  We trace a simple mechanical model of a ratchet, and embed its setup in a
conventional quasi-two-dimensional electron system in a semiconductor
heterostructure. Expressed are two distinct microscopic mechanisms for such
systems to serve as quantum ratchets producing the in-plane directed current
from zero-mean time-dependent electric fields. The effective-mass swap ratchet
is based on modulation of the mobility through alteration of the electron's
effective mass. Modulation of the strength of the effective in-plane electric
field marks the skin-effect ratchet. They can generate both linear and circular
photocurrents, the later taking place in the dissipationless regime.
