\fill[pattern=north west lines, pattern color=gray] (-2, 1) rectangle (2, 2);
\fill[pattern=north west lines, pattern color=gray] (-2,-1) rectangle (2,-2);
\draw[thick] (-2, 1) -- (2, 1);
\draw[thick] (-2,-1) -- (2,-1);
\node[anchor=west] at (2,-1) {$E_{\mathrm{V}}$};
\node[anchor=west] at (2, 1) {$E_{\mathrm{C}}$};

\node[fill=white,inner sep=1.2pt] at (0,-1.5) {Valence band};
\node[fill=white,inner sep=1.2pt] at (0, 1.5) {Conduction band};

\draw[{Latex[round,scale=0.8]}-{Latex[round,scale=0.8]},semithick] 
  (1,1) -- (1,-1) node[ midway
                      , anchor=west
                      , inner sep=1.5pt
                      ] () {$E_{\mathrm{g}}$};

\draw[-{Latex[round]},semithick] 
  (-2.25,-1.75) -- (-2.25,1.75) node[ midway
                                    , rotate=90
                                    , anchor=south
                                    , inner sep=1.5pt
                                    ] () {Electron energy};