Practical takeaway
BJTs and FETs are controlled semiconductor devices, but they control current differently and therefore behave differently in biasing and input impedance.
E6A · Amateur Extra question group
Semiconductors, BJTs & FETs
BJTs and FETs are controlled semiconductor devices, but they control current differently and therefore behave differently in biasing and input impedance.
Amateur Extra exam focus
The explanation below teaches the ideas behind E6A while keeping the important terms recognizable. The Quiz Me button uses the current question-pool wording for this group.
1. A BJT is current-controlled
A bipolar junction transistor uses base current to control a much larger collector current. Current gain describes how a change in base current produces a change in collector current in the active region.
2. A FET is primarily voltage-controlled
A field-effect transistor uses an electric field at the gate to control channel current. The gate draws very little steady-state current in many FET types, giving high input impedance.
3. Device structure determines RF usefulness
Junction capacitance, carrier mobility, breakdown voltage, power dissipation, and package inductance limit high-frequency performance. Device choice depends on frequency and power, not only DC gain.
E6A checklist
- BJT collector current is controlled by base current.
- FET drain current is controlled primarily by gate voltage/electric field.
- FETs typically have high input impedance.
- Parasitics and device geometry limit RF performance.
Exam scope: NCVEC 2024-2028 Amateur Extra Class (Element 4), effective July 1, 2024 through June 30, 2028, with February 4, 2026 fourth errata. The current NCVEC pool is the exam authority; explanatory prose here is SPARKA teaching material. NCVEC pool