Germanium-tin n-channel tunneling field-effect transistor: Device physics and simulation study

We investigate germanium-tin alloy (Ge1−xSnx) as a material for the design of tunneling field-effect transistor (TFET) operating at low supply voltages. Compared with Ge, Ge1−xSnx has a smaller band-gap. The reported band-gap of Ge0.89Sn0.11 is 0.477 eV, ∼28% smaller than that of Ge. More importantly, Ge1−xSnx becomes a direct band-gap material when Sn composition x is higher than 0.11. By employing Ge1−xSnx in TFET, direct band-to-band tunneling (BTBT) is realized. Direct BTBT generally has higher tunneling probability than indirect BTBT. The drive current of TFET is boosted due to the direct BTBT and the reduced band-gap of Ge1−xSnx. Device simulations show that the drive current and subthreshold swing S characteristics of Ge1−xSnx TFETs with x ranging from 0 to 0.2 are improved by increasing the Sn composition x. For Ge0.8Sn0.2 TFET, sub-60 mV/decade S is achieved at a high current level of ∼8 μA/μm. For x higher than 0.11, Ge1−xSnx TFETs show higher on-state current ION compared to Ge TFET at a supply voltage of 0.3 V. Ge1−xSnx alloy is a potential candidate for high performance TFET composed of group IV materials.