  We report on a temperature-induced transition from a conventional
semiconductor to a two-dimensional topological insulator investigated by means
of magnetotransport experiments on HgTe/CdTe quantum well structures. At low
temperatures, we are in the regime of the quantum spin Hall effect and observe
an ambipolar quantized Hall resistance by tuning the Fermi energy through the
bulk band gap. At room temperature, we find electron and hole conduction that
can be described by a classical two-carrier model. Above the onset of quantized
magnetotransport at low temperature, we observe a pronounced linear
magnetoresistance that develops from a classical quadratic low-field
magnetoresistance if electrons and holes coexist. Temperature-dependent bulk
band structure calculations predict a transition from a conventional
semiconductor to a topological insulator in the regime where the linear
magnetoresistance occurs.
