IEDM2024|Research Progress on III-V Materials for RF Devices

Introduction

III-V materials (particularly InGaAs and InAs) possess unique material properties that enable a wide range of applications in radio frequency (RF) electronic devices. These materials feature a low effective mass, which reduces intrinsic capacitance and extends the electron mean free path, enabling low-power and high-frequency operation. They are also applicable in quantum technologies. Indium phosphide high electron mobility transistors (InP HEMTs) have demonstrated outstanding RF performance, with a maximum oscillation frequency exceeding 1 THz. However, further device scaling remains challenging. Integrating high-k gate oxide layers for gate insulation offers a new direction for enhancing device performance and continued scaling [1].

Physical Modeling and Capacitance Analysis

To evaluate the intrinsic RF performance of two-dimensional III-V field-effect transistors, a simplified yet accurate triple-capacitance model can be used, incorporating near-ballistic transport characteristics. In the saturation regime, the intrinsic gate capacitance can be modeled using three series-connected capacitances: the oxide/barrier capacitance (Cox), the quantum/state density capacitance (Cq), and the charge centroid capacitance (Cc). For scaled InGaAs field-effect transistors, all three components significantly impact the total gate capacitance (Cg).

Figure 1 illustrates: (a) the intrinsic and parasitic capacitances of a non-quasi-static small-signal model, (b) fundamental equations for the intrinsic gate capacitance of a quantum well FET, and (c) RF gain model calculation results.

Low-Temperature Performance

Operating III-V metal-oxide-semiconductor field-effect transistors (MOSFETs) at cryogenic temperatures induces significant changes in transistor physics. Electron mobility strongly correlates with the two-dimensional carrier concentration. As electron kinetic energy decreases, scattering increases, causing mobility to drop nearly linearly with carrier density. Additionally, band tail effects lead to blurred band-edge state densities, limiting the minimum subthreshold swing achievable at low temperatures.

Figure 2 shows: (a) changes in low-field mobility at various temperatures, (b) long-channel current and charge characteristics obtained through magnetoresistance measurements, and (c) the minimum subthreshold swing behavior under the influence of band tail effects.

Device Architecture and Implementation

Researchers have developed various III-V MOSFET structures, including planar, lateral nanowire, and vertical nanowire configurations. For lateral FETs, devices fabricated via epitaxial regrowth exhibit excellent intrinsic performance, featuring high transconductance (gm) and low on-resistance (Ron), though the fabrication process remains complex.

Figure 3 provides an overview of III-V FET structures, including: (a) raised n+ structure, (b) TEM view of a planar device, (c) SEM image of a lateral nanowire, (d) MOSHEMT structure, (e) top-down view of a MOSHEMT, (f) vertical nanowire design, and (g) cross-sectional image.

Performance Metrics and Comparison

Recent advancements in III-V MOSFETs have achieved remarkable results. Optimized MOSHEMT structures have attained gate-drain capacitance (Cgd,t) as low as ~0.15  fF/μm, comparable to III-V high electron mobility transistors. Although fabrication-induced damage to the channel may reduce intrinsic transconductance, the reduced gate-drain capacitance contributes to a high MSG20 gain of 19  dB.

Figure 4 presents the performance characteristics, including: (a) MOSHEMT structure, (b) transfer characteristics, (c) capacitance measurements, (d) microwave gain, (e) non-quasi-static effects, and (f) influence of drain-source voltage on gain.

Future Directions

III-V MOSFETs and HEMTs exhibit excellent performance in RF applications and are approaching the performance level of InP HEMTs. The integration of high-k oxide layers on group III nitrides offers new opportunities in power electronics, particularly in vertical structures. Moreover, these devices hold potential in quantum technologies, including integration with superconducting components.