Govt. Exams
Minimum noise figure occurs at optimum source impedance that matches the device's noise characteristics, typically provided in device specifications.
β varies with temperature and VBE changes, following the Ebers-Moll model. Early voltage causes slight VCE dependence.
In tunnel diode's NDR region, as voltage increases, tunneling current decreases (fewer states to tunnel into) while diffusion current increases, causing net decrease in total current.
ni = √(Nc·Nv)·exp(-Eg/2kT) where Nc, Nv depend on effective masses. Small Eg and high density of states both exponentially increase ni. GaAs has higher ni than Si due to smaller Eg.
Contact resistance ρc ∝ exp(φB/kT)/Nc where φB is barrier height. Higher barrier leads to exponentially higher resistance following Thermionic emission theory.
APD operates in high reverse bias where impact ionization (collision ionization) produces secondary electron-hole pairs, creating avalanche multiplication and signal amplification internally.
Fill Factor (FF) = Pmax/(Voc × Isc) = (Vm × Im)/(Voc × Isc). It indicates how close the I-V curve is to a rectangle, typically 0.7-0.85 for practical solar cells.
Forward current I = I₀exp(eV/kT). Ratio = exp(e×0.3/kT) = exp(0.3/0.026) ≈ exp(11.5) ≈ 10^5. For ΔV = 0.3V more, increase is exp(11.5) ≈ 1000 times.
Heterojunctions use materials with different bandgaps to create band offsets that confine carriers (electrons and holes) to specific regions, improving device efficiency. AlGaAs has larger Eg than GaAs, confining carriers in GaAs.
Bandgap of semiconductors decreases with increasing temperature. For Si: dEg/dT ≈ -2.3 meV/K near 300 K. This is derived from Varshni equation and is crucial for device design.
