Explanation of the Electronic Transition in ZrTe$_5$ and HfTe$_5$*

The pentatellurides, ZrTe$_5$ and HfTe$_5$ show strong electronic transitions at 150 K and 70 K, for the two compounds, respectively. This is seen in a strong resistivity peak and a sign change from a large electron-like to a large hole-like thermopower. This is not associated with a structural transition. Here use transport theory and the first principles band structure to show the origin of the transition in the particular electronic structure of these compounds. Results in good accord with experiment are found both for the transition temperatures and the effects in transport.