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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?

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  • \$\begingroup\$ You say: "by increasing the P-doping in an nmos, threshold voltage is decreased". Are you quite sure that's true? I think it's the opposite. 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. Anyone agree? \$\endgroup\$ – PkP Oct 23 '16 at 18:55
  • \$\begingroup\$ Oh, now I get it, do you mean gate ion implantation? If positive ions are implanted to the gate, they also contribute to the electric field between the gate and the substrate, thus decreasing the threshold voltage. Am I right? \$\endgroup\$ – PkP Oct 23 '16 at 19:01
  • \$\begingroup\$ You are right, I edited that. I want to know about how the process of ion implantation for controlling the doping of the channel for changing the threshold voltage is done... \$\endgroup\$ – Fateme Oct 23 '16 at 19:26
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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"

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  • \$\begingroup\$ Thank you, The link wouldn't open though. Can you e-mail it to me please? f_mir.abbasi@yahoo.com @PkP \$\endgroup\$ – Fateme Oct 28 '16 at 10:24

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