In this work, the terahertz transimission spectrum through a one-demensional (1D) photonic crystal made from doped semiconductor n-GaAs/polycarbonate is studied by the transfer-matrix method. We show that, compared with most photonic crystals made from two different dielectrics, the doped semiconductor/dielectrics 1D photonic crystals can not only form the photonic gap, but also enhance the terahertz transmission through n-GaAs. We also propose and research a terahertz modulator based on the 1D photonic crystals, the transimitted terahertz waves through which can be modulated by different carrier densities in the semicondutor by varying the applied voltage.