  The strong coupling effect in the (2+1)-dimensional quantum electrodynamics
(QED_3) description of the $S=1/2$ Heisenberg antiferromagnet is studied in
terms of a canonical transformation which has been used in the small polaron
theory in electron-phonon systems. In the N{\' e}el ordered phase, we show that
the Dirac fermions are localized and the spectral function is of the Gaussian
form due to the coupling to the longitudinal gauge field. The width of the
broad line shape is \sim 4J with J the superexchange interaction energy and the
localization length is sim 3a, with a the lattice constant.
