Compact and coherent source is a key component for various applications of the terahertz wave. We report on our recent results of terahertz oscillators using resonant tunneling diodes (RTDs). The RTD is an InGaAs/AlAs double-barrier structure on InP substrate, and integrated with a planar slot antenna as a resonator and radiator. The output power is obtained from the substrate side through a Si lens. To achieve high-frequency oscillation, a narrow quantum well and an optimized collector spacer thickness were used. The former reduces the electron dwell time in the resonant tunneling region and the latter simultaneously reduces the electron transit time and the capacitance at the collector depletion region. The conduction loss of the slot antenna was also reduced with an optimized antenna length and an improved air bridge structure between the RTD and antenna. By these structures, fundamental oscillation up to 1.92 THz were obtained at room temperature. Oscillation above 2 THz is further expected in theoretical calculation. An oscillator with patch antenna, in which a Si lens is unnecessary, was fabricated. In a preliminary experiment, output power of 55 μW was obtained at 1 THz in a three-element array. Wireless data transmission using direct intensity modulation was demonstrated with the data rate of 30 Gbp/s and the bit error rate below the forward error correction limit. By integrating a varactor into the slot antenna, electrical frequency tuning was achieved with a tuning range of 580-900 GHz in an array device. Application of frequency-tunable RTD oscillators to measurements of absorption spectra was also demonstrated.
Recent progress in room-temperature resonant-tunneling-diode (RTD) terahertz (THz) oscillators and high-electron-mobility- transistor (HEMT) THz receivers is reported in this paper. In this study, oscillations up to 1.86 THz were obtained using an optimized antenna and RTD. Using a two-element oscillator array, high output power of 0.6 mW at 620 GHz was obtained. THz communication up to 3 Gbps was demonstrated. A structure for high-speed direct modulation was fabricated, and the intensity modulation up to 30 GHz was achieved. A novel oscillator structure was proposed and fabricated for extraction of output power without using a Si lens. A short-gate InGaAs HEMT detector integrated with a broadband bow-tie antenna was fabricated, and a high current sensitivity of ~5 A/W was obtained at 280 GHz.
Our recent results of room-temperature THz oscillators using resonant tunneling diodes (RTDs) are reported. This
oscillator is composed of a GaInAs/AlAs double-barrier RTD and a planar slot antenna. The maximum oscillation
frequency in RTDs is limited by the electron delay time across the RTD layers, which consists of the dwell time in the
resonant tunneling region and the transit time across the collector depletion region. The dwell time was reduced by a
narrow quantum well, and a fundamental oscillation up to 1.31 THz with the output power of 10 μW was achieved at
room temperature. Further increase in oscillation frequency is expected by optimized size and materials of the well and
barriers for the dwell time and those of the collector depletion layer for the transit time. By these improvements, a
fundamental oscillation up to around 2 THz is theoretically possible. For high output power, coherent power combining
was demonstrated in a two-element array with offset slot antennas coupled with each other, and 610 μW at 620 GHz was
obtained. Spectral characteristics were measured with a heterodyne detection, and the linewidth of less than 10 MHz was
obtained. A frequency change of 1-5 % with bias voltage was also observed, which is attributed to the bias-dependent
dwell time. Direct intensity modulation and wireless data transmission were demonstrated. A transmission rate of 3 Gbps
with the bit error rate of 3×10<sup>-5</sup> was obtained at 540 GHz in a preliminary experiment, which is limited by the frequency
characteristics of external modulation circuits at present.