In this paper, based on the differential equations of Change rate of hole concentration for deep acceptor trap and excess hole located below quasi-Fermi level under equilibrium or non-equilibrium state (with light illumination), an exponential attenuation equation has been deduced. It is capable of explaining the effect of deep level trap on photo-pulse transient characteristics, satisfactorily. We indicate chat the deep trap is the primary cause of producing the long tail of attenuation curve. The photo-pulse transient characteristic curves of GaAs MESFET, GaAl-As TEGFET and Si JFET were measured. The experimental results could check the exactness of the theory. The effect of deep level depth ET, concentration NT and so on attenuation curves has also been discussed. The dependence of equivalent noise voltage Eeq on frequence for the above three devices were measured. The relations between equivalent noise current Ieq and f, and between Ieq and effect deep level concentration NT,eff were obtained using the equations in ref. (6). In this way, the effect of deep level trap on noise may be obtained. The relation between △NT,eff and drift of drain current △ID could be calculated. They were in well agreement with the experimental results. Hence, the increase of △ID also results from the deep level trap.