Transistors: BJTs & MOSFETs
A transistor lets a small signal control a large current, and that trick underlies modern electronics. In a BJT (bipolar junction transistor), a small current into the base allows a much larger current to flow between collector and emitter, amplified by a factor called beta, often 100 or more. Fed a gentle signal, it amplifies. Driven hard, it snaps fully on or off and works as a switch.
A MOSFET is controlled by voltage instead of current. Charge on its gate opens a conductive channel between drain and source. The gate draws almost no steady current, and a fully-on MOSFET looks like a very small resistance, so little power gets wasted. That’s why MOSFETs are the default for switching power to motors, LEDs, and supply rails, and, in miniature, the building block of every digital chip.
The classic mistake is driving a MOSFET’s gate half-heartedly. Between off and fully on there’s a region where the transistor behaves like a substantial resistor, and P = I² × R turns it into a heater. Driving a MOSFET specified for a 10 V gate from a 3.3 V microcontroller pin lands you exactly there. Logic-level MOSFETs exist for this reason. Switch decisively, fully on or fully off.
Key points
- Transistors use a small input to control a large current, as amplifiers or switches.
- BJTs are current-controlled (base current times beta). MOSFETs are voltage-controlled at the gate.
- MOSFET gates draw almost no steady current, so they’re efficient and easy to drive when driven fully.
- A half-on MOSFET dissipates serious heat. Use logic-level parts with 3.3 V or 5 V control signals.
Practice
0 of 3 answered · Not startedWrong answers just let you try again, and hints are there if you want them. Answering every question first time, without hints, is what earns mastery.
- 1
What fundamentally distinguishes a MOSFET from a BJT?
- 2
Why is RDS(on) the headline specification for a switching MOSFET?
- 3
A MOSFET with RDS(on) of 25 mΩ carries 4 A. How many watts does it dissipate, to two decimal places?
W
