Govt. Exams
At low forward bias, electrons tunnel through the narrow bandgap, creating peak current. As voltage increases, direct band-to-band current dominates, causing current to decrease with voltage (negative differential resistance).
Higher doping concentrations create stronger electric fields at lower voltages, reducing Zener breakdown voltage. This allows designing Zeners with specific breakdown voltages (3.3V, 5.1V, etc.).
Wide bandgap material (AlGaAs) acts as barrier, confining carriers to narrow bandgap GaAs region. This reduces recombination, improves injection efficiency, and is crucial for LEDs and laser diodes.
Quantum confinement in thin layers (nanometers) modifies the density of states from 3D to 2D structure, changing effective masses and enabling bandgap engineering for applications like quantum well lasers and LEDs.
Channel length modulation occurs when the depletion region at the drain expands with increasing Vds, shortening the effective channel length. This causes output current to increase with voltage, reducing output impedance (ro decreases).
Ideal subthreshold swing SS = (kT/q) × ln(10) ≈ 60 mV/decade at 300 K. This is a fundamental limit based on the thermal voltage. Real MOSFETs have SS > 60 mV/decade due to interface states.
Early voltage characterizes the output resistance of a BJT. V_A is inversely proportional to the base width modulation effect (Early effect), which becomes significant in short-base transistors.
Heavy doping narrows the depletion region, allowing quantum tunneling at lower voltages. This shifts breakdown mechanism from avalanche (Zener ~5-6V) to tunneling (~3-4V) in heavily doped junctions.
In Zener breakdown, as the reverse bias increases slightly, the breakdown mechanism (avalanche or tunneling) generates more current, but the voltage across the junction decreases due to voltage regulation.
Is ∝ ni²/(NA·ND), depending on intrinsic carrier concentration squared and inversely on doping concentrations.
