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Semiconductor Devices — BJTs and MOSFETs
Learn2026-01-14

Semiconductor Devices — BJTs and MOSFETs

#electronics#semiconductor#transistor

Overview

This chapter covers the practical behaviour of bipolar junction transistors (BJTs) and MOSFETs, DC biasing, and small‑signal models used for amplifier design and hand calculations.

Prerequisites

  • Basic circuit analysis and familiarity with resistive networks

Learning objectives

  • Explain BJT and MOSFET operating regions and common parameters (β, Vth, Rds(on))
  • Derive small‑signal models (r_pi, gm, ro) and use them for gain/impedance estimates
  • Bias transistors for linear amplification and verify with measurements

Tools & materials

  • Breadboard or PCB, BJTs and MOSFETs, multimeter, oscilloscope

Hands-On Mini Task

  1. Build a common-emitter BJT amplifier with emitter degeneration. Measure DC bias points and small‑signal gain.
  2. Compare measured gain to small‑signal prediction using gm and r_pi.

Expected result: measured gain within ~20% of hand-estimate depending on device variation and loading.

Theory

BJTs: operate in active, saturation, and cutoff. Key small-signal params: β (current gain), r_e ≈ 26mV/I_E, r_pi = β × r_e.

MOSFETs: regions are cutoff, triode, saturation. Key small-signal params: threshold Vth, transconductance gm = 2Id/(Vgs-Vth) for square-law approx, and output resistance r_o.

Device measurement and practical tips

  • Measure DC curves (Ib/Ic or Id) to find operating point. Use a current-limited supply when experimenting.
  • Beware of self-heating when measuring small-signal parameters; allow stabilization time.

Worked example — estimate small-signal gain

  1. For a BJT with β = 100, I_C = 1 mA: r_e ≈ 26 Ω, r_pi ≈ β·r_e ≈ 2.6 kΩ. With R_C = 10 kΩ and emitter degeneration R_E = 200 Ω, approximate voltage gain Av ≈ - (R_C || r_o) / (r_e + (1+β)·R_E/r_pi) (use simplified formulas and verify with simulation).

Troubleshooting

  • If the amplifier saturates, increase supply voltage or reduce gain/load.
  • If bias shifts with temperature, add bias stabilization (constant-current sources or negative feedback).

Further reading

  • Sedra & Smith, "Microelectronic Circuits" — transistor chapters
  • Manufacturer datasheets and application notes for device-specific parameters

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