Explain how the threshold voltage of mosfet depends upon the designed of the device
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the threshold voltage in advanced CMOS is a quite complicated function of both W and L.
In traditional CMOS (say, 0.5um CMOS and earlier), there were manlyi two effects: a roll-off of VT with decreasing L (referred to as short-channel effect), and a roll-up of VT with decreasing W (referred to as narrow-channel effect).
With CMOS technologies using pocket/halo implants near source and drain to mitigate short-channel effects, channel doping is non-uniform along the channel (high close to source and drain, and low at channel center). When using shorter channel lengths, the center part (low doped) is smaller, and hence the average channel doping higher -- hence an increase in VT when reducing channel lengths, called reverse short-channel effect (RSCE). At very short-channel devices, again the traditional roll-off of VT occurs since the short-channel effect is still there. Typically a peak occurs, at a channel length typically about 1.5*Lmin, which can be about 50-100mV above the VT of shortest channel, then drops off towards longer channel length. This is all very much technology-dependent, and no general rule or equation applies to all cases.
In traditional CMOS (say, 0.5um CMOS and earlier), there were manlyi two effects: a roll-off of VT with decreasing L (referred to as short-channel effect), and a roll-up of VT with decreasing W (referred to as narrow-channel effect).
With CMOS technologies using pocket/halo implants near source and drain to mitigate short-channel effects, channel doping is non-uniform along the channel (high close to source and drain, and low at channel center). When using shorter channel lengths, the center part (low doped) is smaller, and hence the average channel doping higher -- hence an increase in VT when reducing channel lengths, called reverse short-channel effect (RSCE). At very short-channel devices, again the traditional roll-off of VT occurs since the short-channel effect is still there. Typically a peak occurs, at a channel length typically about 1.5*Lmin, which can be about 50-100mV above the VT of shortest channel, then drops off towards longer channel length. This is all very much technology-dependent, and no general rule or equation applies to all cases.
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