  By the first-principles electronic structure calculations, we have
systematically studied the electronic structures of recently discovered
extremely large magnetoresistance (XMR) materials LaSb and LaBi. We find that
both LaSb and LaBi are semimetals with the electron and hole carriers in
perfect balance. The calculated carrier densities in the order of $10^{20}$
cm$^{-3}$ are in good agreement with the experimental values, implying long
mean free time of carriers and thus high carrier mobilities. With a
semiclassical two-band model, the perfect charge compensation and high carrier
mobilities naturally explain (i) the XMR observed in LaSb and LaBi; (ii) the
non-saturating quadratic dependence of XMR on external magnetic field; and
(iii) the resistivity plateau in the turn-on temperature behavior at very low
temperatures. The explanation of these features without resorting to the
topological effect indicates that they should be the common characteristics of
all perfectly electron-hole compensated semimetals.
