What Are MODFET Circuits?

MODFET circuits are electronic circuits built around the modulation-doped field-effect transistor, a compound semiconductor device that achieves exceptionally high electron mobility by spatially separating the conducting channel from its ionized dopant atoms. In a conventional FET, electrons accelerated by the gate field scatter frequently off the doping impurities in the channel, limiting mobility and therefore speed. The MODFET resolves this by placing the dopants in a wider-bandgap layer, such as AlGaAs, adjacent to an undoped narrower-bandgap layer, such as GaAs; electrons diffuse into the undoped layer and form a two-dimensional electron gas at the heterojunction, where mobility is dramatically higher because impurity scattering is absent. Circuits exploiting this property operate at microwave and millimeter-wave frequencies with noise figures and gain characteristics that are unavailable in silicon or conventional compound-semiconductor devices.

The device is also known as a high-electron-mobility transistor (HEMT) or heterostructure FET (HFET). Its invention is attributed to Takashi Mimura at Fujitsu, who conceived the structure in 1979 after reading about modulation-doped heterojunctions developed at Bell Labs, and the first MODFET circuits for supercomputer applications were demonstrated using GaAs/AlGaAs in the early 1980s.

Device Structure and Operating Principle

The MODFET structure consists of a semi-insulating substrate, typically GaAs or InP, on which epitaxial layers are grown by MOCVD or molecular beam epitaxy. The active layer sequence includes a spacer layer of undoped wide-bandgap material between the doped supply layer and the channel, which reduces Coulomb scattering from ionized donors. The gate electrode is placed directly above the heterojunction channel, and its voltage controls the two-dimensional electron gas density through the thin barrier layer. Because the channel electrons are confined to a quantum well at the heterointerface, the effective mass is low and the saturation velocity is high, giving the device superior transconductance per unit gate width compared to silicon MOSFETs at equivalent gate lengths. Pseudomorphic HEMTs, which use an InGaAs channel strained to the GaAs substrate, achieve still higher electron velocities and dominate low-noise amplifier applications in the frequency range from 2 GHz to over 100 GHz.

High-Frequency and Low-Noise Performance

The principal figures of merit for MODFET circuits are the unity-current-gain frequency (fT) and the maximum oscillation frequency (fmax), both of which scale inversely with gate length and directly with carrier velocity. Sub-100-nanometer gate-length InGaAs HEMTs have demonstrated fT values exceeding 600 GHz, placing them among the fastest three-terminal devices made. For low-noise amplifier design, the minimum noise figure of a MODFET circuit decreases with increasing fT; at 60 GHz, properly designed pseudomorphic HEMT amplifiers achieve noise figures below 2 dB with several decibels of associated gain. GaN-on-SiC HEMTs occupy a different performance space: the wide bandgap and high breakdown field of gallium nitride allow high-power density HEMT circuits for radar and communications to operate at drain voltages of 28 to 50 volts, delivering power densities of 10 to 40 watts per millimeter of gate width.

Circuit Topologies

MODFET circuits encompass low-noise amplifiers (LNAs), power amplifiers, oscillators, mixers, and switches, each exploiting the device's specific capabilities. LNA design uses single or cascode MODFET stages biased at low drain current to minimize noise contribution while providing sufficient gain. Power amplifiers use large-periphery devices with combined gate widths of millimeters to achieve watt-level output, often with multiple transistors combined using Wilkinson or Doherty combining networks. JEDEC's definition of the modulation-doped field-effect transistor establishes the standardized terminology used across the semiconductor industry for these circuit elements.

Applications

MODFET circuits have applications in a wide range of microwave and millimeter-wave systems, including:

• Low-noise amplifiers for satellite receivers, radio astronomy front ends, and deep-space communication ground stations
• Power amplifiers in cellular base station transmitters and active phased-array radar systems
• Millimeter-wave imaging and automotive radar sensors operating at 77 GHz and 94 GHz
• High-efficiency power stages in point-to-point microwave backhaul links
• Oscillator and frequency synthesis circuits in electronic warfare receivers