What happens if semiconductor is heated to a very high temperature and how does a semiconductor react to temperature changes what actually happens?
It melts or decomposes as usual.
But before that, conductivity rises.
Because one can show conductivity is product of charge carrier density and mobility. With temperature, mobility drops as charge carriers are more often scattered by thermal vibrations, but carrier density rises as more and more carriers are given thermal energy to break from bond of individual atoms (like electrons breaking from valence band to conduction band)
For metals, bandgap being practically zero for all purposes, charge carrier density is near-saturated from very low tempertaure, and fall of mobility with rise of temperature contributes to losing conductivity with temperature.
But for semiconductors, charge carrier density is far from saturated, so heating up to a very significant extant (I cannot tell whether virtually all of break-freeable charge carriers set free earlier or the semiconductor melts earlier as temperture keeps rising ) raises chrage carrier density much much more than lost mobility. So, heating raises conductivity of semiconductors
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