3D structure of MOS transistor

You have probably solved your problem. However, here I send a suggestion to someone else who needs a similar figure in the future.

    
documentclass{standalone}
    usepackage{tikz}
    begin{document}
    begin{tikzpicture}
    tikzset{
        Text/.style={font=normalsizesffamily,text centered, minimum size=1cm, text width=3cm},
    }
 
    node[Text] (p-type) at (5.5, .25) {p-type Body};
    draw (0,0)rectangle (11,3);
    draw (0,3)--(2, 7)--(13, 7)--(11, 3)--(0,3);
    draw (11,0)--(11, 3)--(13, 7)--(13, 4)--(11,0);
    
    draw[fill = gray!50] (4.25,3) -- (1,3) -- (1,2.5) to [out=270,in=180] (1.5,2) -- (3.75,2) to [out=0,in=270] (4.25,2.5) -- (4.25,3);
    draw[fill = gray!50] (1,3)--(3, 7)--(6.25,7)--(4.25,3)--(1,3);  
    node[Text] (n1) at (2.625, 2.3) {n+};
         
    draw[fill = gray!50] (10,3) -- (6.75,3) -- (6.75,2.5) to [out=270,in=180] (7.25,2) -- (9.5,2) to [out=0,in=270] (10,2.5) -- (10,3);
    draw[fill = gray!50] (6.75,3)--(8.75, 7)--(12,7)--(10,3)--(6.75,3); 
    node[Text] (n2) at (8.375, 2.3) {n-};
    draw[<->] (7.5, 3.2)--(9.25,6.8);
    node[Text] (W) at (8.75, 5) {W};
          
    draw[<->] (4.28, 2.75)--(6.71,2.75);
    node[Text] (L) at (5.5, 2.5) {L};
          
    draw[fill= red!50] (4,3.5) rectangle (7,4.5); 
    draw[fill = red!50] (7,3.5)--(7, 4.5)--(9,8.5)--(9,7.5)--(7, 3.5);
    draw[fill = red!50] (4,4.5)--(6, 8.5)--(9,8.5)--(7, 4.5);
    draw[->] (3.88, 4.2)--(3.88,3.7);
    node[Text] (L) at (3.6, 3.7) {$t_{ox}$};
    draw[->] (3.88, 2.3)--(3.88,2.8);
         
    draw[fill = blue!10] (4,3) rectangle (7,3.5);
    draw[fill = blue!10] (7,3)--(7, 3.5)--(9,7.5)--(9,7)--(7, 3);
    end{tikzpicture}
    end{document}