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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 μ = ( ) = 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 μ
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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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