What is the significance of the highly doped n region near drain and source end in mosfet? What would happen if it was not present there?

  • \$\begingroup\$ doping enables higher k charge layers between narrower conductors to reduce RdsOn while increasing Ciss and Coss. this product is an unavoidable constant for a given doping level and similar geometry. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 May 17 '17 at 13:11

Historically, the contacts to the drain and source regions of a transistor used aluminium. This aluminium would be contacting the semiconductor. In the case of an Al-p or Al-p+ contact, we would have a normal, ohmic contact behavior. This is what we desire - if we increase the voltage, the current increases too in a linear manner.

Things are different when we build an Al-n contact. When we do this, we have actually made a Schottky barrier! The contact will behave as a Schottky diode, only conducting current in one direction. To counteract this behaviour, we need to more strongly dope the immediate contact area. While this will not make the contact ohmic, it makes it conduct in both directions in a somewhat linear manner.

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  • \$\begingroup\$ Hence PFETs (Pchannels) were the first 100 fet and 1,000 fet (shift-register) integrated circuits? They needed fewer processing steps, and had more tolerance of photo-graphic misalignment? \$\endgroup\$ – analogsystemsrf May 17 '17 at 14:57
  • \$\begingroup\$ @analogsystemsrf I believe this is also related to VT shift due to impurities. Sodium impurities were common issues in the early years. Sodium impurities will lower the VT of a channel. In a NMOS device, they would pull down the VT, potentially pulling it down so far that the device became a enhancement-mode device. In pmos, lowering the VT would just mean it's harder to turn on. \$\endgroup\$ – Joren Vaes Sep 13 '17 at 11:33

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