  We study two-leg spin ladder systems analytically by using a t-J model which
includes the interchain spin exchange and the interchain hopping term. The spin
part is mapped to a quantum sine-Gordon model via a bosonization method that
fully accounts for the phase fluctuation. The spin gap in the Luther-Emery
phase evolves to gapless Luttinger liquid phases in even-ladders at certain
doping concentration. This transition occurs as doping rate increases and/or
the ratio of the interchain spin exchange and hopping to the intrachain
couplings decreases. We also estimate the transition temperature at which the
conventional electron phase is deconfined to spinon and holon.
