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

\node[] at (1.8,5.25) {ohmic};
\node[] at (4.55,5.25) {saturation};

\draw[gray] (2.7, 2.17269) -- (4, 2.17269) -- (4, 2.2828);
\filldraw (2.7, 2.17269) circle (0.6pt);
\filldraw (4.0, 2.28280) circle (0.6pt);

\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);

\draw[thick] plot[smooth,domain=0:1.5] 
  (\x, {0.7*((3-1.5)*\x-0.5*\x^2)*(1+0.05*\x)+0.25});
\draw[thick] plot[smooth,domain=1.5:5] 
  (\x, {0.25+0.7875*(1+0.05*\x)});

\draw[thick] plot[smooth,domain=0:2.2] 
  (\x, {0.7*((3.7-1.5)*\x-0.5*\x^2)*(1+0.05*\x)+0.25});
\draw[thick] plot[smooth,domain=2.2:5] 
  (\x, {0.25+1.694*(1+0.05*\x)});

\draw[thick] plot[smooth,domain=0:3.3] 
  (\x, {0.7*((4.8-1.5)*\x-0.5*\x^2)*(1+0.05*\x)+0.25});
\draw[thick] plot[smooth,domain=3.3:5] 
  (\x, {0.25+3.8115*(1+0.05*\x)});

\draw[dashed] plot[smooth,domain=0:3.7] 
  (\x, {0.76*0.5*\x^2+0.25});

\node[anchor=south] at (6,0.00) {$V_{\mathrm{DS}}$};
\node[anchor=east]  at (0,5.75) {$I_{\mathrm{D}}$};

\draw[{Latex[round,scale=0.6]}-] (3.35,2.17269) --++ (0,-0.3) 
  node[ anchor=north
      , font=\scriptsize
      , inner sep=2pt
      ] {$\Delta V$};
\draw[{Latex[round,scale=0.6]}-] (4,2.227745) -- (4.5,2) 
  node[ anchor=west
      , font=\scriptsize
      , inner sep=2pt
      ] {$\Delta I$};

\node[anchor=east] at (4.75,3.25) 
  {$g_{\mathrm{DS}} = \frac{\Delta I}{\Delta V}$};