  Existing calculations of heavy quark production in charged-current and
neutral current lepton-hadron scattering are formulated differently because of
the artificial distinction of ``light'' and ``heavy'' quarks made in the
traditional approach. A proper QCD formalism valid for a wide kinematic range
from near threshold to energies much higher then the quark mass should treat
these processes in a uniform way. We formulate a unified approach to both types
of leptoproduction processes based on the conventional factorization theorem.
In this paper, we present the general framework with complete kinematics
appropriate for arbitrary masses, emphasizing the simplifications provided by
the helicity formalism. We illustrate this approach with an explicit
calculation of the leading order contribution to the quark structure functions
with general masses. This provides the basis for a complete QCD analysis of
charged current and neutral current leptoproduction of charm and bottom quarks
to be presented in subsequent papers.
