The terahertz（THz）frequency band（0.3~10 THz）has attracted fast-growing research interests due to its many possible applications，such as imaging systems for radar or security imaging，spectroscopy in chemistry and biotechnology and high-data-rate communications ^［1-6］. The compact and coherent solid-state THz signal sources are considered to be the key components for these applications. Compared with any other traditional electronic devices such as field-effect or bipolar transistors（MOSFETs，HEMTs or HBTs），or diode technologies（IMPATT，Gunn，etc.），the resonant tunneling diodes（RTDs）have also been considered as one of the most promising candidates for THz oscillators at room temperature ^［7-10］. The advantages of RTDs include the facts that they can exhibit the highest oscillation frequency of the inherent negative differential conductance（NDC）due to the resonant tunneling among the electronic devices and consume low power，as well as the output power is easily modulated through the bias network and can operate at room temperature.

In this paper，we propose and fabricate a THz oscillator with InP-based RTD and dielectric lens antenna. The influences of material structure including the doping concentration of emitter，the thickness of barrier layer，space layer，and well layer on the DC characteristics are analyzed by Silvaco software. Excellent DC characteristics of the RTD are demonstrated with J_p = 359.2 kA/cm²，J_v = 135.8 kA/cm²，and PVCR = 2.64. The oscillator is packaged with Si-lens and exhibits the output power of 2.57 μW at an operation frequency above 1 THz.

RTDs are commonly realized in GaAs and InP. Compared with GaAs，InP material system offers better properties in the conduction band offset，the electron effective mass. In_0.53Ga_0.47As can also be highly doped when used as the RTD device electrodes to reduce the Ohmic contact resistance. The material system for the RTD device utilized in this project is InGaAs/AlAs. InP-based RTD device is typically consisted of a narrow bandgap material（4∼6 nm thick quantum well）sandwiched between two thin wide bandgap materials（1∼3 nm AlAs barriers），making up a double barrier quantum well（DBQW）structure. The structure is completed by lightly doped InGaAs spacer layers，n-type emitter/collector layers，and highly doped InGaAs contact layers on each side of the DBQW，as shown in Fig. 1.

The 2D structure of the device is defined in Deckbuild based on the material structure as shown in Fig. 1 by using the Atlas，which a device simulation product originating simulated from Silvaco. The relationship between the epi-layer structure and DC characteristics has been researched on the doping concentration of emitter，the thickness of barrier layer（t_B），well layer（t_W）and spacer layer（t_S）. In order to observe the relationship between the material structure and DC characteristics more intuitively，the peak current（I_P）and peak voltage（V_P）of the device are mainly analyzed，as shown in Fig. 2. The simulation results in Fig. 2 shows that the performance of RTD devices is closely related to the epitaxial material structure. In order to realize a high-power，high-frequency RTD device，the electrical characteristics of the device is optimized mainly by optimizing the negative differential conductance region（NDR）in the I-V curve in this design. After optimization，the RTD material structure designed in this study is shown in Table 1.

The RTD epi-material is grown on a 3-inch wafer using molecular beam epitaxy（MBE）. The photograph of the manufactured InP-based RTD device with an emitter area of 12 μm² is shown in Fig. 3（a）. Characterization of the device was done by on-wafer measurements with B1500A Semiconductor Device Parameter Analyzer at room temperature. The on-wafer measured I-V characteristic of the RTD device is depicted in Fig. 3（b）. The RTD shows peak voltage V_p=0.91 V，peak current I_p=43.1 mA，peak current density J_p=I_p/A=359.2 kA/cm²，and peak-to-valley current ratio PVCR=I_p/I_v=2.64.

Figure 4 shows the small-signal equivalent circuit of the RTD. The total series resistance（R_s）of the diode includes ohmic contact resistance，spreading resistance，and resistance due to the epitaxial layer. The R_s ≈4.8 Ω is estimated from transmission line measurements（TLM）. The self-capacitance（C_n）represents the charging and discharging effect of electrons at the semiconductor depletion regions. The C_n ≈11 fF is measured by B1500A. The quantum-well inductance（L_qw）represents the electron dwell time in quantum well and will not limit the oscillator frequency ^［11］.

The frequency dependent impedance of the RTD is given by Eq. 1.

The maximum oscillation frequency（f_max）of the RTD is defined as the frequency where R（f） becomes zero，as given by Eq. 2.

Equation 2 simplifies to Eq. 3 when L_qw is assumed to be negligibly small.

The RTD negative conductance（G_n）is given by

where △I=I_p-I_v，△V=V_p-V_v.

The maximum available RF output power（P_max）that a RTD-based oscillator can deliver to the load，which is given by

According to Eqs. 3-5，the theoretical f_max of the RTD is evaluated to be 1.49 THz and the theoretical P_max=1.71 mW. The DC measurement results of the RTD device and theoretical oscillation frequency and maximum RF output power are summarized in Table 2.

Bow-tie antenna is a widely used wideband antenna in terahertz band. The proposed bow-tie shape THz antenna is shown in Fig. 5（a）and patterned over 25 µm InP substrate. Due to the lower gain of the proposed antenna，a silicon lens is mounted over the antenna as depicted in Fig. 5（b）. The simulation of the proposed antenna is carried out in CST Microwave Studio. Fig. 6 shows the variation of the Voltage Standing Wave Ratio（VSWR）and gain of the proposed antenna with Si-lens which indicates more than 200 GHz impedance bandwidth spanning from 0.9 to 1.1 THz and13.7 dBi gain.

Figure 7 presents the package configuration of the quasi-optical RTD oscillator. It consists of RTD oscillator by integrating RTD with a bow-tie antenna，a highly resistive Si-lens and a low-frequency PCB. The fabricated chip is mounted on Si-lens and wire-bonded with Au wires with a diameter of 50 μm onto PCB for the measurements of the oscillation characteristics.

Figure 8 shows the RTD oscillator packaged with PCB and the detail of RTD oscillator. The THz signal is generated from the RTD oscillator and radiate into air through Si-lens.

In order to characterize the performances of the fabricated oscillator，the room temperature measurement is performed and the block diagram of the measurement scheme is shown in Fig. 9（a）. Due to lacking 1 THz spectrum analyzer，0.75~1.1 THz horn antenna and WR0.65（1.1~1.7 THz）convert waveguide are used with PM5 for measuring the output power and frequency as shown in Fig. 9（b）. The best performances of the fabricated oscillator are obtained at an applied bias voltage of 0.95 V with a current of 8.7 mA. A peak output power of 2.57 μW is obtained above 1 THz. The DC power consumption of the RTD oscillator is 8.33 mW，while the DC-to-RF power efficiency is 0.0308% at the operation frequency.

A THz RTD oscillator is designed and fabricated based on InP RTD MMIC technology. The RTD device shows theoretically calculated maximum RF output power of 1.71 mW and f_max of 1.49 THz. The oscillator shows an output power of 2.57 μW with a DC power consumption of 8.33 mW above 1 THz. This work shows the potential of the RTD oscillator topology as a practical THz signal source for future THz applications. To our knowledge，this is the first reported oscillator of frequency above 1 THz in domestic.