Hongjuan Wang

Design of GeSn-Based Heterojunction-Enhanced N-Channel Tunneling FET With Improved Subthreshold Swing and ON-State Current

We design a heterojunction-enhanced n-channel tunneling FET (HE-NTFET) employing a Ge_1-xSn_x/Ge_1-ySn_y (x > y) heterojunction located in the channel region with a distance of L_T-H from the source-channel tunneling junction. We investigate the impact of L_T-H on the performance of HE-NTFETs by simulation. HE-NTFETs achieve a positive shift of VONSET, a steeper subthreshold swing (SS), and an enhanced ION 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% ION enhancement is demonstrated in the Ge_0.92Sn_0.08/Ge_0.94Sn_0.06 HE-NTFET with a 4 nm L_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, ION and SS of HE-NTFETs are improved due to the increase in band offsets at the Ge_1-xSn_x/Ge_1-ySn_y interface, which leads to the enhanced modulating effect of heterojunction on BTBT.

Strained GeSn p-Channel Metal–Oxide–Semiconductor Field-Effect Transistors With In Situ Si 2 H 6 Surface Passivation: Impact of Sn Composition

We report a study about the impact of Sn composition on the performance of strained germanium-tin (GeSn) pMOSFETs. GeSn pMOSFETs with Sn compositions of 0.027, 0.040, and 0.075 were fabricated on Ge(001) with an in situ Si₂H₆ passivation. Enhancement in drive current and transconductance is obtained for GeSn pMOSFETs with higher Sn composition due to the smaller capacitance equivalent thickness, the reduced source/drain resistance, and the improved effective hole mobility μeff. Right shift of threshold voltage with Sn composition is observed for the devices. Ge_0.973Sn_0.027, Ge_0.960Sn_0.040, and Ge_0.925Sn_0.075 pMOSFETs demonstrate the peak μ_eff of 340, 378, and 496 cm²/Vs, respectively. At an inversion charge density of 5 × 10¹² cm^-2, Ge_0.925Sn_0.075 pMOSFETs demonstrate 36% and 24% enhancement in μ_eff compared with Ge_0.973Sn_0.027 and Ge_0.960Sn_0.040 devices, respectively. Simulation shows that the enhancement in μeff with Sn composition is resulted from the reduction of hole effective mass and intervalley scattering between heavy and light holes caused by the increased compressive strain.

Germanium-Tin P-channel tunneling field-effect transistors: Impacts of biaxial tensile strain and surface orientation

This work investigates the impacts of biaxial tensile strain and surface orientation on performance of GeSn pTFET. Multi-bands k·p method is used to calculate the band structure of biaxially tensile strained GeSn on various orientations. The electrical characteristics of tensile strained GeSn line- and point-pTFETs are computed implementing the dynamic nonlocal BTBT algorithm. Our simulation demonstrates that 1) tensile strained GeSn pTFETs achieve significantly improved |ION| over relaxed devices; 2) With the same tensile strain, GeSn pTFETs on (011) and (111) orientations demonstrate higher |ION| compared to (001)-oriented device.

Mobility enhancement in undoped Ge0.92Sn0.08 quantum well p-channel metal-oxide-semiconductor field-effect transistor fabricated on (111)-oriented substrate

We report the dependence of the electrical performance on surface orientations of undoped Ge0.92Sn0.08 quantum well (QW) pMOSFETs on Ge(111) and (001) substrates. (111)-oriented Ge0.92Sn0.08 QW pMOSFETs show a peak μeff of 845 cm2V−1 s−1 and demonstrate a μeff improvement of 25% over (001)-oriented control at an inversion charge density of 5 × 1012 cm−2. We also report that undoped Ge0.92Sn0.08 QW pMOSFETs show a higher μeff than the doped GeSn devices reported in the literature. The high μeff achieved in undoped QW devices is enabled by incorporating high biaxial compressive strain (1.43%) and eliminating dopant impurity scattering in the defect-free channel.

Temperature dependent IDS–VGS characteristics of an N-channel Si tunneling field-effect transistor with a germanium source on Si(110) substrate

We fabricated n-type Si-based TFETs with a Ge source on Si(110) substrate. The temperature dependent IDS–VGS characteristics of a TFET formed on Si(110) are investigated in the temperature range of 210 to 300 K. A study of the temperature dependence of ILeakage indicates that ILeakage is mainly dominated by the Shockley-Read-Hall (SRH) generation—recombination current of the n+ drain—Si substrate junction. ION increases monotonically with temperature, which is attributed to a reduction of the bandgap at the tunneling junction and an enhancement of band-to-band tunneling rate. The subthreshold swing S for trap assisted tunneling (TAT) current and band-to-band tunneling (BTBT) current shows the different temperature dependence. The subthreshold swing S for the TAT current degrades with temperature, while the S for BTBT current is temperature independent.

Strained Ge 0.96 Sn 0.04 P-channel MOSFETs with in situ low temperature Si 2 H 6 surface passivation

We developed process flow for GeSn pMOSFET fabrication with in situ low temperature Si₂H₆ passivation module. High performance Ge_0.96Sn_0.04 pMOSFETs were fabricated. At a Q_mv of 6×10¹² cm^-2, a 24% enhancement in μ_eff is demonstrated in Ge_0.96Sn_0.04 pMOSFETs compared to Ge control.