Positive temperature coefficient (PTC) thermistors are widely used in electronics for its switching and power limiting ability. It can also be used for sensing of different parameters through accurate measurement of temperature. For the sensor application of PTC thermistor, its characteristics should be tailored very accurately For this, controlled doping with both acceptor and donor elements is required at a very high degree of accuracy. The method of solid state diffusion of ions has its inherent problems like inhomogeneity or accuracy in amount of addition at this low level of doping. Further, the acceptor concentration is required preferentially at grain boundaries which cannot be achieved through solid state diffusion. In this investigation, a novel method of doping through diffusion by vapour phase is applied to form modified PTC thermistor where accurate control of its electrical characteristics is achieved. This method of vapour phase doping results in preferential distribution of acceptorions attain boundaries resulting in improved characteristics.