## 3 Package and Measurement of RTD  
The packaged RTD configuration with WR10 waveguide output and an SMA connector is shown in Figure 5. The packaged RTD consisted of a DC bias microstrip line (MSL), an RTD chip, an E plane probe transformer, and a metallic cavity with WR 10 waveguide output. The DC bias MSL that was connected to an SMA connector was used to bias the RTD. The MSL was designed on the Rogers 6010 substrate with a relative permittivity of 10.2 and a thickness of 127 \(\upmu\)m. The width of the MSL was 0.1 mm to match with the 50 \(\Omega\) SMA connector. The RTD chip was the same as in the one used in the literature [14], and the indium phosphide (InP) substrate height of the RTD was 600 \(\upmu\)m. The E plane probe transformer was used to radiate the RF signal of the RTD to the WR 10 waveguide. By using gold wire bonding, the signal and ground traces of the RTD were connected to the E plane probe transformer and the DC MSL was bonded to the DC trace of the RTD. A metallic package cavity with a WR-10 standard waveguide output was fabricated. We etched three slots in the cavity and used a conductive silver glue to fix the transformer, the RTD chip, and the DC bias MSL inside the cavity. The photograph and micrograph of the fabricated packaged RTD is shown in Figure 6.  
Figure 7 shows the I-V characteristics of the packaged RTD. The RTD chip in the literature [14] consists of two RTDs that are connected in parallel, and each device is biased individually with resistor \(R_{e}\). Because of the additional current through the shunt resistor that is connected in parallel with the RTD, a negative differential resistance (NDR) region was not observed in the I-V curve [13]. Instead, a small decline of 1.05 V was observed, and 1.05 V was the bias point of the RTD. As proposed in [14], by changing the DC voltage, an RTD can operate in negative differential resistance region or positive resistance region. Therefore, an RTD can be biased to switch between the on and off states with OOK modulation. The voltage-dependent frequency and output power of RTD is shown in Figure 8. The oscillator frequency ranged from 88 to 92 GHz, and the maximum RF output power was \(-8.6\) dBm. When the losses of the cable, the SMA adapters, and the bias-T were removed, the actual output power was at least \(-7\) dBm. Figure 9 shows the output spectrum at 90 GHz when DC = 1.2 V.  
The carrier frequency of the RTD with a waveguide output increased by 15 GHz compared with the RTD chip, an effect that was mainly caused by gold wire bonding. The packaged RTD equivalent circuit, including gold wire bonding, is shown in Figure 10. The RTD oscillator circuit part was the same as [15], and \(-\)G1n and \(-\)G2n were the negative differential conductances of RTD1 and RTD2, respectively, while C1n and C2n were the self-capacitances of each RTD. The gold wire was equivalent to series inductance and parallel capacitance. Generally, the effect of shunt capacitors is negligible due to their smaller value. Oscillator frequency can be determined by equating the admittance imaginary part to zero, as shown in Equation (1):  
Figure 4: Simulated and measured S-parameters of the transition. \[f=\frac{1}{2\pi\sqrt{(C_{1n}+C_{1n})\frac{L_{L_{g}}}{L+L_{g}}}} \tag{1}\]  
where \(L_{g}\) is the series inductance of gold wire. It can be seen that the total inductance in the circuit was decreased compared with that of [15], which led to the increased carrier frequency. Figure 5: The packaged resonant tunneling diode (RTD) configuration. Figure 6: Photograph and micrograph of the RTD with a rectangular waveguide output. Figure 8: Measured voltage-dependent frequency and output power of the RTD. Figure 7: The I–V characteristics of the packaged RTD. Figure 6.0 Measured output spectrum of the RTD oscillator. Figure 11 shows the output spectrum with OOK modulation at 90 GHz when DC = 1.2 V. The modulation signal was a pseudorandom bit sequence (PRBS) with repetition length 215-1, and the data rate was 4 Gbps.