Diodes, transistors & switching
Junctions, diodes & rectification
A diode’s direction-dependent behavior can reshape a signal.
A p-n junction forms a depletion region and an internal electric field. Forward bias encourages carrier injection and current; reverse bias normally leaves small leakage until breakdown. Current changes nonlinearly with voltage. Rectifiers use this directional behavior to select a half-cycle or flip both halves of an AC waveform. A smoothing capacitor reduces ripple, but does not make an unregulated supply perfectly constant.
BJT operating regions & bias
Choose a useful operating point instead of hoping a transistor lands there.
A bipolar transistor has cutoff, active and saturation regions. In forward-active operation, collector current is approximately β times base current, but β varies widely. An amplifier is biased so its signal can swing without reaching cutoff or saturation. Emitter resistance and negative feedback can make the operating point less dependent on transistor gain. A switching circuit instead aims for a suitably off or strongly on state.
MOSFET gate drive & conduction loss
“Starting to turn on” is not the same as being fully enhanced.
A MOSFET uses gate-to-source voltage to control a conducting channel. Its threshold is specified at a small test current and does not guarantee low on-resistance. Choose a device with RDS(on) specified at the gate voltage your driver actually supplies. The insulated gate draws little steady DC current but needs charge moved during switching. Faster switching, gate resistance, layout and driver strength interact.