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https://designers-guide.org/forum/YaBB.pl Modeling >> Semiconductor Devices >> Why does Vth fall with increasing length? https://designers-guide.org/forum/YaBB.pl?num=1140202373 Message started by Marc Murphy on Feb 17th, 2006, 10:52am |
Title: Why does Vth fall with increasing length? Post by Marc Murphy on Feb 17th, 2006, 10:52am Why does the threshold voltage, Vth, fall as channel-length increases (in short-channel devices)? Thanks, Marc |
Title: Re: Why does Vth fall with increasing length? Post by Marc Murphy on Feb 17th, 2006, 11:13am I think I may have found the answer. It is related to Reverse Short Channel Effect (RCSE) Here's what some guy wrote: Arya Raychaudhuri 3rd June 2004 - 05:32 PM You can look at it like this. For example, if you plot the surface potential along the channel region of the MOSFET near but below the threshold voltage (by a device simulator, perhaps), you will observe high surface potential near the source/drain edges (almost 0.55 + phif), and then it gradually falls below the 2*phif value near the middle of the channel. As you know, the inversion of the substrate (channel formation) takes place when the surface potential at the center of the channel reaches 2*phif. But, beyond 2*phif, it requires higher and higher gate voltage increments to increase the band bending (surface potential). For short channel MOSFETs, the gradually falling surface potental gradients adjacent to the central region help increase the band bending at the channel center. So, to an extent, this helps lowering the threshold voltage. At the same time, it has been seen that the substrate doping concentration at the center of the channel region increases slightly when the channel length is decreased. So, this is an effect that should increase the threshold voltage. So, the threshold voltage can go thru a maxima as the channel length is decreased from long channel. --- And here is a more concise explanation that makes more sense to me: RSCE in nMOSFETs is mainly caused by the boron dopants pile-up phenomenon at the edge of the source and drain regions. So, if the channel is really short, there is little space between source and drain; it is mainly made up of these high-concentration regions. High concentration requires a higher Vth. Makes sense to me! :) |
Title: Re: Why does Vth fall with increasing length? Post by pbs681 on Feb 17th, 2006, 5:58pm Vds field will increase body effect for short channel. This also contribute to the increasing Vth as the lenght become shorter. |
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