The effect of the vertical electric field on freeze-out in MOS-structures

It is well known that majority carrier freeze-out at cryogenic temperatures does not occur in the channel region of an enhancement-mode MOSFET. However, no adequate explanation for this lack of freeze-out has thus far been presented. The authors examine the effect of the vertical electric field under the gate of the MOSFET on frozen-out carriers in the region and present a new theory to explain the lack of freeze-out in this area in terms of a Zener-type ionization of frozen-out carriers. Theoretical calculations show that it is likely that the ionization of the frozen-out carriers in the depletion region under the gate of a MOS transistor is indeed due to the effect of the vertical electric field under the gate. It is shown theoretically that a steady state of complete ionization within the depletion region is reached at a sufficiently low gate voltage within an acceptable time for this effect to be the cause of the non-freeze-out phenomenon. The theory provides a possible explanation for anomalous device behavior encountered at cryogenic temperatures during initial measurements on depletion-mode MOSFETS as well as pn-junction diodes.<>