    \begin{tikzpicture}[yscale=0.5]
      \draw[fill=black,opacity=0.1] (1.75,-5) rectangle (3.25,1);
      \draw (-1,0) -- (1.75,0) edge[out=0, in=180] (3.25,-3) (3.25,-3) -- (6,-3)
            (-1,-2) -- (1.75,-2) edge[out=0, in=180] (3.25,-5) (3.25,-5) -- (6,-5);
%            (0,-2.5) edge[dashed] (6,-2.5);
%      \node[anchor=south east] at (6,-2.5) {$E_F$};
      \node[anchor=south east] at (6,-3) {$E_C$};
      \node[anchor=south east] at (6,-5) {$E_V$};
      \draw[dotted] (2.5,-5) -- (2.5,1) (1,-5) -- (1,1) (4,-5)--(4,1);
      \node at (0.5,0.5) {\large P};
      \node at (2.5,0.5) {\large $\longleftarrow \mathcal{E}$};     
      \node at (5.5,0.5) {\large N};
      \foreach \i in {0,1,2,3} {
        \pgfmathparse{0.25*\i};
        \node[circle,draw=black,inner sep=0,anchor=south west] at ($(3,-3)+0.33*(\i,0)$) {\small $-$};
        \node[circle,draw=black,inner sep=0,anchor=north east] at ($(2,-2)-0.33*(\i,0)$) {\small $+$};
      };
      \node[anchor=south west] at (-1,-5) {$V_D = 0$};
    \end{tikzpicture}

    \begin{tikzpicture}[yscale=0.5]
      \draw[fill=black,opacity=0.1] (1.75,-5) rectangle (3.25,1);
      \draw (-1,0) -- (1.75,0) edge[out=0, in=180] (3.25,-2.2) (3.25,-2.2) -- (6,-2.2)
            (-1,-2) -- (1.75,-2) edge[out=0, in=180] (3.25,-4.2) (3.25,-4.2) -- (6,-4.2);
%            (0,-2.5) edge[dashed] (6,-2.5);
%      \node[anchor=south east] at (6,-2.5) {$E_F$};
      \node[anchor=south east] at (6,-2.2) {$E_C$};
      \node[anchor=south east] at (6,-4.2) {$E_V$};
      \draw[dotted] (2.5,-5) -- (2.5,1) (1,-5) -- (1,1) (4,-5)--(4,1);
      \node at (0.5,0.5) {\large P};
      \node at (2.5,0.5) {\large $\longleftarrow \mathcal{E}$};     
      \node at (5.5,0.5) {\large N};
      \foreach \i in {0,1,2,3} {
        \pgfmathparse{0.25*\i};
        \node[circle,draw=black,inner sep=0,anchor=south west] at ($(3,-2.2)+0.33*(\i,0)$) {\small $-$};
        \node[circle,draw=black,inner sep=0,anchor=north east] at ($(2,-2)-0.33*(\i,0)$) {\small $+$};
      };
      \node[anchor=south west] at (-1,-5) {$V_D = V_P$};
      \draw[red,-triangle 45,very thick] (3,-2.0) -- ++(-0.8,0);
    \end{tikzpicture}

    \begin{tikzpicture}[yscale=0.5]
      \draw[fill=black,opacity=0.1] (1.75,-5) rectangle (3.25,1);
      \draw (-1,0) -- (1.75,0) edge[out=0, in=180] (3.25,-1.5) (3.25,-1.5) -- (6,-1.5)
            (-1,-2) -- (1.75,-2) edge[out=0, in=180] (3.25,-3.5) (3.25,-3.5) -- (6,-3.5);
%            (0,-2.5) edge[dashed] (6,-2.5);
%      \node[anchor=south east] at (6,-2.5) {$E_F$};
      \node[anchor=south east] at (6,-1.5) {$E_C$};
      \node[anchor=south east] at (6,-3.5) {$E_V$};
      \draw[dotted] (2.5,-5) -- (2.5,1) (1,-5) -- (1,1) (4,-5)--(4,1);
      \node at (0.5,0.5) {\large P};
      \node at (2.5,0.5) {\large $\longleftarrow \mathcal{E}$};     
      \node at (5.5,0.5) {\large N};
      \foreach \i in {0,1,2,3} {
        \pgfmathparse{0.25*\i};
        \node[circle,draw=black,inner sep=0,anchor=south west] at ($(3,-1.5)+0.33*(\i,0)$) {\small $-$};
        \node[circle,draw=black,inner sep=0,anchor=north east] at ($(2,-2)-0.33*(\i,0)$) {\small $+$};
      };
      \node[anchor=south west] at (-1,-5) {$V_D > V_P$};      
    \end{tikzpicture}
