We report the investigation of negative differential resistance (NDR) in negative capacitance (NC) germanium (Ge) pFETs. The NDR in NC transistors is attributed to the coupling of drain voltage to the internal gate voltage V _int via the gate-to-drain capacitance. It is demonstrated that NDR strongly depends on the matching between the NC induced by ferroelectric capacitance C _FE and the positive capacitance associated with the underlying transistor capacitance C _MOS . For the non-hysteretic devices, NDR gets pronounced with an increased thickness of ferroelectric film t _fe and V _GS . This is attributed to the fact that the drain coupling factor is improved with an increased t _fe and V _GS , leading to the better matching between C _FE and C _MOS . For the hysteretic NC transistors, however, NDR is only obtained at the lower V _GS , but not observed at higher V _GS .

GeSn alloys have already attracted extensive attention due to their excellent properties and wide-ranging electronic and optoelectronic applications. Both theoretical and experimental results have shown that direct bandgap GeSn alloys are preferable for Si-based, high-efficiency light source applications. For the abovementioned purposes, molecular beam epitaxy (MBE), physical vapour deposition (PVD), and chemical vapor deposition (CVD) technologies have been extensively explored to grow high-quality GeSn alloys. However, CVD is the dominant growth method in the industry, and it is therefore more easily transferred. This review is focused on the recent progress in GeSn CVD growth (including ion implantation, in situ doping technology, and ohmic contacts), GeSn detectors, GeSn lasers, and GeSn transistors. These review results will provide huge advancements for the research and development of high-performance electronic and optoelectronic devices.

We report the first ferroelectric (FE) HfZrO _x (HZO) Ge and GeSn pMOSFETs with sub-60 mV/decade subthreshold swing (SS) (40~43 mV/decade), negligible hysteresis, and enhanced Ids. With a RTA at 450 ^o C, FE devices with reduced hysteresis of 40~60 mV demonstrate the significantly improved SS and I _ds characteristics compared to control devices without FE, owing to the negative capacitance (NC) effect induced by HZO. FE Ge and GeSn pFETs achieve 22% and 20% I _ds enhancement than control devices, respectively, at the drive voltage of 1.0 V. NC effect in FE devices is proved by the gate leakage and inversion capacitance characteristics.

Negative capacitance (NC) GeSn pFETs integrated with HfZrO _x (HZO) ferroelectric film is demonstrated with sub-20 mV/decade subthreshold swing (SS) over two orders of magnitude of IDS. The ratio of remnant polarization to coercivity in HZO is significantly improved with the increasing of the annealing temperature from 400°C to 500°C, which contributes to the effective reduction of hysteresis in ferroelectric NC GeSn transistors. Ferroelectric NC GeSn pFET annealed at 500°C achieves a hysteresis of 70 mV while maintaining a steep SS dramatically lower than 60 mV/decade, and an improved IDS over control device without HZO.

In this letter, we proposed and experimentally demonstrated a metal-insulator-semiconductor (MIS) ferroelectric capacitor with non-volatile programmable capacitance. By switching the polarization of the Al-doped HfO2 ferroelectric insulator with the external electric field, the depletion width of the semiconductor can be modulated, leading to the continuously adjustable capacitance of the device. Due to the non-volatility of ferroelectricity, the programmed capacitance is stable without the constant DC bias. We also proposed some potential applications of the capacitor in the data storage and circuit design.