Bandwidth is everything, even for short-distance communications, and the terahertz frequency range promises a great deal of bandwidth to those who can realize practical communications devices to send and receive information. With that motivation, researchers at Northrop Grumman Corp., with funding from the DARPA THz Electronics Program and U.S. Army Research Laboratory (ARL) under DARPA contract HR0011-09-C-0062, developed receiver and transmitter front ends using a new generation, 25-nm indium phosphide (InP) high electron mobility transistor (HEMT) process capable of direct low-noise and power signal amplification at 850 GHz.
The receiver has two front-end low-noise amplifiers (LNAs) followed by a downconverting subharmonic mixer (SHM). The mixer translates the frequency band from 830 to 865 GHz to an intermediate-frequency (IF) range of 16 to 51 GHz, with the mixer driven by a local oscillator (LO) multiplier chain. The LO channel uses a 45.22-GHz input frequency which is then multiplied by a factor of nine to an output frequency of 407 GHz. The transmitter uses an upconverting SHM to translate frequencies from the IF range to the RF range. Three amplifiers are used to provide signal amplification and power gain, resulting in as much as −2.2 dBm output power from 830 to 865 GHz.
Measurements at these high frequencies were made with a number of test sets and accessories—mainly an 8510XF vector network analyzer (VNA) from Agilent Technologies (now Keysight Technologies) for microwave measurements from 10 to 110 GHz, along with WR1.5 and WR1.0 waveguide frequency extender test sets to make signal measurements at 500 to 700 GHz and 750 to 1000 GHz, respectively. The VNA measurements were calibrated using on-wafer thru-reflect-line (TRL) calibration kits.
To make this terahertz monolithic integrated circuit (TMIC) technology practical, the researchers realized the importance of developing a packaging technology that would enable low-loss transfer of signals from semiconductor chips to package waveguide input and output ports. The packaging approach does away from wirebonds and the losses they impose at such high frequencies. Transitions between the chip conductors and the package ports are formed by using a reactive-ion-etching (RIE) process.
The front-end receiver LNA is an impressive design of itself, using a 10-stage common-source configuration to achieve high gain at 850 GHz. Each stage employs a transistor with 10-μm gate periphery. Because of the short wavelengths at such high frequencies, the line lengths for the interstage matching networks are quite short—typically only 10 to 15 μm or about 19 to 30 deg. electrical length at 850 GHz. Coplanar-waveguide (CPW) transmission lines are used for the matching circuitry, with on-chip EM transitions used to directly interconnect the TMIC with the waveguide ports. The LNA exhibits peak gain of about 14.8 dB at 830 GHz and 13.6 dB at 850 GHz, with about a 12-dB noise figure. It draws 45 mA from a +1.2 V dc supply, for power dissipation of about 55 mW.
The TMIC is an impressive design and points to the possibilities of creating practical device solutions at terahertz frequencies for medical, industrial, and commercial communications applications. This particular device marks the highest-frequency integrated receiver and transmitter based on transistors (rather than on diodes). The researchers are aware of steps that can be taken to optimize performance, such as additional filtering to reduce LO leakage, but their work represents a major step towards the commercialization of THz frequency bands.
See “850 GHz Receiver and Transmitter Front-Ends Using InP HEMT,” IEEE Transactions on Terahertz Science and Technology, Vol. 7, No. 4, July 2017, p. 466.