In this paper the potentiality of impact avalanche transit time (IMPATT) devices based on different semiconductor materials such as GaAs, Si, InP, 4H-SiC and Wurtzite-GaN (Wz-GaN) has been explored for operation at terahertz frequencies. Drift–diffusion model is used to design double-drift region (DDR) IMPATTs based on different materials at millimeter-wave (mm-wave) and terahertz (THz) frequencies. The performance limitations of these devices are studied from the avalanche response times at different mm-wave and THz frequencies. Results show that the upper cut-off frequency limits of GaAs and Si DDR IMPATTs are 220 GHz and 0.5 THz, respectively, whereas the same for InP and 4H-SiC DDR IMPATTs is 1.0 THz. Wz-GaN DDR IMPATTs are found to be excellent candidate for generation of RF power at THz frequencies of the order of 5.0 THz with appreciable DC to RF conversion efficiency. Further, it is observed that up to 1.0 THz, 4H-SiC DDR IMPATTs excel Wz-GaN DDR IMPATTs as regards their RF power outputs. Thus, the wide bandgap semiconductors such as Wz-GaN and 4H-SiC are highly suitable materials for DDR IMPATTs at both mm-wave and THz frequency ranges.

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Prospects of IMPATT devices based on wide bandgap semiconductors as potential terahertz sources

Basic principles of operation, fundamental power-generation capabilities, and fabrication technologies are reviewed for three groups of two-terminal devices, i.e., resonant-tunneling diodes (RTDs), transferred-electron devices (TEDs), and transit-time diodes. The paper focuses on devices for frequencies above 30 GHz, and an overview of recent research in this area and of various state-of-the-art laboratory results is given. As an outlook, the potential of some new material systems for high-power devices is discussed.

Two-terminal millimeter-wave sources

A selective etching technology has been established for 94 GHz GaAs IMPATT diodes operating on diamond heat sinks. Using this technology diodes were realized delivering an output power of 270 mW. The corresponding efficiency was 5.7% at an oscillation frequency of 95 GHz.

Selective etching technology for 94 GHz GaAs IMPATT diodes on diamond heat sinks

CW GaAs double-Read IMPATT diodes for D-band frequencies are designed and tested. For reproducible RF impedance matching, the module encapsulation technique is applied. Ohmic losses of the active device are reduced by a titanium-Schottky contact instead of an alloyed ohmic n/sup +/-contact. At 144 GHz 100 mW RF power with a conversion efficiency of 5% is realised.

140 GHz GaAs double-Read IMPATT diodes

The broad applications of ultrawide-band signals and terahertz waves in quantum measurements1,2, imaging and sensing techniques3,4, advanced biological treatments5, and very-high-data-rate communications6 have drawn extensive attention to ultrafast electronics. In such applications, high-speed operation of electronic switches is challenging, especially when high-amplitude output signals are required7. For instance, although field-effect and bipolar junction devices have good controllability and robust performance, their relatively large output capacitance with respect to their ON-state current substantially limits their switching speed8. Here we demonstrate a novel on-chip, all-electronic device based on a nanoscale plasma (nanoplasma) that enables picosecond switching of electric signals with a wide range of power levels. The very high electric field in the small volume of the nanoplasma leads to ultrafast electron transfer, resulting in extremely short time responses. We achieved an ultrafast switching speed, higher than 10 volts per picosecond, which is about two orders of magnitude larger than that of field-effect transistors and more than ten times faster than that of conventional electronic switches. We measured extremely short rise times down to five picoseconds, which were limited by the employed measurement set-up. By integrating these devices with dipole antennas, high-power terahertz signals with a power-frequency trade-off of 600 milliwatts terahertz squared were emitted, much greater than that achieved by the state of the art in compact solid-state electronics. The ease of integration and the compactness of the nanoplasma switches could enable their implementation in several fields, such as imaging, sensing, communications and biomedical applications.

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Nanoplasma-enabled picosecond switches for ultrafast electronics

High-performance GaAs double-drift Read IMPATT diodes have been demonstrated at 60 GHz. 1.24-W CW output power at 11.4-percent dc to RF conversion efficiency was obtained with a junction temperature rise of 225°C. The doping profiles and test circuits have not yet been optimized and we expect that still higher power and efficiency should be achievable.

M.G. Adlerstein

Papers

GaAs IMPATT diodes for 60 GHz