ZnO/Si and ZnO/Ge bilayer tunneling field effect transistors: Experimental characterization of electrical properties

In this study, we experimentally characterize the electrical properties of a bilayer tunneling field effect transistor (TFET) with a heterotunneling junction composed of an oxide-semiconductor source and a group-IV-semiconductor channel in detail. Bilayer TFETs with a n-ZnO/p-Si or n-ZnO/p-Ge heterotunneling junction with type-II energy band alignment are fabricated by pulsed-laser deposition of a zinc oxide (ZnO) layer on Si or Ge with various impurity concentrations. The evidence of the TFET operation are examined through a comparison of the electrical characteristics with ZnO thin film transistors as well as the tunneling junction area dependence, which is important for clarifying the operating mechanism. The source material and its impurity concentration significantly affect the I_d-V_g and I_d-V_d characteristics of the bilayer TFETs, with reducing tunneling barrier height and tunneling distance. The influence of the source materials and doping concentrations is also studied by simulation. As a result, the minimum subthreshold swing (SS) of 71 mV/dec and the I_on/I_off ratio of ∼6 × 10⁸ have been achieved for n-ZnO/p-Si and n-ZnO/p-Ge TFETs, respectively, at room temperature. It is also found that the fabricated TFET shows weak measurement temperature dependencies of I_on and SS, which are expected for TFETs, with the extremely low off-state current in a fA/μm range. These characterizations of the electrical characteristics of the bilayer TFETs are important not only for a physical understanding of the operating mechanism but also for further improvement of the TFET performance.