  The main purpose of this paper is to provide an effective procedure to study
rigorously the relationship between unipolar and bipolar Euler-Poisson system
in the perspective of mass. Based on the fact that the mass of an electron is
far less than that of an ion, we amplify this property by letting
$m_e/m_i\rightarrow 0$ and using two different singular limits to illustrate
it, which are zero-electron mass limit and infinity-ion mass limit. We use the
method of asymptotic expansion to handle the problem and find that the limiting
process from bipolar to unipolar system is actually the process of decoupling,
but not the vanishing of equations of the corresponding other particle.
