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

We investigate germanium-tin alloy (Ge_1-xSn _x) as a material for the design of tunneling field-effect transistor (TFET) operating at low supply voltages. Compared with Ge, Ge_{1- x}Sn_x has a smaller band-gap. The reported band-gap of Ge_0.89Sn_0.11 is 0.477 eV, ∼28% smaller than that of Ge. More importantly, Ge_1-xSn_x becomes a direct band-gap material when Sn composition x is higher than 0.11. By employing Ge_1-xSn_x 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 Ge_1-xSn_x. Device simulations show that the drive current and subthreshold swing S characteristics of Ge_1-xSn_x TFETs with x ranging from 0 to 0.2 are improved by increasing the Sn composition x. For Ge_0.8Sn _0.2 TFET, sub-60 mV/decade S is achieved at a high current level of ∼8 μA/μm. For x higher than 0.11, Ge_1-xSn _x TFETs show higher on-state current I_ON compared to Ge TFET at a supply voltage of 0.3 V. Ge_1-xSn_x alloy is a potential candidate for high performance TFET composed of group IV materials.