N-type has spare electrons, P-type has holes, and reverse bias pulls them apart
Doping adds impurity atoms to silicon. Donor atoms add free electrons: N-type. Acceptor atoms leave electron vacancies called holes: P-type. Join them and carriers near the junction cancel, leaving an empty depletion region.
Reverse bias: carriers pulled away from the junction, depletion region widens, no current.
RememberReverse bias separates holes and electrons, widening the depletion region, so no current. Forward bias squeezes it shut.
GaAs is for microwaves, silicon for everything else
Silicon is the everyday material. Gallium arsenide (GaAs) carries electrons faster, so it works at microwave frequencies.
Beta: a small base current controls a large collector current
A silicon NPN transistor turns on when the base-to-emitter junction is forward biased: about 0.6 to 0.7 volts, a voltage, not a resistance. Beta is collector current change per base current change. Alpha is the common-base gain; the alpha cutoff frequency is where it falls to 0.7 of its 1 kHz value.
Collector current = beta × base current. A small base current steers a large collector current.
Collector current4 mA
RememberBeta = ΔIc ÷ ΔIb. Silicon base-emitter on: 0.6 to 0.7 V.
A FET gate takes voltage, not current
The gate of a FET is insulated (MOSFET) or reverse-biased (JFET), so it draws almost no current: very high input impedance, unlike a BJT base. A depletion-mode FET already conducts at zero gate volts. An enhancement-mode FET does not.
Depletion mode: conducts with zero gate voltage; gate voltage squeezes the channel shut.
Drain current (relative)25 %
The MOSFET gate oxide is extremely thin, so static can punch through it. Zener diodes from gate to source clamp the voltage and protect the gate from static damage.
The zener clamps the spike below the oxide limit.
RememberFET: high input impedance. Depletion: current at zero gate volts. Zener at the gate: static protection.
FET symbols: the arrow shows N or P, extra gate lines show dual-gate
The arrow tip pointing at the channel bar is N-channel ("points iN"). Pointing away is P-channel. A JFET has its gate lead touching the channel bar through the arrow. A MOSFET has an insulated gate plate set apart from the bar. A dual-gate MOSFET has two gate leads, G1 and G2.
Arrow pointing at the channel bar means N-channel. Pointing away means P-channel.
Memory hookArrow Points iN: N-channel. Two gate leads: dual gate.