Sub-threshold energy

The sub-threshold current in a MOSFET is given by ([12]):

$$I_{DS_{\text{sub--th}}}=\mu_0\frac{W}{L}\frac{kT}{q}Q(V_S)\left[ 1-e^{-\frac{qV_{DS}}{\xi kT}}\right]$$

where

$$Q(V_S)\approx -\frac{kT}{q}\sqrt{\frac{q\epsilon_s N_a}{\vert\Phi_p\vert}}\; e^{\frac{q(V_G-V_T)}{\xi k T}}$$

and

$$\xi = 1 + \frac{1}{2C_{\text{ox}}}\sqrt{\frac{\epsilon_s N_a}{\vert\Phi_p\vert}}.$$

This current is proportional to the MOSFET width $W$, but, usually is negligible. However, with the scaling down of the dimensions and hence of the threshold voltage this current may become no more negligible, and with low $V_G$ and higher $V_D$, the current becomes independent from $V_G$.
Moreover, while the short-circuit current is limited by the switching times of the circuit, the sub-threshold current is not limited in time, so its dissipation can be comparable to the short-circuit dissipation.