Modeling and Analysis of Bow-Tie Antenna Integrated Resonant-Tunneling-Diode Relaxation Oscillators for Wireless Radio Applications

We propose a self-complementary bow-tie antenna-integrated resonant-tunneling-diode relaxation oscillator and investigate its oscillation/radiation characteristics. In the investigation, we establish a physics-based equivalent circuit model of the oscillator for taking the all physical phenomena related to the diode and the antenna into consideration simultaneously. In this paper, we report the equivalent circuit modeling and the large-signal oscillation/ radiation analysis of the oscillator. Figure 1 Schematics of a proposed oscillator. Figure 2 Block expressions of an equivalent circuit for the oscillator shown in Figure 1. doi: 10.11605/cp.nmc2017.01001 Nano-Micro Conference Published by Nature Research Society http://nrs.org Nano-Micro Conf., 2017, 1, 01001 | 2 posed of several RLC elements which can be explained by the electromagnetic properties: the surface impedance due to the skin effect [5], the straight micro stripline [6], fringe capacitance [7], and parasitic components regarding the semi-insulating substrate. The circuit elements involved in Zf(ω,D,wshunt) and Zb(ω,D,wshunt) are evaluated by the EM field distribution in the vicinity of the circuits calculated by the finite element method-based simulator, namely, COMSOL. Their approximate numerical values of the elements are also estimated by the physical interpretation based on the structural and material properties. More precise values are numerically de-embedded by using the optimization method, namely, the particle swarm optimization [8]. More details regarding the circuit identification process have also been reported in Ref. [2]. Figure 3 summarizes the typical fitting results of the imFigure 3 Typical fitting results regarding the impedance characteristics of (a) the peripheral circuit in front, (b) that in back, and (c) the entire oscillator, and (d) the radiation characteristics. Radiation efficiency, η(ω,D,wshunt), is defined by the ratio of the oscillation power on the semiconductor layers to the radiation power on the BTA. Solid lines are depicted by using the de-embedded values of the circuit parameters. Dash lines are calculated by the physical/material parameters. The antenna size, D, and the line width, wshunt, are set to 300 and 10 μm, respectively. Figure 4 (a) Measured/theoretical current density-voltage (J-V) characteristics of the employed RTD and the calculated results of the timedependent orbit of the current density, irtd(t)/S, in the RTD. (b) Waveforms of the radiation voltage and (c) corresponding frequency spectra. Chain and dotted lines indicate the relaxation and the sinusoidal oscillation mode, where the line width, wshunt, is set to 10 and 20 μm, respectively. Antenna size, D, is set to 300 μm. doi: 10.11605/cp.nmc2017.01001 Nano-Micro Conference Nano-Micro Conf., 2017, 1, 01001 | 3 Published by Nature Research Society http://nrs.org pedance and radiation characteristics regarding the peripheral circuit/entire oscillator. From Figure 3, the equivalent circuit expression is quantitatively valid for the evaluation of the radiation/oscillation characteristics by the circuit analysis around the first resonance frequency of the oscillator. Oscillation Analysis The non-linear oscillation analysis of the proposed oscillator is performed by using the equivalent circuit above mentioned. The non-linearity of the RTD is involved by considering the measured current-voltage (I-V) charact-eristics [9]. The supplied voltage, Vb, is set to a constant value to keep the negative differential conductance of the RTD maximum. More details regarding the oscillation analysis methodology have been reported in Refs. [10, 11]. Figure 4 displays the current density-voltage (J-V) charact-eristics of the RTD and the time-dependent orbit of the current density, irtd(t)/S, in the RTD, where S indicates the mesa area. We classify the oscillation modes depicted the dotted and chain lines in Figure 4 designated as the "sinusoidal" and the "relaxation" mode, respectively. The modes are quantitatively distinguished by the cycle number of the irtd(t)/S trajectory in Figure 4; if the cycle number is unity, the mode is a “sinusoidal mode”; else, it is a “relaxation mode’’. The entire emission RF power of the relaxation mode shown in Figure 4 is ~ 6 dBm greater than that of the sinusoidal mode when the upper limit value of band is approximately set to 400 GHz. It is suggested that the relaxation wave can compensate for the shortage of the emission RF power if we employ a certain band appropriately. Moreover, it is found that the oscillation mode can be designed by adjusting the two parameters, D and wshunt, appropriately. According to Shannon-Hartley theorem [12], the capacity of a wireless channel is directly proportional to the channel bandwidth. Therefore, the wideband-spectrum relaxation wave can contribute to the possibility of large-capacity wireless transmissions together with compensating for the RF