In this work, LDMOS transistors with different drift length and drift doping are characterized at cryogenic temperature. The physics behind the LDMOS transistor behavior at this extreme environmental condition is studied by analyzing device parameters like threshold voltage, carrier mobility and carrier freeze-out. It is shown that the carrier freeze-out in lightly doped drift region occurs at 140K and is responsible for increased drift region resistance. The increase in carrier mobility at lower temperature and carrier freeze-out affects the device characteristics in the linear region. In contrast, the carrier mobility plays a significant role in the saturation region. The probability of impact ionization at different temperatures is also evaluated by considering the electron-phonon interaction in high electric field regions.