In this work, we theoretically investigate the tunable photonic band gap (PBG) in a semiconductor-dielectric photonic crystal made of highly doped n-type silicon (Si) layers alternating with silicon oxide layers. The tunable characteristic is studied by changing the donor impurity concentration in Si layer. The PBG is numerically analyzed in the near infrared frequency region from the reflectance calculated by the transfer matrix method. The of filling factor in Si layer on the photonic band gap is also illustrated. These tunable properties in such a photonic crystal provide some information that could be of technical use to the semiconductor optoelectronics, especially in communication applications.