CONCEPTS

Hi..

Difference between High vt and low vt cells

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High vt means :- high threshold voltage

Low vt means :- low threshold voltage                       (vt => threshold voltage)

High Vt cells will have more oxide thickness where as low vt cells have less oxide thickness.

For high vt cells the leakage current is less because there is more oxide thickness, but for low vt cells there is more leakage current..

But for high vt cells the swithching speed is less i.e, high vt cells are slower where as low vt cells are faster because they switch faster...

"1) What are High-Vt and Low-Vt cells?

Ans: Hvt cells are MOS devices with less leakage due to high Vt but they have higher delay than low VT, where as the low Vt cells are devices, which have less delay, but leakage is high. The threshold (t) voltage dictates the transistor switching speed, it matters how much minimum threshold voltage applied can make the transistor switching to active state, which results to how fast we can switch the transistor. Disadvantage is it needs to maintain the transistor in a minimum sub threshold voltage level to make it switch fast so it leads to leakage of current in turn loss of power."

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      Assume this is a low vt cell(transistor), I want to make it high vt 

         * for this i have to increase the threshold voltage.....

           If i increase the threshold voltage for the same thickness of oxide(t_ox), then to make the transistor on we need to give more vgs(V)

                               but Electric field(E) = Voltage applied(v) / oxide thickness(t_ox)

                                    since the v_gs (V) is increased to make the transistor on ,(but the oxide thickness is still same....) There is huge amount of electric field passing in the capacitor, this makes the capacitor to break down.

                                   There fore , since E = V / t_ox   ....increase the oxide thickness to reduce the electric field....

                                    Therefore  high Vt transistors are designed with larger oxide thickness...