Electronics based on perovskite oxides, a class of materials with unparalleled wealth of physical functionalities, possesses high potential to go beyond the present semiconductor-based technologies. Towards universal and scalable oxide-based electronics, an important milestone is to realise both N- and P-type conduction regions – the two fundamental blocks of most of electronic devices – on the same oxide substrate surface. However, in contrast to the case of conventional semiconductors, the formation of planar PN junctions is highly challenging in oxide materials owing to difficulties in carrier doping. Here, we show that high-mobility PN junctions can be formed on a surface of SrTiO3 (STO), one of the most versatile oxide materials, in a robust and low-cost manner by simply depositing Angstrom-thin metal layers on top of an STO substrate near room temperature. Furthermore, by forming planar N-P-N junctions, we successfully demonstrate a new type of oxide-based tunnelling field effect transistor (TFET), which enables an extremely sharp switching with a subthreshold swing value S ~ 38 mV/dec and a large current ON/OFF ratio of 108. This high-performance FET operation is obtained by a new mechanism where a gate voltage strongly modulates the tunnelling probability through the depletion layers at the PN interfaces, utilising the unique strong nonlinear electric-field dependence of the permittivity of STO. Our simple method for selectively forming P and N-type regions monolithically on STO is potentially applicable to a wide range of oxide-based electronic systems, from single devices to integrated circuits, and even to flexible electronics.