NTT has developed a 300 GHz wireless transceiver with an integrated power amplifier (PA) that uses indium phosphide high-electron-mobility transistor (InP-HEMT) technology to support ultra-high-speed wireless communication beyond 5G. The design addresses two challenges in terahertz integrated circuits: RF signal loss caused by high-current DC wiring and unwanted electromagnetic-wave propagation through the semiconductor substrate. The amplifier was integrated into a wireless transceiver that demonstrated 120 Gbit/s data transmission over 9.8 m.
The research, published in NTT Technical Review, focuses on improving amplifier gain and output power at frequencies where small circuit structures can significantly affect signal transmission. At approximately 300 GHz, conventional chip layouts can introduce losses that reduce the RF power available for wireless transmission.
Key Research Findings
Reducing RF Loss with Backside DC Distribution
A high-power RF amplifier uses multiple transistors to increase the strength of a transmitted signal. NTT’s design combines several amplifier stages and parallel signal paths, requiring a total drain current of approximately 1.8 A. Supplying this current through conventional on-chip wiring requires wide DC metal lines, several hundred micrometers across.
In a conventional layout, long RF transmission lines must cross or overlap these wide DC conductors. At 300 GHz, the overlapping metal structures alter the electromagnetic environment around the RF lines, increasing transmission loss. As a result, part of the RF signal power is lost before it reaches the amplifier output, reducing both gain and available output power.
NTT addressed this problem using a backside DC line (BDCL) structure. Instead of routing the wide DC distribution line across the RF circuitry on the front of the chip, the researchers placed it on the backside of the InP substrate. This eliminates the problematic overlap between the RF transmission lines and the wide DC conductors while retaining the amplifier’s current supply.
Electromagnetic simulations showed that the backside DC distribution increased amplifier gain by approximately 3 dB across 280–320 GHz compared with the conventional layout. The improvement comes from reducing losses in the RF signal path rather than changing the fundamental operation of the transistors.
Suppressing Substrate Modes with Dense TSVs
Another challenge at 300 GHz is the propagation of unwanted electromagnetic waves inside the InP substrate. These are known as substrate modes. They can occur when the substrate thickness becomes comparable to the wavelength of the RF signal inside the semiconductor.
A conventional InP substrate is approximately 600 µm thick, while the wavelength of a 300 GHz signal inside InP is below 500 µm. This allows electromagnetic energy to propagate through the substrate and couple between parts of the circuit that should remain isolated. In a power amplifier, unwanted coupling from the output back to the input can create feedback and potentially cause oscillation.
NTT reduced the substrate thickness to 55 µm and introduced densely spaced through-substrate vias (TSVs). These metal-filled connections restrict the substrate regions through which unwanted electromagnetic waves can propagate.
(a) InP chip with substrate thinning and dense TSV formation, substrate-mode propagation for TSV pitches of (b) 100 μm and (c) 50 μm, Image Credit: NTT
Simulations compared TSV edge-to-edge spacings of 100 µm and 50 µm. The 50 µm spacing sufficiently suppressed substrate-mode propagation and was adopted for the amplifier design. This helped maintain isolation between amplifier ports and supported stable operation at 300 GHz.
Combining Eight InP-HEMT Amplifier Paths
The amplifier uses NTT’s 80 nm-gate InP-HEMT technology, incorporating a composite InGaAs channel. The transistors demonstrated a transition frequency of 300 GHz and a maximum oscillation frequency of 700 GHz, providing the high-frequency performance required for the circuit.
Each unit power amplifier consists of six cascaded common-source amplifier stages. The first three stages use two-finger HEMTs to provide higher gain, while the final three use four-finger HEMTs to handle greater RF power. Low-impedance interstage matching helps reduce losses between successive stages.
Eight unit amplifier paths are connected in parallel at the output stage to combine their RF power. This arrangement increases the total output capability while the backside DC distribution supplies the current required by the multiple transistor paths.
NTT packaged the amplifier in a metal module with WR3.4 rectangular waveguide interfaces covering 220–325 GHz. A ridge coupler transfers signals between the rectangular waveguide and the chip’s coplanar waveguide transmission lines. The measured coupling loss was below 1 dB across 220–303 GHz, helping preserve the RF power delivered to and from the amplifier.
(a) 300-GHz PA chip and (b) module.
Measured Gain and Output Power
The fabricated amplifier module achieved a maximum small-signal gain of 20.5 dB at 295 GHz. Its reverse transmission remained below -40 dB across the WR3.4 waveguide band, indicating strong isolation between the output and input.
Large-signal measurements showed a saturated output power of 12 dBm at 296 GHz, with an output 1 dB compression point of 6 dBm. The saturated output power remained above 10 dBm from 278 to 302 GHz. The below graphs (a) and (b) illustrate the characteristics.
These results demonstrate the combined contribution of the backside DC line, substrate-mode suppression, multi-path power combining and low-loss waveguide transition.
Demonstrating 120 Gbit/s Wireless Transmission
NTT integrated the amplifier into a 300 GHz-band heterodyne transceiver containing frequency mixers, local-oscillator amplifiers and receiver amplification circuitry. The system used a 270 GHz local oscillator, a 20 GHz intermediate frequency and an RF operating frequency around 290 GHz.
The researchers used 16-state quadrature amplitude modulation (16QAM), which carries four bits per symbol. In a back-to-back test, where the transmitter and receiver were connected through an attenuator rather than a wireless path, the system achieved 124 Gbit/s at 31 Gbaud.
For the wireless experiment, the transmitter and receiver used 50 dBi lensed horn antennas separated by 9.8 m. The system successfully demonstrated 120 Gbit/s wireless transmission using 16QAM at 30 Gbaud.
The experiment also used a variable attenuator to emulate the additional path loss of longer links. At 30 Gbaud, the measured signal-to-noise ratio supported an equivalent link distance of 10.5 m at the specified bit-error-rate threshold of 10-3. Lower symbol rates supported greater equivalent distances.
Test Setup and Evaluation of Wireless Link Performance
The results connect the amplifier’s circuit-level improvements with system-level wireless performance. By reducing RF loss in high-current DC distribution, suppressing unwanted substrate propagation and combining eight amplifier paths, NTT demonstrated a practical approach to generating the RF power required for high-data-rate communication around 300 GHz.
Measured SNR vs. baud rate of 300-GHz-band TRX in 9.8-m wireless-communication experiment.