Hi Berti,

The older technologies has thicker poly for the gate. Supposedly, this made harder to use implant techniques to dope them, and so they were doped during the poly deposition process, and so all were the same type (n+).

With newer processes, it became possible to dope the thinner poly with implant and anneal methods.

Just Googled on buried channel pmos, and found the info on this link:

http://ece.iisc.ernet.in/~navakant/E3-238/lecture13.pdf

Regards,

Vivek

Hi Berti,

If you look at the link I sent in the earlier post, you will see the reason.

In short, the counter doping which leads to formation of buried channels was an extra step which was necessary in the older technologies to achieve a reasonable (small) level of threshold voltage. This was due to the flatband voltage which results when using an n+ poly-Si gate for a PMOS.

In the new processes where it is possible to have a p+ gate for PMOS, the flatband voltage is much better and there is no need for the extra step which was responsible for buried channel formation in PMOS. In fact, I doubt if you could actually achieve anything with counter doping anymore. It was the large amount of counterdoping required in older processes which led to formation of buried channels.

So, it is not that the modern processes avoid buried channels per se, but more that the older processes could produce nothing but buried channels due to the use of an n+ gate for both PMOS and NMOS.

Regards,

Vivek

Berti -

buried channel devices are avoided in modern technologies due to poor short-channel effects (as compared to surface channel devices), caused by 2D electrostatics. This was not an issue in old technologies with long channel/gate lengths, where short channel effects were non-existent.

Buried channel devices have their own advantages - higher mobility (due to lower normal electric field and lower surface carrier scattering), lower noise (carriers are less interacting with traps/defects at the Si/SiO2 interface), lower NBTI degradation for pMOS, etc.

Extra implant step for buried channel devices is not an issue - all modern MOSFETs have this extra implant step that is used to adjust
Vt - threshold voltage.

  Maks
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