Transistor (NPN, CE)

A bipolar junction transistor (BJT) is a 3-terminal device with two p-n junctions: NPN or PNP. Terminals: Emitter (E), Base (B), Collector (C).

NPN transistor in 'active' mode: E-B junction forward-biased, C-B junction reverse-biased.

Emitter (heavily doped) injects electrons into Base (lightly doped, thin); most diffuse across to the Collector (moderately doped).

Small base current I_B controls large collector current I_C — transistor is a CURRENT AMPLIFIER.

Current gain β (or h_FE) = I_C/I_B; typically 50-300. α = I_C/I_E ≈ 0.99.

Three operating regions: (i) Cutoff (both junctions reverse, transistor OFF). (ii) Active (E-B forward, C-B reverse, amplifies). (iii) Saturation (both forward, transistor 'ON' as a switch).

Three common configurations: (i) Common-Emitter — high current AND voltage gain; (ii) Common-Base — voltage gain only; (iii) Common-Collector (emitter follower) — current gain, unity voltage gain, high input impedance.

Applications: amplifiers, switches, oscillators, logic gates. Modern MOSFETs (field-effect) dominate ICs.