We design a heterojunction-enhanced n-channel tunneling FET (HE-NTFET) employing a Ge1−xSnx/Ge1−ySny ( $x>y$ ) heterojunction located in the channel region with a distance of $L_{\rm T-H}$ from the source-channel tunneling junction. We investigate the impact of $L_{\rm T-H}$ on the performance of HE-NTFETs by simulation. HE-NTFETs achieve a positive shift of $V_{\rm ONSET}$ , a steeper subthreshold swing (SS), and an enhanced $I_{\mathrm{{\scriptscriptstyle ON}}}$ compared with homo-NTFETs, which is attributed to the modulating effect of heterojunction on band-to-band tunneling (BTBT). At a supply voltage of 0.3 V, 304% $I_{\mathrm{{\scriptscriptstyle ON}}}$ enhancement is demonstrated in the Ge0.92Sn0.08/Ge0.94Sn0.06 HE-NTFET with a 4 nm $L_{\rm T-H}$ over Ge0.92Sn0.08 homo-NTFET due to the steeper average SS. The impact of Sn composition on the performance of HE-NTFETs is also studied. As we increase the difference in Sn composition $x-y$ across the heterojunction, $I_{\mathrm{{\scriptscriptstyle ON}}}$ and SS of HE-NTFETs are improved due to the increase in band offsets at the Ge1−xSnx/Ge1−ySny interface, which leads to the enhanced modulating effect of heterojunction on BTBT.