Two-dimensional (2D) transition metal dichalcogenides (TMDs) have been rapidly established as promising building blocks for versatile atomic scale circuits and multifunctional devices. However, the high contact resistance in TMDs based transistors seriously hinders their applications in complementary electronics. In this work, we show that an Ohmic homojunction n-type tungsten diselenide (WSe₂) transistor is realized through spatially controlling cesium (Cs) doping region near the contacts. We find that the remarkable electron doping effect of Cs stimulates a semiconductor to metal (2H to 1T’) phase transition in WSe₂, and hence the formation of 2H-1T’ hetero-phase contact. Our method significantly optimizes the WSe₂ transport behavior with a perfect low subthreshold swing of ~ 61 mV/dec and ultrahigh current on/off ratio exceeding ~ 10⁹. Meanwhile, the electron mobility is enhanced by nearly 50 times. We elucidate that the ideal n-type behavior originates from the negligible Schottky barrier height of ~ 19 meV and low contact resistance of ~ 0.9 kΩ·μm in the 2H-1T’ homojunction device. Moreover, based on the Ohmic hetero-phase configuration, a WSe₂ inverter is achieved with a high gain of ~ 270 and low power consumption of ~ 28 pW. Our findings envision Cs functionalization as an effective method to realize ideal Ohmic contact in 2D WSe₂ transistors towards high performance complementary electronic devices.