An investigation of a suppressed-drain cylindrical gate-all-around retrograde-doped heterospacer steep-density-film tunneling field-effect transistor

The effect of retrograde doping with a high-density layer on the performance of a cylindrical (cyl.) gate-all-around (GAA) tunneling field-effect transistor (TFET) has been investigated. The proposed device design incorporates retrograde doping (RD) along with a heterospacer (HeTS) on a silicon-germanium substrate. The proposed device also includes a suppressed drain (SD) and steep density film (SDF) to achieve the optimal ON-current (I_ON) but suppressed OFF-current (I_OFF) across the drain tunneling junctions. Since a high gate potential results in band bending in the source and drain regions due to the fringing fields, the heterospacer dielectric causes steep energy-band bending on the source side, leading to a high current drivability. Furthermore, to mitigate reliability issues, the trap charges are analyzed. Enhanced switching and radiofrequency (RF) characteristics are achieved when retrograde doping is incorporated with a steep density film. The device design and simulation are performed using Synopsys three-dimensional (3D) technology computer-aided design (TCAD) software. Direct-current (DC) and transient characteristics such as I_OFF, I_ON, the gate–source capacitance (C_gs), the gate–drain capacitance (C_gd), the transconductance (g_m), the electric field intensity, and the average subthreshold swing (SS_avg) are analyzed for the three proposed structures. The simulation results reveal that that the SD GAA RD HeTS SDF TFET exhibits enhanced performance compared with other device structures, showing a high I_ON of 1.64 × 10^–5 A/µm, a diminished I_OFF of 3.48 × 10^–18 A/µm, a g_m of 57 µS/µm at V_gs = 0.6 V, and the steepest subthreshold slope (SS) of 29 mV/decade, but without degradation of the current drivability.