  The development of semiconductor electronics is shortly reviewed, beginning
with the development of germanium devices (band gap $E_g=0.66$ eV) after world
war II. Quickly a tendency to alternative materials with wider band gap became
apparent, starting with silicon ($E_g=1.12$ eV). This improved the signal/noise
ratio for classical electronic applications. Both semiconductors have
tetrahedral coordination, and by isoelectronic alternative replacement of Ge or
Si with carbon or several anions and cations other semiconductors with wider
$E_g$ are obtained, that are transparent for visible light and belong to the
group of wide band gap semiconductors. Nowadays some nitrides, especially GaN
and AlN, are the most important materials for optical emission in the
ultraviolet and blue spectral region. Oxide crystals, such as ZnO and
$\beta$-Ga$_2$O$_3$, offer similarly good electronic properties but suffer
still from significant difficulties in obtaining stable and technically
sufficient $p$-type conductivity.
