Bipolar Junction Transistors and Field-Effect Transistors
What this domain tests
This domain tests the two workhorse three-terminal devices. For the BJT you need current gain, regions of operation and biasing; for the FET, the voltage-controlled channel, pinch-off and the very high input resistance that makes it a good buffer.
Core rules
- BJT terminals are emitter, base and collector. The emitter is heavily doped, the base thin and lightly doped, the collector large to dissipate heat.
- α = Ic/Ie, β = Ic/Ib, β = α/(1 − α), and Ie = Ic + Ib always.
- Regions: cut-off (both junctions reverse biased, no current), active (BE forward, CB reverse, used for amplification), saturation (both forward, VCE ≈ 0.2 V, used as a switch).
- Silicon VBE ≈ 0.7 V when conducting; base current sets collector current in the active region.
- CE amplifier output is 180° out of phase with the input; CB has low input impedance; CC (emitter follower) has gain near 1 and buffers.
- FET: the gate draws almost no current and VGS controls ID. A JFET is depletion-mode with pinch-off voltage Vp and IDSS at VGS = 0. MOSFETs are depletion or enhancement type; enhancement needs VGS above threshold.
- Transconductance gm = ΔID/ΔVGS.
Worked example
A BJT has IB = 20 µA and β = 100. Then IC = βIB = 2 mA, IE = 2 + 0.02 = 2.02 mA, and α = 2/2.02 = 0.99. With a 2.2 kΩ collector resistor the drop is 4.4 V; with VCC = 9 V, VCE = 4.6 V, so the device is in the active region, not saturation.
Traps that cost marks
Assuming β is a fixed constant; using 0.7 V for a germanium device; describing saturation as "no current"; confusing JFET pinch-off with MOSFET threshold voltage; omitting IB when calculating α.