What is a Semiconductor?: Technical Interview Question Guide

Chase Na

26 May 2025 — 9 min read

Imagine you're in the final interview at your dream company and they ask: "What is a semiconductor? Please explain in detail"

1. Band Theory Explanation

A semiconductor is a solid material with electrical conductivity between that of a conductor (metal) and an insulator. From a band structure perspective, semiconductors are characterized by an energy gap (band gap) of approximately 0.5-3 eV between the conduction band and valence band.

reference: https://www.geeksforgeeks.org/fermi-energy/

At absolute zero (0K), semiconductors behave like insulators with a completely filled valence band and empty conduction band. However, at room temperature, the band gap is small enough that thermal energy can excite some carriers (electrons or holes) to higher energy states.

Key Process: Electrons excited to the conduction band and the holes left behind in the valence band both contribute to electrical conduction.

Summary: While semiconductors don't conduct electricity as well as metals, their conductivity can be easily controlled through external energy or doping, which dramatically changes the carrier concentration.

Generally, semiconductors are materials that have a small band gap at room temperature with the Fermi level positioned within the band gap region.

2. Intrinsic vs. Extrinsic Semiconductors

2.1 Intrinsic Semiconductors

Intrinsic semiconductors are pure semiconductors without impurities. In a silicon crystal, when an electron is excited to the conduction band, a hole is created in the valence band in a 1:1 ratio.

At equilibrium, the electron concentration (n) equals the hole concentration (p), known as the intrinsic carrier concentration. This intrinsic carrier concentration is very small at room temperature.

Pure silicon has low conductivity (resistivity ~10⁵ Ω·cm) due to the minimal number of free electrons and holes.

2.2 Extrinsic Semiconductors

To achieve higher conductivity, small amounts of impurities are added through doping to create extrinsic semiconductors. Doping significantly alters either electron or hole concentrations.

n-type Doping:

Adding pentavalent donor atoms (P, As, Sb) to silicon (tetravalent)
Donors easily ionize, providing electrons to the conduction band
Electrons become majority carriers
Fermi level shifts upward toward the conduction band

p-type Doping:

Adding trivalent acceptor atoms (B, Al) creates electron deficiency
Acceptors capture electrons from neighboring atoms, leaving holes
Holes become majority carriers
Fermi level shifts downward toward the valence band

At room temperature, impurities are typically fully ionized, so the majority carrier concentration approximates the doping concentration, resulting in significantly improved conductivity compared to intrinsic semiconductors.

Industrial Context: Commercial semiconductors are typically extrinsic semiconductors with completed doping.

3. Temperature Effects on Semiconductors

Temperature changes significantly affect carrier concentration and mobility in semiconductors.

Intrinsic Excitation: In pure semiconductors, increasing temperature dramatically increases thermal excitation, promoting more electrons to the conduction band and creating more holes.

Freeze-Out: At low temperatures, even doped semiconductors experience insufficient ionization of donors or acceptors, causing most free carriers to disappear.

Extrinsic Range: In intermediate temperature ranges, donors and acceptors are fully ionized, making majority carrier density approximately equal to the doping concentration. While carrier count remains relatively constant with temperature, lattice vibrations (phonons) increase, reducing mobility and slightly decreasing conductivity.

Intrinsic Takeover: At higher temperatures, intrinsic carriers exceed the doping concentration. The semiconductor essentially behaves intrinsically, with conductivity increasing sharply with temperature. This causes the rapid increase in leakage current at high temperatures.

Commercial Standard: Semiconductor operating conditions are typically designed for -40°C to 125°C.

4. Why Silicon Became the Dominant Semiconductor Material

Crystal Structure & Purity

Silicon crystallizes in a diamond cubic structure
Large-diameter, high-purity single crystals can be grown using processes like Czochralski
Wide-area wafers are more easily obtainable compared to other elements
Silicon is abundant in Earth's crust, making it cost-effective

Band Gap & Electrical Properties

Silicon has an indirect band gap of ~1.12 eV (at 300K)
Intrinsic carrier concentration is ~10¹⁰ cm⁻³ at room temperature
Electron mobility: ~1400 cm²/V·s
Hole mobility: ~600 cm²/V·s
These properties are sufficient for most logic device implementations

SiO₂ Formation

High-temperature oxidation of silicon forms silicon dioxide (SiO₂)
SiO₂ has excellent insulating properties with minimal semiconductor-oxide interface defects
Stable oxidation process with minimal degradation maintains high yield
This became fundamental for MOS structure development and high-density integration
Other semiconductors (like GaAs) cannot form such high-quality oxide layers cost-effectively

Process Maturity

Decades of silicon process technology development
Extensive infrastructure for doping, oxidation, lithography, etching, and deposition
Good mechanical strength and thermal conductivity for high-temperature processing

Bottom Line: Silicon's combination of affordability, appropriate band gap, good mobility, and process advantages made it the industry standard.

