Current Voltage Relation in Mosfet
Your plot is (qualitatively) valid in the near subthreshold region, VGS &lt; Vth.
The threshold voltage depends on the acceptor concentration in the substrate. For a heavier doped substrate, the threshold voltage Vth of an n-type mosfet is higher. The current voltage formula for VDS ≥ (VGS – Vth) and VGS &lt; Vth is
$$
i_D ≈ I_{D0}·exp\left({-{κ·(V_{th}-V_{GS}) - V_{BS})}\over{V_T}}\right)
$$
where VGS is the gate/source potential, VBS is the substrate (bulk/source) potential, κ is a &quot;capacitance channel divider&quot; coefficient Cox/(Cox + CD), CD is the depletion layer capacitance.
In principle, the depletion layer capacitance depends on the channel's acceptor concentration, so the subthreshold slope also changes. Pay attention: for a &quot;classical&quot; transistor layout (not tunneling, 3D nanowire or else) it is greater than 60 mV even with a lightly doped substrate, still more for a heavier doped substrate case.