Semiconductors
Core concepts
A semiconductor has a valence band nearly filled and a narrow forbidden gap, so thermal energy frees a few electrons and leaves holes. Intrinsic material conducts poorly; doping creates n-type material (donor atoms, electrons as majority carriers) or p-type material (acceptor atoms, holes as majority carriers). At a p-n junction, diffusion forms a depletion region with a barrier potential. Forward bias narrows the depletion region and reduces the barrier; reverse bias widens it and allows only small leakage current. A diode conducts appreciably only above its forward knee: about 0.6-0.7 V for silicon and 0.2-0.3 V for germanium. A Zener diode is normally operated in reverse breakdown, where its voltage is nearly constant. An LED is forward biased and emits light; a photodiode is usually reverse biased and conducts more when illuminated. In a BJT, base current controls collector current; in a FET, gate-source voltage controls channel current.
Traps
Candidates often confuse conventional current with electron flow: conventional current runs from positive to negative, electron flow the other way. Do not say the depletion region disappears under forward bias; it narrows. Do not forward bias a Zener for regulation. Do not omit the LED series resistor. Remember that the diode drop is a voltage, not a resistance.
Worked example
Choose a series resistor for a 2.0 V LED at 10 mA from a 5 V supply: R = (5 - 2.0) / 0.010 = 300 ohm. Use the nearest preferred value above this, such as 330 ohm, then check current: (5 - 2.0)/330 = 9.1 mA.