5. Compound Semiconductors and Specialized Applications

Compound semiconductors consist of two or more elements and serve specialized roles that silicon cannot effectively perform.

Gallium Arsenide (GaAs)

Band gap: ~1.42 eV
High luminous efficiency for LEDs and lasers
Higher electron mobility than silicon for RF applications
Limited by substrate growth challenges, oxide formation difficulties, and mechanical strength compared to silicon

Silicon Carbide (SiC)

Band gap: ~3.26 eV (4H-SiC)
High breakdown field (~2-3×10⁶ V/cm)
Excellent thermal conductivity
Ideal for high-voltage, high-temperature, high-power devices
Applications: EV drive MOSFETs, IGBTs, Schottky diodes

Gallium Nitride (GaN)

Band gap: 3.4 eV
Excellent electron mobility (especially in AlGaN/GaN heterostructures with 2D electron gas)
High-frequency, high-power HEMT applications
Blue/UV wavelength LEDs and laser diodes

Other Compound Semiconductors

Indium Phosphide (InP), GaSb, CdTe, ZnSe
Used in optical communications, sensors, and solar cells

Historical Note: Shuji Nakamura received the Nobel Prize for developing blue LEDs using compound semiconductors.

6. P-N Junctions: The Foundation of Modern Electronics

When p-type and n-type semiconductors are brought together, they form a p-n junction—the fundamental building block of virtually all semiconductor devices. The behavior of this junction under different bias conditions enables the creation of diverse electronic and optoelectronic devices.

6.1 P-N Junction Physics

Formation and Equilibrium: When p-type and n-type materials are joined, majority carriers diffuse across the junction due to concentration gradients. Electrons from the n-side diffuse to the p-side, while holes from the p-side diffuse to the n-side. This creates a space charge region (depletion region) where mobile carriers are depleted, leaving behind ionized dopant atoms.

Built-in Potential: The charge separation creates an electric field that opposes further diffusion, establishing equilibrium with a built-in potential (Vbi) given by:

Vbi = (kT/q) × ln(NaNd/ni²)

where Na and Nd are acceptor and donor concentrations, respectively, and ni is the intrinsic carrier concentration.

Depletion Width: The depletion region width (W) depends on the built-in potential and doping levels:

W = √(2εs(Na + Nd)Vbi/q(NaNd))

6.2 Bias Conditions and I-V Characteristics

Forward Bias (p-side positive, n-side negative):

Reduces the electric field across the junction
Lowers the potential barrier for majority carrier injection
Exponential current increase described by the Shockley equation:

I = Is(e^(qV/nkT) - 1)

where Is is the saturation current and n is the ideality factor.

Reverse Bias (p-side negative, n-side positive):

Increases the electric field and widens the depletion region
Majority carriers are swept away from the junction
Only small reverse saturation current flows due to minority carriers
Junction capacitance decreases with increasing reverse bias

https://www.allaboutcircuits.com/textbook/semiconductors/chpt-2/the-p-n-junction/

7. Semiconductor Devices and Applications

7.1 Bipolar Junction Transistors (BJTs)

Structure: Three-layer devices (npn or pnp) with two p-n junctions

Emitter-Base Junction: Forward biased in active mode
Base-Collector Junction: Reverse biased in active mode

Operation Principle: In an npn transistor, electrons injected from the emitter into the base are collected by the collector. The base current controls the collector current through current gain (β):

IC = β × IB

Applications: Amplifiers, switches, and analog circuits where high current gain and low noise are required.

7.2 Field-Effect Transistors (FETs)

MOSFET Structure: Metal-Oxide-Semiconductor structure with gate, source, and drain

Gate Oxide: Provides electrical isolation between gate and channel
Channel Formation: Gate voltage modulates channel conductivity

Operation Modes:

Enhancement Mode: Channel forms when gate voltage exceeds threshold voltage (Vth)
Depletion Mode: Existing channel is modulated by gate voltage

Key Equations:

Linear Region: ID = μnCox(W/L)[(VGS - Vth)VDS - VDS²/2]
Saturation Region: ID = (μnCox/2)(W/L)(VGS - Vth)²

Advantages: High input impedance, low power consumption, excellent scalability for digital circuits.

7.3 Photodetectors

Operating Principle: Reverse-biased p-n junctions where incident photons generate electron-hole pairs

Photocurrent: Proportional to incident optical power
Spectral Response: Determined by band gap and absorption coefficient
Quantum Efficiency: η = (Number of collected carriers)/(Number of incident photons)

Types:

PIN Photodiodes: Intrinsic region between p and n regions for enhanced sensitivity
Avalanche Photodiodes (APDs): Internal gain through impact ionization
Phototransistors: Optical input controls base current

Applications: Optical communications, imaging systems, light detection and ranging (LiDAR).

