An equivalent circuit of transistors operating in saturation region is suggested. This circuit contains a transistor operating in active region and a diode biased in forward direction. By using this equivalent circuit, the physical meaning of storage time of transistors may be explained more intuitively. With this, the storage time in drift transistor, Which is common base connected, has been investigated. By solving the continuity equation, the steady state and the transient components of the densities of minority carriers in diode near the p-n-junction is obtained. By setting these two components equal in magnitude and opposite in sign, we get a formula, from which the storage time may be determined. The storage time in some special cases has been calculated. The results show that the storage time depends upon the life time of minorities and the surface recombination velocities both in base region and in collector region. This may be a guide for design a transistor with more short storage time.