Transistor Basics
Part of Semiconductors
How bipolar transistors work, why they amplify, and how to use them in fundamental circuit configurations.
Why This Matters
The transistor is the fundamental active component of modern electronics. Every amplifier, switch, oscillator, and logic gate uses transistors as its active elements. Understanding transistor basics — not just the rules, but why they work — enables you to design circuits from first principles, diagnose failures correctly, and innovate beyond copied circuits.
For a rebuilding civilization, transistor basics is the course from which semiconductor technology branches into all directions: radio receivers, audio amplifiers, power regulators, digital logic, computers. This is the central node of the technology tree. Mastering it unlocks dozens of downstream capabilities.
The explanations here focus on bipolar junction transistors (BJTs) — the type made by alloy or diffusion junction processes accessible early in rebuilding. Field-effect transistors (FETs) are conceptually simpler in some ways but harder to fabricate. BJTs first.
The Transistor as a Current Valve
A BJT transistor has three terminals: base (B), collector (C), and emitter (E). The fundamental relationship:
IC = hFE × IB
where IC is collector current, IB is base current, and hFE (current gain, also called β) typically ranges from 20 to 500 depending on device type.
This is the transistor’s core function: a small base current controls a large collector current. The transistor is a current amplifier — or equivalently, a valve where base current is the control input.
Physically, in an NPN transistor:
- Forward-biasing the base-emitter junction injects electrons from the emitter into the base.
- These electrons are minority carriers in the (p-type) base. Most diffuse across the thin base and reach the collector junction.
- The reverse-biased collector junction sweeps them into the collector circuit.
- Collector current IC ≈ emitter current IE for high-gain transistors.
- Base current IB is the small fraction of emitter electrons that recombined in the base rather than reaching the collector.
The base must be thin for high gain — most injected carriers must make it across without recombining. hFE = (fraction reaching collector) / (fraction recombining in base) = α / (1 - α), where α is the transport factor. If 99% cross the base: α = 0.99, hFE = 99. If 99.5% cross: hFE = 199. Base thickness and carrier lifetime determine α.
Three Operating Regions
Active region: VBE ≈ 0.7V (forward biased), VCE > 0.3V (collector junction reverse biased). IC = hFE × IB. Both junctions correctly biased. This is the amplification region — small changes in IB produce large changes in IC.
Saturation region: Both junctions forward biased. VCE drops below ~0.2V (silicon). IC is limited by external circuit, not by IB. The transistor is “fully on” — behaves like a small resistor (~10-100 Ω) between collector and emitter. Used for switch-on state in digital circuits.
Cutoff region: IB ≤ 0. Both junctions reverse biased (or base-emitter junction not forward biased). Only ICEO leakage flows. Transistor is “off.” Used for switch-off state.
Breakdown region: VCE exceeds BVCEO. Avalanche at collector junction. Destructive if current is not limited. Avoid.
The transition between active and saturation is the boundary between analog and digital behavior. Analog amplifiers operate in active region; digital switches alternate between saturation and cutoff.
The Four Circuit Configurations
Common emitter (emitter connected to ground/reference): Signal applied base-to-ground; output taken collector-to-ground. Voltage gain is -(RC / re) where re = 26 mV / IC. Gain is negative (inverts signal). Moderate input impedance (~hFE × re), moderate output impedance (~RC). The most common configuration for voltage amplification.
Common collector (collector connected to supply): Signal applied base-to-supply; output taken emitter-to-ground. Voltage gain ≈ 1 (unity). Current gain = hFE + 1 (high). High input impedance (~hFE × RE), low output impedance (~RE / hFE). Also called emitter follower. Used for impedance transformation: drive a low-impedance load from a high-impedance source without signal loss.
Common base (base connected to ground via capacitor for AC): Signal applied emitter-to-ground; output taken collector-to-base. Voltage gain is (RC / re) — same magnitude as common emitter but non-inverting. Very low input impedance (~re, typically 26 Ω at 1 mA), high output impedance. Used for high-frequency applications (the low input impedance gives the best high-frequency performance) and for current buffers.
Each configuration trades different parameters. Common emitter: medium Z_in, high Z_out, high voltage gain, inverting. Common collector: high Z_in, low Z_out, unity voltage gain, non-inverting. Common base: low Z_in, high Z_out, high voltage gain, non-inverting. Choosing the right configuration for the application is fundamental design knowledge.
Small-Signal Model
For analog design, the transistor is modeled by its small-signal equivalent circuit — a linear model valid for small signals around the operating point:
rπ (input resistance, base-emitter): rπ = hFE / gm = hFE × VT / IC. At IC = 1 mA and hFE = 100: rπ = 100 × 0.026 = 2.6 kΩ.
gm (transconductance): gm = IC / VT = IC / 0.026. At IC = 1 mA: gm = 38.5 mA/V. A 1 mV input voltage produces 38.5 µA output current change.
ro (output resistance): finite collector-emitter resistance in active region. ro = VA / IC where VA is the Early voltage (typically 50-200V for BJTs). At IC = 1 mA and VA = 100V: ro = 100 kΩ. Usually much larger than RC — often ignored in first-order calculations.
The small-signal model converts transistor circuits into standard linear circuits solvable by Ohm’s law and Kirchhoff’s laws. Apply it: replace transistor with rπ (base-emitter), gm×vπ (controlled current source, collector-emitter), and ro (output). Solve the resulting linear circuit.
Voltage gain from model: Common emitter with collector resistance RC: Av = -gm × (RC || ro) ≈ -gm × RC (if ro >> RC) = -(IC/VT) × RC = -(IC × RC) / VT
At IC = 1 mA, RC = 4.7 kΩ: Av = -(1×10^-3 × 4.7×10^3) / 0.026 = -181.
This confirms the earlier formula Av = -(RC/re) since re = 1/gm = VT/IC.
Switching Applications
Transistors as switches operate between cutoff and saturation — digital applications.
Switch-off: IB = 0 (or slightly negative). IC = ICEO (leakage, typically < 1 µA). VCE ≈ Vcc — the transistor supports full supply voltage with minimal current. Effective open circuit.
Switch-on: Apply base current to drive transistor into saturation. Calculate required IB: if the load needs 50 mA through it when the transistor is on, and hFE = 100, then IB_min = 50 mA / 100 = 0.5 mA. Use 3-5× overdrive to ensure saturation: IB = 1.5-2.5 mA. VCEsat ≈ 0.1-0.3V. Most of Vcc appears across load.
Switching speed: Falling edge (switch-off) is slower than rising edge because stored minority carriers in base must be removed. Apply a brief reverse base current (negative IB) at switch-off to actively sweep out stored carriers — called “active turn-off.” This dramatically improves switching speed, from microseconds to nanoseconds.
Base resistor calculation: For a transistor driving a relay coil (50 mA) from a 5V logic output: IB needed = 50 mA / (hFE_min = 50) = 1 mA, with overdrive → 3 mA. RB = (5V - VBE) / IB = (5V - 0.7V) / 3 mA = 1.43 kΩ → use 1.2 kΩ standard value.
Transistor switching is the foundation of digital logic. NAND gates, flip-flops, counters, and entire computers are built from transistor switches. The path from a single transistor understanding to a working computer is long but direct.