The temperature-dependent effect of residual charge carrier(no),at the Dirac point,on mobility is studied.We fabricate and characterize a graphene Geld effect transistor(GFET) using 7nm TiO_2 as tie top-gate dielectric.The temperature-dependent gate voltage-drain current and room temperature gate capacitance are measured to extract the carrier mobility and to estimate the quantum capacitance of the GFET.The device shows the mobility value of 900 cm~2/V-s at room temperature and it decreases to 45 cm~2/V-s for 20 K due to the increase of n_0.These results indicate that the phonon scattering is not the dominant process for the unevenness dielectric layer while the coulomb scattering by charged impurities degrades the device characteristically at low temperature. The temperature-dependent effect of residual charge carrier (no), at the Dirac point, on mobility is studied. We fabricate and characterize a graphene Geld effect transistor (GFET) using 7nm TiO_2 as tie top-gate dielectric. Temperature- dependent gate voltage-drain current and room temperature gate capacitance are measured to extract the carrier mobility and to estimate the quantum capacitance of the GFET. The device shows the mobility value of 900 cm ~ 2 / Vs at room temperature and it decreases to 45 cm ~ 2 / Vs for 20 K due to the increase of n_0.These results that that phonon scattering is not the dominant process for the unevenness dielectric layer while the coulomb scattering by charged like degrades the device characteristically at low temperature.