\begin{axis}[ xmin=0
            , xmax=1.15
            , ymin=0
            , ymax=1.2
            , xtick={0}
            , ytick={0}
            , yticklabels={,}
            , xticklabels={,}
            , extra x ticks={0,0.3,0.5,0.7,1.0}
            , extra x tick labels={ \SI{0}{\volt}
                                  , $V_{\mathrm{T,L}}$
                                  , $\nicefrac{V_{\mathrm{DD}}}{2}$
                                  , $V_{\mathrm{T,H}}$
                                  , $V_{\mathrm{DD}}$
                                  }
            , extra y ticks={ 0.015
                            , 0.515
                            , 1.015
                            }
            , extra y tick labels={ \SI{0}{\volt}
                                  , $\nicefrac{V_{\mathrm{DD}}}{2}$
                                  , $V_{\mathrm{DD}}$
                                  }
            , inner axis line style={semithick,black}
            , axis lines=center
            , axis line style={-Latex[round]}
            , every axis x label/.style={ at={(ticklabel* cs:0.975)}
                                        , anchor=south
                                        , inner sep=2.5pt
                                        }
            , every axis y label/.style={ at={(ticklabel* cs:0.975)}
                                        , anchor=west
                                        , inner sep=2.5pt
                                        }
            , clip=false
            , major tick length=1.75pt
            , xlabel=$V_{\mathrm{I}}$ in \si{\volt}
            , ylabel= $V_{\mathrm{O}}$ in \si{\volt}
            , width=0.5\textwidth
            , height=0.35\textwidth
            , every tick/.style={black}
            , tick align=outside
            ]

  \draw [densely dashed,gray] 
          (axis cs:0,0.5+0.015) 
       -- (axis cs:1.0,0.5+0.015); 
  \draw [densely dashed,gray] 
          (axis cs:0,0.0+0.015) 
       -- (axis cs:0.3,0.0+0.015); 

  \draw [densely dashed,gray] 
          (axis cs:0.5,0.0) 
       -- (axis cs:0.5,1.05);    

  \draw [thick] 
        (axis cs:0.0,1.0+0.015) 
     -- (axis cs:0.3,1.0+0.015)
     to node[currarrow,rotate=90] {} (axis cs:0.3,0.0+0.015)
     -- (axis cs:0.7,0.0+0.015)
     -- (axis cs:1.0,0.0+0.015);

  \draw [thick] 
        (axis cs:0.0,1.0+0.015) 
     -- (axis cs:0.7,1.0+0.015)
     to node[currarrow,rotate=270] {} (axis cs:0.7,0.0+0.015)
     -- (axis cs:1.0,0.0+0.015);                   
\end{axis}