Semiconductor Devices: Diodes, Bipolar Transistors, and Field-Effect Structures
Diode Transport
Shockley equation and departures
The ideal diode equation is , but real diodes exhibit series resistance, recombination current, tunneling, and high-injection effects. Understanding which transport channel dominates is essential for interpreting I-V curves.
The ideality factor in a diode equation is often greater than because of:
Depletion-region recombination and other nonidealities increase the apparent ideality factor.
Correct answer: Recombination in the depletion region and nonideal transport
What does the saturation current represent physically?
It is not strictly constant in all real devices, but it captures reverse-saturation behavior in the ideal model.
Correct answer: It represents the small reverse-bias current set by minority-carrier injection and recombination processes.
BJT and MOSFET Physics
Minority-carrier versus majority-carrier devices
BJTs rely on minority-carrier injection and gain through current amplification, while MOSFETs exploit electrostatic control of a majority-carrier channel. Device scaling has shifted emphasis toward short-channel electrostatics, mobility degradation, velocity saturation, and quantum confinement.
BJT vs MOSFET
BJT
- Current-controlled minority-carrier device
- High transconductance per current in some regimes
- Sensitive to recombination and base transport
MOSFET
- Voltage-controlled majority-carrier device
- Dominant in digital VLSI
- Strongly impacted by electrostatics and interface quality
A key advantage of MOSFETs in digital logic is that they:
MOSFET operation is based on gate-controlled channel formation, which enables low static power in logic.
Correct answer: Are inherently electrostatically controlled by a gate
Name one major short-channel effect in advanced MOSFETs.
Any one of these is acceptable; they are classic scaling issues.
Correct answer: Drain-induced barrier lowering, velocity saturation, threshold-voltage roll-off, or subthreshold leakage.
Quantum Confinement in Device Structures
Low-dimensional channels
In ultra-scaled devices, carriers can be confined in one or more dimensions, leading to subbands, modified density of states, and altered threshold behavior. Quantum confinement can improve electrostatic control while also increasing sensitivity to interface scattering and roughness.
Quantum confinement in a thin semiconductor channel typically causes:
Confinement discretizes motion in one or more directions and changes device electrostatics.
Correct answer: Subband formation and altered threshold voltage