Master the Basics: What is a Field-effect Transistors

Master the Basics: What is a Field-effect Transistors

Unlike bipolar junction transistors (BJTs) that are current-controlled, field-effect transistors (FETs) are voltage-controlled. This guide introduces the most common FET type—MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors)—and how they’re used to control current in modern electronics.

What Is a MOSFET?

A MOSFET works by using an electric field to control the conductivity of a semiconductor material. The three terminals on a MOSFET are:

  • Gate (G): control input
  • Source (S): reference terminal (typically tied to ground or V+)
  • Drain (D): output path for current

The voltage between the gate and source (VGS) determines whether the transistor is ON or OFF.

Types of MOSFETs

There are four main types, but two are most common:

  • N-channel Enhancement Mode (normally OFF, turns on with positive VGS)
  • P-channel Enhancement Mode (normally OFF, turns on with negative VGS)

Less common types:

  • N-channel Depletion Mode (normally ON)
  • P-channel Depletion Mode (normally ON)

We’ll focus on enhancement mode MOSFETs since they are widely used in digital and switching applications.

How an N-Channel MOSFET Works

  1. The source is connected to ground.
  2. The drain connects to the load and a positive power supply.
  3. When VGS > threshold voltage, an electron-rich channel forms, turning the transistor on.
  4. As VGS increases, the channel becomes more conductive.
  5. When VGS < threshold, the transistor is off.

The rDS(on) (resistance between drain and source when fully on) is a key spec, often less than 100 milliohms for low-power MOSFETs.

N-Channel vs. P-Channel

  • N-Channel MOSFETs: Turn on with positive VGS, usually drawn with source at the bottom
  • P-Channel MOSFETs: Turn on with negative VGS, usually drawn with source at the top

In both cases, current flows through the drain, which is typically connected to the load.

Enhancement vs. Depletion Mode

  • Enhancement mode: Normally off, requires VGS to turn on
  • Depletion mode: Normally on, requires VGS to turn off

MOSFET Symbols

MOSFET symbols vary but usually include:

  • Three terminals: gate, source, drain
  • An arrow indicating conventional current flow
  • A broken channel line for enhancement mode (normally off)
  • A solid channel line for depletion mode (normally on)
  • Sometimes a fourth terminal for the body (bulk), typically tied internally to the source

A diode symbol between drain and source may also appear to indicate internal ESD protection.

Controlling MOSFETs

MOSFETs are voltage-controlled, meaning they draw almost no current at the gate. This makes them ideal for microcontroller-based circuits where current availability is limited.

  • N-channel: ON when gate voltage is above source
  • P-channel: ON when gate voltage is below source

If gate and source voltages are equal, the MOSFET is off.

Packages

MOSFETs come in common packages like:

  • TO-92: small, good for breadboarding
  • TO-220: large, used for power MOSFETs with heatsinks

Key Takeways

MOSFETs are incredibly versatile, efficient, and widely used in modern electronics for switching and amplification. Compared to BJTs, they:

  • Operate with very high input impedance
  • Are easier to control from microcontrollers
  • Allow for more efficient switching

By learning how N- and P-channel enhancement mode MOSFETs work, you’ll be equipped to build and understand a wide variety of electronic circuits with ease.

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