BJT & FET Notes

Last reviewed 16 Sept 2026 · 8 min read

Bipolar junction transistor (BJT)

A BJT has three doped regions forming two PN junctions: emitter, base and collector.

Region	Doping	Size	Function
Emitter	Heavily doped	Medium	Emits (injects) majority carriers
Base	Lightly doped	Very thin	Controls carrier flow; most carriers pass through
Collector	Moderately doped	Largest (to dissipate heat)	Collects carriers

NPN — electrons are majority carriers (faster, more common); PNP — holes.
Symbol arrow on the emitter points in the direction of conventional current (out for NPN, in for PNP).
"Bipolar" — both electrons and holes take part in conduction.

Working (NPN in active region)

Emitter–base junction forward biased, collector–base junction reverse biased.
Electrons from the emitter enter the thin, lightly doped base; only a small fraction recombine (base current); most are swept into the collector.

FormulaCurrent relations

I_{E} = I_{B} + I_{C}

Common-base current gain $α = \frac{I _{C}}{I _{E}}$ (slightly less than 1, e.g. 0.95–0.995) Common-emitter current gain $β = \frac{I _{C}}{I _{B}}$ (typically 50–300)

β = \frac{α}{1 - α}, α = \frac{β}{1 + β}

Including leakage: $I_{C} = β I_{B} + (1 + β) I_{C B O}$

Transistor configurations

Parameter	Common base (CB)	Common emitter (CE)	Common collector (CC) / emitter follower
Input / output	Emitter / collector	Base / collector	Base / emitter
Current gain	α (< 1)	β (high)	$1 + β$ (high)
Voltage gain	High	High	≈ 1 (slightly less)
Power gain	Moderate	Highest	Moderate
Input resistance	Very low	Medium	Very high
Output resistance	Very high	Medium	Very low
Phase shift (input to output voltage)	0°	180°	0°
Applications	High-frequency amplifiers	General-purpose amplifiers (most used)	Buffer / impedance matching

Characteristics (CE)

Input characteristics — $I_{B}$ vs $V_{B E}$ at constant $V_{C E}$ (like a diode).
Output characteristics — $I_{C}$ vs $V_{C E}$ for various $I_{B}$ — flat lines in active region; Early effect (base-width modulation) gives slight slope.

Regions of operation

Region	E–B junction	C–B junction	Use
Active	Forward	Reverse	Amplifier
Cutoff	Reverse (or zero)	Reverse	Switch OFF ( $I_{C} \approx 0$ )
Saturation	Forward	Forward	Switch ON ( $V_{C E (s a t)} \approx 0.2$ V)
Inverse active	Reverse	Forward	Rarely used

Transistor as a switch: driven between cutoff and saturation (digital circuits, relay drivers, LED drivers). For saturation, $I_{B} > \frac{I _{C (s a t)}}{β}$ .

Biasing and stability

Biasing sets a stable operating point (Q-point) — $I_{C}$ and $V_{C E}$ — in the active region so that the signal is amplified without distortion.

DC load line

For a CE circuit: $V_{C E} = V_{C C} - I_{C} R_{C}$ (plus $I_{E} R_{E}$ if emitter resistor present).

End points: $I_{C (s a t)} = V_{C C} / R_{C}$ (at $V_{C E} = 0$ ) and $V_{C E (c u t o f f)} = V_{C C}$ (at $I_{C} = 0$ ).
The Q-point should normally lie near the middle of the load line for maximum symmetrical swing.

Why stabilisation?

$β$ varies between transistors and with temperature; $I_{C B O}$ doubles about every 10 °C → $I_{C}$ increases → more heating → possible thermal runaway.

Biasing methods

Method	Circuit	Stability
Fixed (base resistor) bias	$R_{B}$ from $V_{C C}$ to base: $I_{B} = \frac{V _{C C} - V _{B E}}{R _{B}}$	Poor — Q-point depends directly on β
Collector-to-base (feedback) bias	$R_{B}$ from collector to base	Better — negative feedback
Emitter bias / with emitter resistor	$R_{E}$ provides negative feedback	Good
Voltage divider bias	$R_{1}$ – $R_{2}$ divider sets base voltage; $R_{E}$ in emitter	Best and most widely used — nearly independent of β

FormulaVoltage divider bias (approximate analysis, β large)

V_{B} = V_{C C} \frac{R _{2}}{R _{1} + R _{2}}, V_{E} = V_{B} - V_{B E}, I_{C} \approx I_{E} = \frac{V _{E}}{R _{E}}

V_{C E} = V_{C C} - I_{C} (R_{C} + R_{E})

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Bipolar junction transistor (BJT)

Working (NPN in active region)

Transistor configurations

Characteristics (CE)

Regions of operation

Biasing and stability

DC load line

Why stabilisation?

Biasing methods

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