Two-dimensional (2D) materials, with their extraordinary physical and chemical properties, have gained extensive interest for physical, chemical and biological sensing applications. However, 2D material-based devices, such as field effect transistors (FETs) often show high contact resistance and low output signals, which severely affect their sensing performance. In this study, we develop a new strategy to combine metallic and semiconducting polymorphs of transition-metal dichalcogenides (TMDCs) to solve this critical issue. We applied a phase engineering methodology to integrate large-scale and spatially assembled multilayers of 2H MoTe2 FETs with coplanar metallic 1T´MoTe2 contacts. Such in-plane heterophase based FETs exhibit an ohmic contact behavior with an extremely low contact resistance due to the coplanar and seamless connections between 2H and 1T´ phases of MoTe2. These 1T´/2H/1T´ based FETs were successfully demonstrated for detecting NH3 with current outputs increased up to microamp levels without using any conventional interdigital electrodes, which is compatible with the current CMOS circuits for practical applications. Furthermore, the as-fabricated sensor can detect NH3 gas concentrations down to 5 ppm at room temperature. Our research demonstrates a new strategy of applying the heterophase MoTe2 based nanoelectronics for high-performance sensing applications.