Much has been written recently about the growing need for components, devices, and circuits at millimeter-wave (mmWave) frequencies from 30 to 300 GHz. The need is fueled by applications such as advanced driver-assistance systems (ADAS) and 5G cellular communications systems that are taking advantage of the wide bandwidths available at those frequencies, especially when compared to the very occupied bandwidths at lower RF and microwave frequencies.
But what happens when the mmWave range also becomes congested? Researchers from the Department of Electrical Engineering and Computer Science at the Massachusetts Institute of Technology (Cambridge, Mass.) are looking ahead to when even higher-frequency devices will be in demand, in the terahertz (THz) frequency range, and whether available semiconductor technologies can provide such devices to serve THz applications.
Because of the need to generate and detect THz-range frequencies (300 to 3000 GHz), transistors fabricated on silicon (Si) or silicon-germanium (SiGe) materials cannot simply be scaled or miniaturized to achieve sufficiently high maximum frequency of oscillation (fmax) to generate those frequencies. Silicon-based devices have been used for amplifier designs at frequencies of about 0.2 to 0.3 THz, but signal generation at frequencies above that range—when using silicon-based devices—requires harmonic generation.
Despite the limitations of silicon-based substrate materials, the researchers point to several innovative device designs on SiGe to hint at the possibilities of silicon for THz applications. This includes a 1-THz radiation source based on 130-nm SiGe heterojunction-bipolar-transistor (HBT) technology. The device integrates 91 coherent radiators within a 0.1-mm2 chip area, with a multifunctional slot resonator that has a fundamental oscillation frequency (f0) of 250 GHz. Radiation is used at multiples of the fundamental frequency to produce the 1-THz output frequency.
The high density of such coherent radiation arrays, when fabricated on relatively small chip sizes of about 10 mm2, yield devices with relatively narrow bandwidths at 1 THz. Material parameters such as consistency of thickness and dielectric constant are critical to achieving performance consistency at 1 THz. But a THz laser chip of this type could provide the signal generation and sensing functions needed for practical short-range communications (within tens of meters) at extremely high data rates for wireless communications systems beyond the mmWave frequency range.
See “Filling the Gap,” IEEE Microwave Magazine, April 2019, pp. 80-93.