The moment of inertia of a uniform semicircular disc of mass M and radius r about a line perpendicular to the plane of the disc through the centre is 
 

We know that the moment of inertia of a circular disc about an axis perpendicular to the plane of disc and passing through centre is $(1/2) {\mathrm{Mr}}^2.$
For semi-circular disc
$\begin{array}{l}\mathrm{I}=\frac{1}{2}\left[\frac{1}{2}\left(2\mathrm{M}\right){\mathrm{r}}^2\right]\\ =\frac{1}{2}{\mathrm{Mr}}^2\end{array}$