Different Types of VT Devices
Threshold voltage plays a significant role in switching and leakage. Standard cells are designed with various VT layer devices.
High threshold voltage means more control in leakage current and less speed and vice versa. If it requires faster switching, then a lower threshold voltage device is preferred because it will give a higher drive current. If the requirement is to lessen the leakage, then a higher threshold voltage device is preferred. A standard threshold voltage device is used as a trade-off between switching and leakage.?
Changing the channel doping in a MOS transistor affects the threshold voltage of a cell. When the doping in the channel goes up, the gate threshold voltage also increases. This happens because a higher gate voltage is required to deplete more of the majority carriers before a channel with minority carriers can be created.
Increasing order of channel doping, Low to High: LVT→SVT→HVT.
Channel doping is higher in HVT than in SVT.?LVT has the least channel doping compared to SVT and HVT.
Increasing order of output drive current of a cell, Low to High (of the same cell size): HVT→SVT→LVT.?
LVT has the highest output drive current compared to HVT and SVT.?
The formula for drain current is as follows:
The lower the threshold voltage, the more the drive current. Since in LVT, the threshold voltage is lower than in HVT and SVT, the drive current is higher in LVT.?
Cell delay reduces in this order, High to Low: HVT→SVT→LVT
Cell leakage increases in this order, Low to High: HVT→SVT→LVT