Electrical performances of gate-all-around (GAA) tunneling field-effect transistors (TFETs) based on a silicon germanium (Si_1-xGe_x) layer have been investigated in terms of subthreshold swing (SS), on/off current ratio, on-state current (I_on). Cut-off frequency (f_T) and maximum oscillation frequency (f_max) were demonstrated from small-signal parameters such as effective gate resistance (R_g), gate-drain capacitance (C_gd), and transconductance (g_m). According to the technology computer-aided design (TCAD) simulation results, the current drivability, f_T, and f_max of GAA TFETs based on Si_1-xGe_x layer were higher than those of GAA TFETs based on silicon. The simulated devices had 60nm channel length and 10nm channel radius. A GAA TFET with x =0.4 had maximum I_on of 51.4µA/µm, maximum f_T of 72GHz, and maximum f_max of 610GHz. Additionally, improvements of performance at the presented device with PNPN junctions were demonstrated in terms of I_on, SS, f_T, and f_max. When the device was designed with x =0.4 and n⁺ layer width (W_n) =6nm, it shows I_on of 271µA/µm, f_T of 245GHz, and f_max of 1.49THz at an operating bias (V_GS=V_DS=1.0V).