Heterojunction Tunneling Transistors Using Gate-Controlled Tunneling Across Silicon–Germanium/Silicon Epitaxial Thin Films

Tunneling transistors that incorporate in the gated source an elevated p⁺ Si_0.6Ge_0.4/n^- Si heterojunction and a HfO₂/WN gate stack are constructed. XTEM images show in tact epitaxial SiGe with sub-10-nm thickness. The current/voltage characteristics within 77 K-300 K show behavior consistent with gate-controlled tunneling over several decades of current. Simulations using a nonlocal tunneling model support a tunneling process that occurs across the heterojunction. There is sufficient gate modulation of the surface potential at the p⁺ SiGe/gate-insulator interface to provide the band overlap and band bending for band-to-band tunneling (BTBT). The transfer and output characteristics are considerably improved over previous devices that used buried SiGe films, ion-implanted junctions, and SiO₂ di-electrics, resulting in a reduced minimum subthreshold slope and an increased current drive with identical biasing. Also, due to the asymmetry in this structure and suppression of drain BTBT, I_on/I_off >; 10⁵ is achieved with V_dd = 2.5 V.