\useasboundingbox (-1,-0.5) rectangle (6.25,6);
\draw[dashed] (1.5,0) -- (1.5,5);
\draw[dashed] (3.0,0) -- (3.0,5);
\draw[dashed] (5.0,0) -- (5.0,5);

\node[] (off) at (0.75,4.75) {off};
\node[] (sat) at (2.25,4.75) {saturation};
\node[] (ohmic) at (4.00,4.75) {ohmic};

\node[anchor=north] at (ohmic.south) {$\sim V_{\mathrm{GS}}$};
\node[anchor=north] at (off.south) {$=0$};
\node[anchor=north] at (sat.south) {$\sim V_{\mathrm{GS}}^2$};

\node[anchor=north] at (0,0) {0};
\draw[thin] (0,0) -- (0,-0.075);
\node[anchor=east] at (0,0.25) {0};
\draw[thin] (0,0.25) -- (-0.075,0.25);


\node[anchor=north] at (1.5,0) {$V_{\mathrm{T}}$};
\draw[thin] (1.5,0) -- (1.5,-0.075);

\node[anchor=north] at (3.0,0) {$V_{\mathrm{DS,1}}+V_{\mathrm{T}}$};
\draw[thin] (3.0,0) -- (3.0,-0.075);


\node[anchor=north] at (5.0,0) {$V_{\mathrm{GS,max}}$};
\draw[thin] (5.0,0) -- (5.0,-0.075);

\draw[{Latex[round,scale=1.2]}-{Latex[round,scale=1.2]},thick] 
  (0,5.75) -- (0,0) -- (6,0);

\draw[thick] (0,0.25) -- (1.5,0.25);
\draw[thick] plot[smooth,domain=1.5:3] (\x, {0.7*0.5*(\x-1.5)^2+0.25});
\draw[thick] plot[smooth,domain=3:5] (\x, {0.7*((\x-1.5)*1.5-0.5*1.5^2)+0.25});

\node[anchor=south] at (6,0) {$V_{\mathrm{GS}}$};
\node[anchor=east] at (0,5.75) {$I_{\mathrm{D}}$};