What is HVT, SVT（RVT） & LVT cells?
The threshold voltage, Vt of a cell depends on the channel doping of a MOS transistor. As doping in the channel increases the gate threshold voltage increases because the gate voltage needs to deplete more majority carriers before a minority carrier channel can form.
LVT → Low Vt cells
SVT → Standard Vt or Nominal/Regular Vt cells
HVT → High Vt cells
Some facts about Vt:
Channel doping increasing order
LVT→SVT→HVT
Cell delay reduces in this order
HVT→SVT→LVT
Cell leakage increases in this order
HVT→SVT→LVT
It allows you to use LVT and SLVT in timing-critical paths for high speed, and HVT for normal operation.
What is the difference between high VT and low VT cells?
High-Vt cells, characterized by higher threshold voltage, introduce more delays compared to Std-Vt (standard-Vt) and Low-Vt (low-Vt) cells. Therefore, High-Vt cells are typically deployed in areas where timing constraints are less stringent or relaxed. Conversely, Std-Vt and Low-Vt cells, which have lower threshold voltages, are preferred in timing-critical sections of the design. The ultimate goal is to achieve timing targets while optimizing for area and power consumption.
Why do LVT cells have more leakage?
The gate tunneling current is higher for LVT cells because the thickness of the gate oxide is less, resulting in more current flowing through the gate.