Barrier Potential
1. Diffusion: Right after the junction forms, mobile free electrons from the n-side diffuse across into the p-side, while holes from the p-side diffuse across into the n-side.
2. Recombination: Near the junction boundary, these electrons and holes combine and neutralise each other.
3. Depletion Layer: This creates a narrow region around the junction that is completely empty (depleted) of mobile charge carriers.
4. Fixed Ions: Behind the boundary, immobile positive donor ions (\(+\)) remain on the n-side and immobile negative acceptor ions (\(-\)) remain on the p-side.
5. Barrier Potential: These fixed charges set up an internal electric field and a potential difference across the junction, known as the barrier potential (approximately \(0.6\text{–}0.7\text{ V}\) for silicon). This potential opposes any further movement of majority carriers across the junction.
Forward Bias vs. Reverse Bias
To control the flow of current through a p–n junction, an external direct current (DC) voltage is applied:
A. Forward Bias (Current Flows):
• Connect the positive terminal of the DC supply to the p-side and the negative terminal to the n-side.
• The applied external voltage directly opposes and overcomes the internal barrier potential.
• The depletion layer becomes narrower.
• Once the applied voltage exceeds the threshold/turn-on voltage (approx. \(0.6\text{–}0.7\text{ V}\)), a large electric current flows easily across the junction.
B. Reverse Bias (No Current Flows):
• Connect the positive terminal of the DC supply to the n-side and the negative terminal to the p-side.
• The external voltage pulls electrons away from the junction on the n-side and holes away on the p-side.
• The depletion layer becomes wider, and the barrier potential increases.
• Majority charge carriers cannot cross. Only a tiny, negligible leakage current flows due to minority charge carriers.
4. Semiconductor Applications in Health and Life Sciences
Semiconductor technology plays a vital role in clinical diagnostics, patient monitoring, and medical instrumentation:
1. Rectification in Power Supplies:
A single p–n junction diode allows current to flow in only one direction. This property is used in rectifier circuits to convert alternating current (AC) from mains electricity into smooth direct current (DC) required to safely power sensitive biomedical equipment (e.g. ECG monitors, infusion pumps).
2. Light Emitting Diodes (LEDs) & Photodiodes:
• LEDs: Emit light of precise wavelengths when forward-biased as electrons recombine with holes.
• Photodiodes: Semiconductor devices that detect light and convert it into an electrical signal.
• Clinical Uses: Combined in pulse oximeters (which emit red and infrared light through a patient's fingertip to measure blood oxygen saturation) and in spectrophotometers (used in biochemistry labs to measure the concentration of substances in blood or urine samples).
3. Thermistors (NTC):
Negative Temperature Coefficient (NTC) thermistors are semiconductor components whose resistance drops sharply and predictably as temperature rises. They are widely used in medical electronic thermometers and neonatal incubators for rapid, high-precision body temperature monitoring.
Section Takeaway: Forward-biasing a p–n junction allows large current flow by shrinking the depletion layer; reverse-biasing widens it. These properties enable essential medical tools like rectifiers, pulse oximeters, and clinical thermistors.
5. Summary & Examiner Pitfalls Checklist
Keep these common exam misconceptions in mind when answering questions on Unit AS 5:
• Pitfall 1 (Bandgap sizes): Remember that semiconductors have a smaller bandgap than insulators, but conductors have no gap (or overlapping bands).
• Pitfall 2 (Charge neutrality): Both n-type and p-type semiconductors have a net electric charge of zero. Do not say n-type is negatively charged or p-type is positively charged.
• Pitfall 3 (Doping groups): Group IV is silicon/germanium. Group V (donor) makes n-type. Group III (acceptor) makes p-type. Do not swap Group III and Group V.
• Pitfall 4 (Temperature vs Resistance): In metals, resistance goes up with heat. In intrinsic semiconductors, resistance goes down with heat because more charge carriers jump into the conduction band.
Quick Review Summary
• Intrinsic semiconductor: Pure semiconductor with narrow forbidden bandgap.
• N-type: Doped with Group V (donor); majority carriers = electrons.
• P-type: Doped with Group III (acceptor); majority carriers = holes.
• Depletion layer: Formed at p–n junction by carrier recombination, creating fixed ions and a barrier potential (\(\approx 0.6\text{–}0.7\text{ V}\)).
• Forward bias: Positive to p-side, negative to n-side; narrows depletion layer; allows current.
• Reverse bias: Positive to n-side, negative to p-side; widens depletion layer; blocks current.
• Key health applications: AC to DC rectification, pulse oximetry (LEDs & photodiodes), precision temperature sensing (NTC thermistors).