\useasboundingbox (-1,-0.5) rectangle (6.25,6);

\node[] (ohmic) at (1.8,5.25) {ohmic};
\node[] (sat) at (4.55,5.25) {saturation};
\node[anchor=north] at (ohmic.south) {$\sim V_{\mathrm{DS}}^2$};
\node[anchor=north] at (sat.south) {$= \mathrm{const.}$};

\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 (5.0,0) {$V_{\mathrm{DS,max}}$};
\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) -- (5,0.25);
\node[anchor=south east, font=\small, inner sep=1pt,xshift=5mm] 
  at (3.5,0.25) {$V_{\mathrm{GS}}=V_{\mathrm{GS,1}}$};

\draw[thick] plot[smooth,domain=0:1.5] (\x, {0.7*((3-1.5)*\x-0.5*\x^2)+0.25});
\draw[thick] (1.5,1.0375) -- (5,1.0375);
\node[anchor=south west, font=\small, inner sep=1pt,xshift=5mm] 
  at (3.5,1.0375) {$V_{\mathrm{GS}}=V_{\mathrm{GS,2}}$};

\draw[thick] plot[smooth,domain=0:2.2] (\x, {0.7*((3.7-1.5)*\x-0.5*\x^2)+0.25});
\draw[thick] (2.2,1.944) -- (5,1.944);
\node[anchor=south west, font=\small, inner sep=1pt,xshift=5mm] 
  at (3.5,1.944) {$V_{\mathrm{GS}}=V_{\mathrm{GS,3}}$};

\draw[thick] plot[smooth,domain=0:3.35] (\x, {0.7*((4.85-1.5)*\x-0.5*\x^2)+0.25});
\draw[thick] (3.35,4.177875) -- (5,4.177875);
\node[anchor=south west, font=\small, inner sep=1pt,xshift=5mm] 
  at (3.5,4.177875) {$V_{\mathrm{GS}}=V_{\mathrm{GS,max}}$};

\draw[dashed] plot[smooth,domain=0:3.9] (\x, {0.7*0.5*\x^2+0.25});

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