(Invited) A Ferroelectric Semiconductor Field-Effect Transistor
Authors: Si Mengwei , Ye Peide D 
Published: February 2020

A ferroelectric semiconductor field-effect transistor (FeS-FET) is proposed and experimentally demonstrated. In this novel FeS-FET, a 2D ferroelectric semiconductor α-In2Se3 is used to replace conventional semiconductor as channel. α-In2Se3 is identified due to its proper bandgap, room temperature ferroelectricity, the ability to maintain ferroelectricity down to a few atomic layers and the feasibility for large-area growth. An ALD Al2O3 passivation method was developed to protect and enhance the performance of the α-In2Se3 FeS-FETs. The fabricated FeS-FETs exhibit high performance with a large memory window, a high on/off ratio over 108, maximum on-current of 671 μA/μm, high electron mobility with μFE= 311.8 cm2/V∙s in forward sweep and μFE= 488.1 cm2/V∙s in reverse sweep, and the potential to exceed the existing Fe-FETs for non-volatile memory applications.
In the FeS-FET, a ferroelectric semiconductor is employed as the channel material while the gate insulator is dielectric. The two non-volatile polarization states in FeS-FETs exist in the ferroelectric semiconductor. Therefore, a high quality amorphous gate insulator can be used instead of the common crystallized ferroelectric insulator. Meanwhile, the mobile charges in the semiconductor may screen the depolarization field across the semiconductor. Thus, the charge trapping and leakage current through FE insulator in conventional Fe-FETs can potentially be eliminated. As a result, it may improve provide potential performance improvement over the conventional Fe-FETs for non-volatile memory applications.
α-In2Se3 is a recently discovered 2D ferroelectric semiconductor, which is employed in this work to demonstrate the FeS-FET. α-In2Se3 bulk crystals were grown by melt method with a layered non-centrosymmetric rhombohedral R3m structure. Photoluminescence (PL) confirms the semiconducting properties of α-In2Se3 in this work with a bandgap of ~1.39 eV. Piezoresponse force microscopy (PFM) is used to characterize the ferroelectricity in α-In2Se3. PFM phase and PFM amplitude versus voltage hysteresis loop show clear ferroelectric polarization reversal under external electric field. The PL measurement of bandgap and PFM measurement of polarization reversal together confirm the α-In2Se3 used in this work is a ferroelectric semiconductor. As mobile charges exist in a semiconductor, such charges may screen and prevent the electric field to penetrate into the body of the semiconductor, so that the ferroelectric polarization switching may be different in a metal-oxide-semiconductor (MOS) structure. Therefore, it is important to test whether the polarization in α-In2Se3 can be switched by an external electric field in a MOS structure. PFM phase and PFM amplitude versus voltage hysteresis loop of Al2O3/α-In2Se3 on Ni/SiO2/Si structure are measured. The ferroelectric hysteresis loop suggests that α-In­Se3 has switchable polarization in a MOS device structure. Thus, it is viable to applied α-In2Se3 as the channel of a FeS-FET.
α-In2Se3 FeS-FETs are experimentally demonstrated. The electrical performance of the α-In2Se3 FeS-FET is systematically measured. The transfer curve shows clear clockwise hysteresis loop, as expected. A high on/off ratio over 108 at VDS=1 V is also achieved, suggesting a high-quality oxide/semiconductor interface. The large memory window and high on/off ratio suggest the α-In2Se3 FeS-FET is a competitive device concept for non-volatile memory applications. A maximum drain current of 671 μA/μm is achieved. Considering the long channel length (Lch=1 μm) used here, the α-In2Se3 FeS-FETs can have higher on-current at shorter channel length and is potential for high speed applications. The field-effect mobility (μFE) is calculated using maximum gm in forward sweep to be 311.8 cm2/V·s and in reverse sweep to be 488.1 cm2/V·s. The performance of the α-In2Se3 FeS-FETs are significantly improved by the 10 nm Al2O3 ALD passivation, comparing to the unpassivated devices (μFE=19.3 cm2/V·s in forward sweep and μFE=68.1 cm2/V·s in reverse sweep).