What Are Schottky Gate Field Effect Transistors?

Schottky gate field effect transistors are semiconductor amplifying and switching devices in which a metal-semiconductor junction, rather than an oxide layer or p-n junction, controls the flow of current through a conducting channel. The gate contact forms a Schottky barrier with the underlying semiconductor, and varying the voltage on this gate modulates the width of the depletion region beneath it, thereby controlling channel conductance. This class of transistor is most widely known under the name metal-semiconductor field-effect transistor, or MESFET, and its commercial importance rests primarily on its realization in gallium arsenide and related III-V compound semiconductors, materials whose electron mobilities are several times that of silicon and that allow operation well into the microwave and millimeter-wave frequency bands.

Gate Structure and Operating Principle

In a Schottky gate field effect transistor, the gate metal is deposited directly on the surface of a lightly doped semiconductor channel layer without an intervening oxide. This direct metal-semiconductor contact forms a Schottky barrier whose depletion region extends into the channel. A negative gate voltage on an n-channel device widens the depletion region, reducing the thickness of the conducting layer and increasing channel resistance; a sufficiently negative voltage pinches the channel off entirely. Because the gate draws essentially no current in forward bias up to the forward conduction threshold of the Schottky diode, and because no oxide charging or hot-carrier effects are present, the structure achieves very low gate leakage and high transconductance. The absence of an oxide layer is both an advantage and a constraint: gate leakage increases if the gate is driven far into forward bias, setting a practical upper limit on the input voltage swing. Technical details of MESFET construction and operation in GaAs are documented in NASA JPL's microwave and millimeter-wave integrated circuit reference.

High-Frequency Performance

The primary reason for the dominance of Schottky gate field effect transistors in microwave electronics is the high electron mobility of gallium arsenide, approximately 8,500 square centimeters per volt-second at room temperature, compared with roughly 1,400 for silicon. Higher mobility translates directly into shorter electron transit times beneath the gate, which raises the cutoff frequency at which the transistor can amplify a signal. Production GaAs MESFETs routinely operate at frequencies up to 45 GHz, and related high-electron-mobility transistors, which share the Schottky gate structure while adding a heterojunction to further confine carriers, push useful gain into the millimeter-wave regime above 100 GHz. The electronics community's reference guide on MESFET and GaAs FET devices summarizes the frequency-capability comparison between GaAs MESFET and silicon MOSFET technologies across application bands.

Fabrication in Compound Semiconductors

Schottky gate field effect transistors are fabricated predominantly in III-V semiconductors: gallium arsenide for most commercial microwave circuits, indium phosphide for very high frequency and low-noise applications, and gallium nitride for high-power amplifiers where wide bandgap and high breakdown voltage are needed. Gate lengths, which determine transit time and thereby maximum frequency, have been scaled to sub-100-nanometer dimensions through electron-beam lithography. The semi-insulating substrate characteristic of GaAs eliminates parasitic conduction paths and simplifies integration of multiple transistors on a monolithic microwave integrated circuit chip. Nanohub instructional material on Metal-Semiconductor Field-Effect Transistors documents the relationship between gate length, doping profile, and device parameters across compound semiconductor material systems.

Applications

Schottky gate field effect transistors have applications in a range of systems, including:

• Low-noise amplifiers at the front end of microwave and satellite communication receivers
• Power amplifiers in cellular base stations and radar transmitters operating in the 1 to 40 GHz range
• High-speed digital logic in compound-semiconductor integrated circuits
• Oscillators and mixers in test and measurement instrumentation
• Monolithic microwave integrated circuits for military and aerospace radar systems