Bipolar Junction Transistor (BJT)

Why it matters

A bipolar junction transistor uses a thin base between emitter and collector to control current. BJTs are used in amplifiers, current mirrors and switching circuits.

Structure and operation

An NPN transistor has an n-type emitter, p-type base and n-type collector. The emitter is heavily doped and the base thin enough that many injected electrons reach the collector. PNP devices reverse the polarities and carrier roles. Both electrons and holes participate, hence bipolar.

For an NPN in forward active operation, the base–emitter junction is forward biased and the base–collector junction reverse biased. In saturation both junctions are forward biased; in cutoff neither junction is appreciably forward biased. Do not confuse BJT saturation with MOSFET saturation: a BJT amplifier usually operates in forward active mode.

Equations

Using current magnitudes, I_E = I_C + I_B. In forward active operation, I_C ≈ β I_B, α = I_C/I_E = β/(β+1), and I_C ≈ I_S exp(V_BE/V_T), with V_T = kT/q. These approximations neglect leakage, Early effect and high-level injection. β varies with device, bias and temperature.

For a small signal about the bias point, g_m = I_C/V_T and r_π = β/g_m. If the Early voltage V_A is modeled, r_o ≈ V_A/I_C. A 0.7 V base–emitter drop is a convenient silicon approximation, not a fixed physical constant.

Worked example

An NPN has β = 100, I_B = 20 μA, emitter grounded, V_CC = 10 V and R_C = 2 kΩ. Assume forward active operation and V_BE = 0.7 V.

I_C = 100 × 20 μA = 2 mA. I_E = 2.02 mA. V_CE = 10 − (0.002)(2000) = 6 V. The collector is above the base, so the base–collector junction is reverse biased and the active assumption is consistent.

At V_T = 25 mV, g_m = 0.002/0.025 = 0.08 S and r_π = 100/0.08 = 1.25 kΩ. α = 100/101 ≈ 0.9901.

Common mistakes

• Applying I_C = βI_B after the transistor has saturated.
• Treating β or V_BE as constant across all operating conditions.
• Confusing DC current gain with small-signal voltage gain.
• Ignoring the emitter current when calculating an emitter resistor drop.

For GATE EC

Identify the operating region before using a model. Check both junction biases and the collector load limit. Linearize only after finding the DC bias point.

Quick check

1. What are the junction biases in forward active NPN operation?
2. If β = 49, what is α?
3. Which small-signal parameter is I_C/V_T?

Answers: 1. Base–emitter forward, base–collector reverse. 2. 0.98. 3. Transconductance g_m.