Semiconductor Physics Interview Questions

Q1. What is the difference between intrinsic and extrinsic semiconductors?

An intrinsic semiconductor is a pure material like silicon where charge carriers are generated only by thermal excitation across the band gap, giving equal electron and hole concentrations. In extrinsic semiconductors, deliberate doping with atoms like phosphorus (n-type) or boron (p-type) creates a dominant carrier type; for example, doping silicon with 10^16 phosphorus atoms/cm³ raises electron concentration far above intrinsic levels. This controlled imbalance is what makes practical devices like diodes and transistors possible.

Follow-up: How does temperature affect carrier concentration in an intrinsic semiconductor?

Q2. What is band gap and why does it determine whether a material is a conductor, semiconductor, or insulator?

Band gap is the energy difference between the valence band top and conduction band bottom, and it determines how easily electrons can be thermally excited into the conduction band. Silicon has a band gap of 1.12 eV, which allows moderate thermal excitation at room temperature, while diamond's 5.5 eV gap keeps it insulating. A smaller band gap means more thermally generated carriers and higher conductivity, which is why GaAs (1.42 eV) is used in high-speed devices where specific carrier concentrations are engineered.

Follow-up: Why does silicon's band gap decrease slightly with increasing temperature?

Q3. Explain the concept of drift and diffusion currents in semiconductors.

Drift current flows when an electric field accelerates free carriers, while diffusion current flows due to a concentration gradient pushing carriers from high to low concentration regions. In a forward-biased p-n junction diode, both mechanisms operate simultaneously — minority carriers injected across the junction diffuse into the neutral regions, and drift current supports the built-in field. Understanding both is critical when analyzing the base transport of a BJT, where minority carrier diffusion across the base sets the collector current.

Follow-up: Under what condition are drift and diffusion currents equal and opposite?

Q4. What is the Einstein relation and what does it connect?

The Einstein relation connects carrier mobility and diffusion coefficient: D = (kT/q) × µ, where kT/q is the thermal voltage (~26 mV at 300 K). For electrons in silicon, if µ_n is 1350 cm²/V·s, then D_n is approximately 35 cm²/s. This relation is useful when designing base widths in BJTs, because it links how fast minority carriers drift under bias to how fast they diffuse, both of which affect transit time and hence frequency response.

Follow-up: How does the Einstein relation change for a degenerate semiconductor?

Q5. What is carrier mobility and what factors reduce it?

Carrier mobility is the drift velocity per unit electric field, measured in cm²/V·s, and it determines how fast carriers respond to an applied voltage. Electron mobility in silicon is about 1350 cm²/V·s, higher than hole mobility (~480 cm²/V·s) because of effective mass differences. Mobility is reduced by lattice scattering (dominant at high temperatures) and ionized impurity scattering (dominant at high doping levels like >10^18/cm³), which is why heavily doped regions in ICs have higher resistivity than expected.

Follow-up: Why is electron mobility higher than hole mobility in silicon?

Q6. Describe the formation and role of the depletion region in a p-n junction.

The depletion region forms at the p-n junction when electrons from the n-side diffuse to fill holes on the p-side, leaving behind ionized donor and acceptor atoms that create a built-in electric field opposing further diffusion. In a silicon p-n junction with typical doping of 10^16/cm³ on each side, the built-in potential is around 0.6–0.7 V and the depletion width is on the order of 0.5–1 µm. This region acts as the gate-keeping mechanism of the diode — it widens under reverse bias and shrinks under forward bias.

Follow-up: How does the depletion width change with doping concentration?

Q7. What is the Hall effect and how is it used to characterize semiconductors?

The Hall effect is the development of a transverse voltage when a current-carrying conductor is placed in a perpendicular magnetic field, because the Lorentz force deflects carriers to one side. In semiconductor characterization, measuring the Hall voltage allows calculation of both carrier type and concentration; for a silicon sample with current 1 mA, magnetic field 0.1 T, and thickness 0.5 mm, the Hall coefficient directly gives carrier density. It is the standard method used in labs to verify whether a wafer is n-type or p-type and to extract mobility after measuring resistivity.

Follow-up: How do you determine carrier type from the sign of the Hall voltage?

Q8. What is recombination in semiconductors and why does it matter for device design?

Recombination is the process by which an electron in the conduction band falls back to recombine with a hole, releasing energy as heat (Auger or SRH recombination) or light (radiative recombination as in LEDs and lasers). In indirect band gap materials like silicon, radiative recombination is inefficient because momentum must also be conserved, which is why silicon LEDs are impractical while GaAs LEDs are efficient. For BJT design, the minority carrier lifetime in the base — which is governed by recombination — directly determines current gain, so high-lifetime base material gives higher beta.

