Semiconductor Technology from A to Z

Everything about semiconductors and wafer fabrication

1. Shared Operating Principle

All field-effect transistors (FETs) control a conductive channel between two terminals (source and drain) via an electric field generated by a third electrode (gate). Unlike the bipolar transistor, where a base current controls the collector current, a FET draws practically no steady-state gate current – control is purely voltage-driven, through the gate-source voltage VGS. The three main FET families differ mainly in how the gate is electrically isolated from the channel:

  • MOSFET (Metal-Oxide-Semiconductor FET): gate separated from the channel by a thin oxide layer (insulator, e.g. SiO2 or a high-k dielectric)
  • JFET (Junction FET): gate separated from the channel by a reverse-biased pn junction
  • MESFET (Metal-Semiconductor FET): gate separated from the channel by a Schottky contact (a direct metal-semiconductor junction with no intermediate layer)

The classic MOSFET structure is described in detail in Construction of a Field-Effect Transistor; the other two families are covered in the following chapters.

2. Depletion-Mode vs. Enhancement-Mode

A second important distinction is the operating mode. Depletion-mode FETs (“normally-on”) already have a conductive channel at VGS = 0 V; an applied gate voltage progressively pinches this channel off. Enhancement-mode FETs (“normally-off”) have no channel at VGS = 0 V – one must first be induced by a gate voltage. While MOSFETs can be built in either mode (enhancement-mode dominates in digital logic), JFETs operate exclusively in depletion mode by design. MESFETs are predominantly fabricated as depletion-mode devices (D-MESFETs), though enhancement-mode variants (E-MESFETs) also exist for digital GaAs logic.

3. Comparison Table

The table below compares the three FET families directly:

FeatureMOSFETJFETMESFET
Isolation mechanismOxide (SiO2/high-k)pn junction (depletion layer)Schottky contact
Available operating modesEnhancement & depletionDepletion onlyPredominantly depletion, some enhancement
Gate driveIsolated gate, practically no gate currentGate reverse-biased, small saturation leakage currentGate reverse-biased, small Schottky leakage current
Input impedanceExtremely highHighHigh
Typical materialSi (also high-k/metal-gate)Si, GaAsGaAs, InP (III-V)
Cutoff frequency / noiseGood, but oxide-related 1/f noiseVery low noiseVery high cutoff frequency
Main applicationDigital/analog circuits, power electronicsLow-noise analog amplifiers, current sourcesRF/microwave technology

4. Context and Outlook

Historically, the JFET was among the first practically realized field-effect transistors, but it was quickly displaced in digital logic by the MOSFET, whose oxide isolation allows far greater scaling. The MESFET, in turn, arose from the need to realize FET principles in III-V semiconductors lacking a stable native oxide, and long dominated GaAs RF technology – though in many of these applications it is increasingly being replaced today by the more capable HEMT (High-Electron-Mobility Transistor).

Inhalt