In this paper, a two-dimensional (2D) model of the terminating-type power sensor is established under different input powers. The 2D heat transfer equation is applied to describe the temperature distribution, and Fourier series is used to obtain the solution based on the boundary conditions. In order to demonstrate the validity of the 2D model, finite-element method (FEM) simulation using ANSYS software was performed. The sensitivity from the 2D model and FEM is 0.25 mV mW −1 and 0.28 mV mW −1 respectively, while the sensitivity from the 1D model is 0.34 mV mW −1 , which indicates that the presented 2D model is closer to the simulation than the 1D model. The terminating-type power sensor was designed and fabricated by MEMS technology and the GaAs MMIC process. The measured return loss is less than −26 dB for a frequency up to 10 GHz. The power measurement was performed and a good linearity of the output thermovoltage with respect to the input power is obtained. The measured sensitivity is close to 0.26 mV mW −1 , 0.23 mV mW −1 and 0.16 mV mW −1 at 0.1, 1 and 10 GHz, respectively. Moreover, the frequency dependence measurement demonstrates that the measured thermovoltage decreases with increasing the frequency. The measurements demonstrate that the measured results agree with the presented 2D model for low frequency while the measured thermovoltage deviates from the expectation at high frequency. The reason is that the electromagnetic coupling loss of the coplanar waveguide and the parasitic loss of the load resistor become higher at high frequency.

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In this paper, a novel capacitive power sensor based on the microelectromechanical systems (MEMS) cantilever beam at 8-12 GHz is proposed, fabricated and tested. The presented design can not only realize a cantilever beam instead of the conventional fixed-fixed beam, but also provide fine compatibility with the GaAs monolithic microwave integrated circuit (MMIC) process. When the displacement of the cantilever beam is very small compared with the initial height of the air gap, the capacitance change between the measuring electrode and the cantilever beam has an approximately linear dependence on the incident radio frequency (RF) power. Impedance compensating technology, by modifying the slot width of the coplanar waveguide transmission line, is adopted to minimize the effect of the cantilever beam on the power sensor; its validity is verified by the simulation of high frequency structure simulator software. The power sensor has been fabricated successfully by Au surface micromachining using polyimide as the sacrificial layer on the GaAs substrate. Optimization of the design with impedance compensating technology has resulted in a measured return loss of less than −25 dB and an insertion loss of around 0.1 dB at 8-12 GHz, which shows the slight effect of the cantilever beam on the microwave performance of this power sensor. The measured capacitance change starts from 0.7 fF to 1.3 fF when the incident RF power increases from 100 to 200 mW and an approximate linear dependence has been obtained. The measured sensitivities of the sensor are about 6.16, 6.27 and 6.03 aF mW −1 at 8, 10 and 12 GHz, respectively.

In this paper, an 8-12GHz capacitive power sensor based on MEMS cantilever beam is proposed. When the displacement of the cantilever beam is far less than the initial height of the air gap, the capacitance between the measuring electrode and the cantilever beam has a linear dependence on the input power of the RF signal. The novelty of the presented design provides not only negligible disturbance of the input signal but also an improved sensitivity because of small elastic coefficient of the cantilever beam. Impedance compensating technology by modifying the slot width of the CPW transmission line is adopted and the validity is verified by the simulation of HFSS software. The presented power sensor has been designed, optimized and fabricated using GaAs MMIC process. The measured return loss is less than -25dB and the insertion loss is around 0.1dB at 8-12GHz. DC voltage measurement and RF power measurement have been performed to test against the validity of the design. A sensitivity of 6aF•mW -1 is obtained which is limited by the thickness of the cantilever beam, the initial height of the air gap and the area of the measuring electrode.

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