This dissertation presents the layout design, nonlinear modeling and sensing application in terms of active device. The active device, such as MOSFET or BJT in semiconductor, is one of the most important components of a wireless communication or sensing system generally. It plays a significant part in determining the overall performance, cost, and reliability of these application systems. In the world of RF wireless communications, the base-stations and long range transmitters use silicon laterally-diffused MOS (LDMOS) high power transistors almost exclusively. To achieve lower on-resistance and a more compact device size, this study adopted an annular structure in the layout design. According to the measurement results, the smaller drain parasitic resistance in the annular structure could be the key factor for improving ft and fmax. In additional to the small-signal analysis, the large-signal characteristics, such as power gain and power added efficiency, were also improved compared to the transitional structure of LDMOS. In addition to high power device design, the behavior model of the nonlinear characteristics for active device is also crucial. In this study, we analyze the polyharmonic distortion model (PHD) and use this model to predict the nonlinear behavior of active device. By way of the PHD model extracted using on-wafer nonlinear vector network analyzer (NVNA), the large-signal validation of this model also shows a good match with measurements at 1.9 GHz without optimization and curve fitting. In another part of this thesis, we discussed and analyzed the sensor design completely using CMOS active device and SAW delay-line device. Their electrical characteristics are evaluated as well as vapor sensing results. The sensing experimental results show that the maximum oscillation frequency shift between gas on and off is approximately 10 kHz with 50 x 103 ppm alcohol vapor concentration. Conclusions on the sensing device and system, and recommendations concerning potential improvements of these components are discussed, finally.