  We present a comprehensive timing and spectral studies of Be/X-ray binary
pulsar EXO 2030+375 using extensive Rossi X-ray Timing Explorer observations
from 1995 till 2011, covering numerous Type I and 2006 Type II outbursts. Pulse
profiles of the pulsar were found to be strongly luminosity dependent. At low
luminosity, the pulse profile consisted of a main peak and a minor peak that
evolved into a broad structure at high luminosity with a significant phase
shift. A narrow and sharp absorption dip, also dependent on energy and
luminosity, was detected in the pulse profile. Comparison of pulse profiles
showed that the features at a particular luminosity are independent of type of
X-ray outbursts. This indicates that the emission geometry is solely a function
of mass accretion rate. The broadband energy spectrum was described with a
partial covering high energy cutoff model as well as a physical model based on
thermal and bulk Comptonization in accretion column. We did not find any
signature of cyclotron resonance scattering feature in the spectra obtained
from all the observations. A detailed analysis of spectral parameters showed
that, depending on source luminosity, the power-law photon index was
distributed in three distinct regions. It suggests the phases of spectral
transition from sub-critical to super-critical regimes in the pulsar as
proposed theoretically. A region with constant photon index was also observed
in ~(2-4) x 10^37 erg/s range, indicating critical luminosity regime in EXO
2030+375.
