By increasing the P-doping in an NMOS, threshold voltage is increased. How does it happen? And why? And what elements do they use for this? can anyone give me a reference or explain?
An NMOS transistor works by having an electric field, from the gate, through the gate oxide, towards the silicon substrate, attract electrons to the substrate to form a conductive layer called an 'electron inversion channel'. In order for the channel to be formed, positively charged holes must be pushed away from the substrate below the gate so that free electrons from the source can get into the substrate.
As the P doping in the P substrate is increased, more holes need to be pushed away from the substrate in order for the inversion layer to be formed. Thus increasing the dopage will increase the threshold voltage.
Additionally, the threshold can be changed by ion implantation. For more information, see for example these slides: "MOSFET threshold adjusting" at http://web.eng.fiu.edu/npala/EEE6397ex/EEE_6397_Ch7_FETs_PART3.pdf, especially slide 6, "Threshold Adjustment by Ion Implantation"