We have temperature inversion effect in 40nm and below technology. At 65nm hand early technology, the worst timing corner are alway low voltage, high temperature corener. But in 40nm and below, the timing worst corner is at low voltage, cold temperature. This is the Temperature Inversion Effect.
Following are 2 main affect while Temperature increase:
In 65nm and above, the mobility is dominate the current/timing. While in 40nm and below technology, at low voltage situation, Vth became the main part of current/timing.
As a result we got the temperature inversion effect.
Vth & Temperature Curve
Mobility & Temperature
Invert Delay vs. Temperature