Follow-up: What is the difference between Shockley-Read-Hall and Auger recombination?

Q9. Explain the concept of Fermi level and how doping shifts it.

The Fermi level is the energy level at which the probability of finding an electron is exactly 50%, and its position relative to the band edges determines carrier concentration. In intrinsic silicon, the Fermi level sits near the middle of the band gap; n-type doping with phosphorus shifts it toward the conduction band, while p-type doping with boron shifts it toward the valence band. In a heavily doped n+ region like the emitter of a BJT, the Fermi level can actually enter the conduction band, creating a degenerate semiconductor with metal-like conductivity.

Follow-up: What happens to the Fermi level at the junction of a p-n diode at equilibrium?

Q10. What is generation current and when does it dominate in a diode?

Generation current arises from thermally generated electron-hole pairs in the depletion region, and it flows as a reverse leakage current in a reverse-biased diode. Unlike diffusion-based saturation current (I0) which depends on minority carrier properties in the neutral regions, generation current scales with depletion width and hence increases with reverse voltage. In lightly doped silicon diodes at room temperature, generation current often exceeds ideal diffusion current, making the diode's reverse characteristic non-ideal and causing the reverse current to increase slightly with voltage rather than saturating sharply.

Follow-up: How does generation current change with temperature?

Q11. What is effective mass of a carrier and why is it different from the free electron mass?

Effective mass is a parameter that accounts for how a carrier in a crystal lattice responds to an external force, lumping all band-structure effects into a single modified mass term in Newton's second law. In silicon, the electron effective mass is about 0.26 m0 and hole effective mass is about 0.37 m0, which are different from the free electron mass m0 = 9.11×10^-31 kg. These differences directly explain why electron mobility in silicon is about three times higher than hole mobility, and why NMOS transistors are faster than PMOS for the same geometry.

Follow-up: How does effective mass affect the density of states in a semiconductor?

Q12. What is the significance of the intrinsic carrier concentration (ni) and how does it change with temperature?

Intrinsic carrier concentration ni is the density of thermally generated electron-hole pairs in an undoped semiconductor, and it equals about 1.5×10^10/cm³ in silicon at 300 K. It increases exponentially with temperature as ni ∝ T^(3/2) × exp(-Eg/2kT), so at 125°C (common junction temperature limit) ni rises to about 10^12/cm³, which can swamp the intentional doping in lightly doped regions. This temperature sensitivity is why silicon-based power devices have a rated maximum junction temperature beyond which leakage currents become unacceptable.

Follow-up: Why does silicon lose its semiconductor properties at very high temperatures?

Q13. Explain the difference between majority and minority carriers with a practical example.

Majority carriers are the dominant charge carriers introduced by doping — electrons in n-type and holes in p-type — while minority carriers are the thermally generated opposite-type carriers present in much smaller numbers. In a p-type silicon base of a BJT doped at 10^17/cm³, holes are majority carriers at that same concentration, but electrons (minority carriers) exist at only about ni²/Na ≈ 2×10³/cm³. The operation of BJTs, diodes, and solar cells fundamentally depends on injecting and controlling minority carriers, making minority carrier lifetime a critical parameter in device performance.

Follow-up: How is minority carrier lifetime measured experimentally?

Q14. What is avalanche breakdown and how does it differ from Zener breakdown?

Avalanche breakdown occurs at high reverse voltages when accelerated carriers gain enough energy to ionize silicon atoms through impact ionization, creating a multiplicative carrier generation process that causes current to surge. It typically occurs in lightly doped junctions at voltages above ~6–7 V for silicon, while Zener breakdown dominates in heavily doped junctions below ~5 V through direct quantum mechanical tunneling. In a 1N4001 diode rated at 50 V reverse voltage, the dominant breakdown mechanism is avalanche, not Zener, despite both being called 'Zener' colloquially in basic courses.

Follow-up: Which breakdown mechanism has a positive temperature coefficient and which has a negative one?

Q15. What is the significance of the depletion approximation in semiconductor analysis?

The depletion approximation assumes that the depletion region is completely swept of mobile carriers and has an abrupt boundary, allowing Poisson's equation to be solved with a simple piecewise linear electric field profile. For a silicon p-n junction with Na = Nd = 10^16/cm³, this gives a built-in potential of ~0.67 V and depletion widths of ~300 nm each side, which closely match numerical solutions. Without this simplification, analytical formulas for diode capacitance, built-in voltage, and MOSFET threshold voltage would be intractable, making it one of the most productive approximations in device physics.

Follow-up: Where does the depletion approximation break down?