The junction FET used as a SAW detector in ZnO-SiO2-Si structures

The junction FET (JFET) is considered as an active surface acoustic wave (SAW) detector. In contrast to FET detectors, the JFET can be designed without any metal gate structures in the SAW propagation path, because the gate consists of an implanted or diffused semiconductor structure. This results in a smooth surface, which reduces intertap reflections and improves the growth of the ZnO layer. The JFET fabrication is fully IC compatible, and electronic drive and control circuitry can be added to obtain a fully integrated, programmable filter. Monolithic test devices operating at 95 MHz have been realized in a ZnO-SiO₂-Si layered structure. The p⁺ gates of the n-channel JFETs are ion-implanted through a 0.1-μm-thick oxide in an n^--type epilayer. Source and drain areas are formed by n⁺ diffusions. The metal contacts to the source, drain, and gate are placed outside the SAW propagation path. The untuned conversion efficiency of the tested JFET at 95 MHz was -49 dB, but it can be reduced significantly by optimizing the geometry of the JFET detector and using a high-frequency BIFET process