ElectronicsI_L8 - Lecture 8 iD vs. vDS The current in the...

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Lecture 8
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i D vs. v DS The current in the triode region is: The current in the saturation region is theoretically constant, equal to the value of the current at the upper edge of the Triode region: In the semiconductor fabrication process, the combination of the surface mobility and oxide capacitance is know as the process transconductance parameter. We can also see that the current through a device is dependent on the aspect ratio of the width to the channel length. – The values of W and L can be selected by the circuit designer. – However every fabrication process has a limit on how small L can be. – This measure is used to describe the fab. process, say 0.09 μ m technology (currently used) () () 2 2 1 t GS ox n D V v L W C i = μ ox n n C k = () = 2 2 1 DS DS t GS n D v v V v L W k i () 2 2 1 t GS n D V v L W k i = () () = 2 2 1 DS DS t GS ox n D v v V v L W C i
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Small = fast • The gate oxide thickness also scales with the channel length. Currently oxides are 2nm thick! • Why do we care about short channels? – If we want a faster transistor, it needs to handle more current. – The way to think about that is the speed of a transistor is fundamentally limited to how long it takes an electron to cross the channel. – Faster crossing times means higher speed, or shorter distances. If we try for higher speed, then that means higher current (i=q X velocity) • So both ways of looking at it demand a shorter channel if we want our computers to keep getting faster
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Example • Consider a MOSFET with W/L=8 μ m/0.8 μ m, t ox =8 nm, μ n =450 cm 2 /Vs, and V t =0.7V.
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ElectronicsI_L8 - Lecture 8 iD vs. vDS The current in the...

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