7.4 Solar Cells (Photovoltaic Devices)

Energy Conversion Mechanism: Photons with energy ≥ band gap create electron-hole pairs

Open-Circuit Voltage (Voc): Maximum voltage under illumination
Short-Circuit Current (Isc): Maximum current under illumination
Fill Factor (FF): Ratio of maximum power to Voc × Isc

Efficiency Limitations:

Shockley-Queisser Limit: Theoretical maximum efficiency (~33% for single junction)
Thermalization Losses: Excess photon energy converted to heat
Transmission Losses: Sub-bandgap photons not absorbed

Advanced Architectures:

Tandem Cells: Multiple junctions with different band gaps
Concentrator Systems: Optical concentration to increase efficiency
Perovskite-Silicon Tandems: Emerging high-efficiency technology

7.5 Light-Emitting Diodes (LEDs)

Electroluminescence Process: Forward-biased p-n junction where recombining electron-hole pairs emit photons

Emission Wavelength: λ = hc/Eg (where Eg is the band gap)
External Quantum Efficiency: ηext = ηint × ηextraction

Material Systems:

Red/IR: AlGaAs, GaAsP
Green/Yellow: AlGaInP, GaN
Blue/UV: InGaN/GaN quantum wells
White Light: Blue LED + phosphor or RGB combination

Advanced Structures:

Quantum Wells: Carrier confinement for improved efficiency
Distributed Bragg Reflectors: Enhanced light extraction
Micro-LEDs: Miniaturized devices for display applications

7.6 Laser Diodes

Stimulated Emission: Population inversion in the active region enables coherent light emission

Threshold Current: Minimum current for lasing action
Optical Cavity: Fabry-Perot or distributed feedback (DFB) structures
Mode Control: Single-mode operation for communications

Key Parameters:

Differential Efficiency: Slope of L-I curve above threshold
Linewidth Enhancement Factor: Coupling between amplitude and phase modulation
Relative Intensity Noise (RIN): Spectral density of intensity fluctuations

Applications: Optical communications, laser processing, spectroscopy, LiDAR.

8. Advanced Device Physics and Emerging Technologies

8.1 Quantum Effects in Nanoscale Devices

Quantum Confinement: When device dimensions approach the de Broglie wavelength, quantum effects become significant

Quantum Wells: 1D confinement affects density of states
Quantum Wires: 2D confinement creates 1D electronic systems
Quantum Dots: 3D confinement leads to discrete energy levels

Tunneling Phenomena:

Tunnel Diodes: Negative differential resistance from band-to-band tunneling
Resonant Tunneling Diodes: Quantum well structures with resonant states
Tunnel FETs: Steep subthreshold slope through tunneling injection

8.2 High-Electron-Mobility Transistors (HEMTs)

Heterostructure Engineering: Different band gap materials create 2D electron gas (2DEG)

AlGaN/GaN HEMTs: High power density for RF applications
AlGaAs/GaAs HEMTs: Low noise for microwave circuits
InAlAs/InGaAs HEMTs: Ultra-high frequency performance

Performance Metrics:

Cutoff Frequency (fT): Unity current gain frequency
Maximum Oscillation Frequency (fmax): Unity power gain frequency
Power Added Efficiency (PAE): (Pout - Pin)/PDC

8.3 Power Electronics Devices

Wide Bandgap Semiconductors: SiC and GaN enable high-voltage, high-temperature operation

SiC MOSFETs: Automotive traction inverters, industrial motor drives
GaN HEMTs: Power adapters, wireless charging, data center power supplies
SiC Schottky Diodes: Fast switching with minimal reverse recovery

Figure of Merit: Baliga's Figure of Merit (BFOM) = εμE³c quantifies material performance for power switching.

8.4 Emerging Device Concepts

Negative Capacitance Transistors: Ferroelectric materials provide voltage amplification Spin-Based Devices: Magnetic tunnel junctions for non-volatile memory Neuromorphic Devices: Memristors and synaptic transistors for brain-inspired computing Topological Insulators: Edge states for quantum computing applications

9. Device Fabrication and Integration Challenges

9.1 Scaling Limitations

Short Channel Effects: Threshold voltage roll-off, drain-induced barrier lowering
Quantum Tunneling: Gate leakage through ultra-thin oxides
Variability: Statistical dopant fluctuations, line edge roughness

9.2 Advanced Process Technologies

FinFET Architecture: 3D channel control for improved electrostatics
Gate-All-Around (GAA): Nanowire/nanosheet devices for ultimate scaling
Monolithic 3D Integration: Vertical stacking of active devices

9.3 Heterogeneous Integration

System-in-Package (SiP): Combining different semiconductor technologies
Chiplet Architecture: Disaggregated design with advanced packaging
Photonic Integration: Co-integration of electronics and photonics

Conclusion: The p-n junction serves as the fundamental building block that enables an extraordinary diversity of semiconductor devices. From simple diodes to complex quantum devices, the ability to control carrier transport through junction engineering and bias conditions has revolutionized modern technology