Why BJTs Are Current-Driven and MOSFETs Are Voltage-Driven

 A plain-language explanation of how these transistors work

Transistors are tiny electronic switches that help control the flow of electricity in circuits. There are two main types: BJTs and MOSFETs. Even though they both act like switches or amplifiers, they work in very different ways.

Let’s break it down.

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First, What Does “Driven” Mean?

When we say a transistor is current-driven or voltage-driven, we’re talking about what it needs at its input to start working — to "turn on" or conduct electricity.

• Current-driven means you need to send current into the transistor’s input to make it work.

• Voltage-driven means you just need to apply a voltage at the input — no real current has to flow.

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BJTs: The Current-Driven Transistor

BJT stands for Bipolar Junction Transistor. It has three parts: base, collector, and emitter.

To turn on a BJT, you need to push a small current into the base pin. This current controls a much larger current flowing from the collector to the emitter.

Think of it like a faucet handle. If you press the handle (the base current), it lets a lot of water (collector current) flow. No press, no flow.

So, BJTs are like valves that open when a small current is applied.

That’s why we call them current-driven — they need a continuous input current to work.

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MOSFETs: The Voltage-Driven Transistor

MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. It also has three parts: gate, drain, and source.

Here’s the difference: To turn on a MOSFET, you don’t need to push current into the gate. Instead, you just apply voltage to the gate.

The gate is separated from the rest of the transistor by a very thin layer of insulation, so no current flows in — like pressing a button that sends a signal through air, not a wire.

Once the voltage is high enough, the MOSFET turns on and allows current to flow between the drain and source.

So, MOSFETs are more like electronic switches that flip when they "feel" a certain voltage, not when they receive current.

That’s why we say they’re voltage-driven.

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Why Does This Matter?

This difference affects how we use each transistor:

• BJTs use more energy because they need current at their input all the time.

• MOSFETs use almost no energy at their input — they’re great for power saving.

• MOSFETs are often better for fast switching, like in computers or high-efficiency power circuits.

• BJTs are still useful in simple circuits and analog amplifiers.

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In Simple Terms

• A BJT is like a water tap you open by letting a bit of water in — you need a small current to control a big one.

• A MOSFET is like a touch-sensitive light switch — just a bit of voltage (like your finger) can flip it on, no current needed.

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