Threshold voltage

The threshold voltage ($v_t$) is an intrinsic property of MOSFETs that determines when the conducting current channel is induced between the drain and source terminals. We define it as the value of $v_{GS}$ where a sufficient amount of electrons accumulate to form the channel; usually between 0.3 V and 1.0 V. For PMOS transistors, $v_t>0$. For NMOS, $v_t<0$. In either case, $v_{GS}\ge |v_t|$ to turn on the transistor.

Why do we care about this? In digital CMOS circuits, we have a pull-up network with an PMOS and a pull-down network with an NMOS. If we switch them around, we get weak ones and weak zeroes. Weak ones happen when there’s a voltage level too close to $v_t$, such that it may cause unintended switching due to signal noise. Weak zeroes happen when the 0 isn’t driven low enough, which can also cause switching problems.