The transient decay of large excess-carrier densities was investigated in ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te} (x=0.205)$ at 77 K. Excess carriers were generated by bulk impact ionization in strong electric fields. A strong variation of the carrier lifetime with excess-carrier density was observed in the transient excess-carrier decay. The comparison with calculated values shows that the carrier decay is determined by the Auger recombination process. To our knowledge this is the first observed Auger-dominated transient carrier decay in a semiconductor. A typical Auger-dominated lifetime for carriers at excess-carrier densities not far from thermal equilibrium is 460 nsec for ${n}_{0}=9\ifmmode\times\else\texttimes\fi{}{10}^{14}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and ${\ensuremath{\mu}}_{H}=2.26\ifmmode\times\else\texttimes\fi{}{10}^{5}$ ${\mathrm{cm}}^{2}$/V sec at 77 K. This lifetime decreased to 90 nsec at an excess-carrier density equal to ${n}_{0}$. Photoconductivity experiments indicate that the majority-carrier lifetime can be enlarged due to minority-carrier trapping near contacts. This was established in photoresponse measurements with ${\mathrm{CO}}_{2}$ laser pulses. In addition to this, the Hall mobility was measured and analyzed between 4.2 and 200 K. Calculations for combined ionized-impurity and polar-optical-phonon scattering indicate that above 77 K the polar-optical-phonon scattering limits the mobility. This electron-phonon interaction was calculated taking into account the two-mode behavior of ${\mathrm{Hg}}_{1\ensuremath{-}x}{\mathrm{Cd}}_{x}\mathrm{Te}$. The high-field measurements have shown that in electric fields above 150 V/cm impact ionization occurs. Values of the carrier generation rate of ${\mathrm{Hg}}_{0.8}$${\mathrm{Cd}}_{0.2}$ Te were deduced from the time dependent $j(E)$ characteristics.