The integration of sensing, memory and computing (IoSMC) is of vital importance to meet the challenges encountered by Internet-of-Things (IoT) and Artificial Intelligence (AI)^1-3. Extended gate (EG) ion-sensitive field-effect transistors (ISFETs) lay a solid foundation for pursuing the development of such highly desired smart biochemical sensor platforms⁴. That is, by isolating the transistors from the test environment, EG ISFETs offer whole chip circuit design and system integration for improved sensitivity, long-term stability, and reliability^5,6. Specific detection can be customized by functionalization of the EG. Thanks to the separation of the sensing environment from the chips, EG ISFETs can not only mitigate the drift, degradation and aging, suppress noise, and ensure stable and reliable sensing response, but also facilitate cleaning and recycling of the sensor, which can thus reduce costs and minimize the impact on the environment⁷.

Negative capacitance ferroelectric field-effect transistors (NC FeFETs) are expected to overcome the limitations of traditional metal-oxide-semiconductor FETs (MOSFETs), specifically the Boltzmann-defined lowest subthreshold swing (SS), i.e., 60 mV/dec at room temperature^8-11. In addition, external electric field-triggered polarization switching in ferroelectric materials can contribute to distinct non-volatile states (‘ON’ and ‘OFF’ states)^12-14, thus opening up new possibilities on IoSMC by utilizing the gating effect of the ferroelectric polarization field on the charge transport of the semiconducting channel.

Nevertheless, traditional ferroelectric materials suffer from the critical dimension problem, which limits their valuable ferroelectricity at the nanometer scale and represents an obstacle for improving the transistor performance as well as the computing and sensing capabilities^14-17. Therefore, NC FeFETs based on two-dimensional (2D) ferroelectric ultrathin films scaled-down to several nanometers are highly promising¹⁸. Particularly, hafnium zirconium oxide (HfZrO_x, HZO), which is achieved via complementary MOS (CMOS)-compatible atomic layer deposition (ALD) technology^19-21, possesses relatively high-k, large bandgap and serves as an innovative candidate for the development of modern FeFETs^22-26 and advanced sensors^27-29.

In this work, a highly sensitive and power-efficient ISFET was proposed based on a metal-ferroelectric-insulator (MFI) gate stack. Here, a 6 nm HZO was adopted as the ferroelectric layer, along with a 2 nm thick AlO_x buffer layer, to construct an MFI gate stack on p-type silicon or highly n-doped germanium. This design allows to harvest the NC effect and the ferroelectric memory effect with reduced operation voltage for sensing applications³⁰. The NC effect was successfully characterized, which achieved valuable sub-60 mV/dec SS (the lowest point SS = 40 mV/dec) and the desired ferroelectric loop in the fabricated MFI-semiconductor (MFIS) FeFETs. Further ionic-sensing measurements were applied on this NC FeFET with EG. Schematic illustration of the sensing platform and the polarization states originated from different gating effects are shown in Fig. 1a and b, respectively. In principle, the ferroelectric polarization state in the HZO layer changes according to the polarity of the applied gate voltage. Furthermore, a different sensing environment with charged molecules around the reference electrode can modulate the applied voltage on the HZO layer according to the charge detection principle of ISFETs. Therefore, the ferroelectric polarization will be modulated by the environment. Fig. 1c illustrates a typical transfer curve of NC FeFET with a clockwise ferroelectric memory window. The ‘OFF’ state of the device exhibits dominant downward polarization. In the presence of charged molecules, including pH, ions, DNAs and proteins, the ‘OFF’ state device can be switched to ‘ON’ under the same gate voltage, as represented by the dashed red line interception, which is mainly resulted from a potential positive shift of the threshold voltage (V_th). Such an upward polarization state is non-volatile and can be maintained unless it is intentionally erased or programed by applying an external gate voltage that surpasses the positive coercive reset voltage. Therefore, the presence of charged molecules can be detected and recorded even if the sensing environment comes back to its original conditions, signifying the potential of FeFETs for smart sensing. At the end of the study, it is found that when the potassium-ion concentrations changes from 1 mM to 1 M, it reveals a high sensitivity of 62 (±2) mV/dec, which is superior to that of 48 mV/dec of its MOSFET counterparts^31,32. Along with configurable ferroelectric memory windows, this work presents an outlook for the potential development of NC FeFETs towards future IoSMC applications.

In Fig. 2a, the optical image and the cross-section schematic structure of the MFIS FeFET arrays were presented. A sub-10 nm gate dielectric stack constituted of 2 nm AlO_x and 6 nm HZO was adopted on the either p-type Si or n-type Ge substrates with the adoption of an ALD. Prior to deposition, the pure n-type Ge (or p-type Si) surface was firstly cleaned with acetone, methanol and deionized water in an ultrasonic bath for 15 min, passivated with 10 cycles of Al₂O₃, and then treated with O₃ atmosphere for 20 min to form a 2 nm AlO_x layer. Here, AlO_x was built-up as a gate buffer layer to provide performance enhancement. Two major restrictions in FeFETs including gate leakage current and charge traps can be better improved through the AlO_x buffer layer^12,33-35. An extra 6 nm high-quality HZO layer with a Zr/Hf ratio of 1 : 1 was grown on the AlO_x layer by ALD. A TaN layer (100 nm) was deposited by sputtering as the gate electrode. Boron ions (B⁺) and phosphorous ions (P⁻) were implanted into the source/drain (S/D) electrode areas for Ge and Si channel transistors, respectively. The implantation energy was 20 KeV, and the dose was 10¹⁵ cm². Ni (20 to 30 nm) was deposited into the S/D regions to form the metallic S/D contacts. Finally, the devices were accomplished by a 30 s post-annealing at 450 °C. Indeed, in our prior studies, we have examined the orthorhombic phase in HZO³⁶. X-ray diffraction (XRD) curves that have been obtained suggest that mixed phases within HZO after underwent annealing at 450 °C for 30 s. This includes the presence of a partial orthorhombic phase. To confirm the MFIS gate dielectric core structure, cross-sectional transmission electron microscopy (TEM) was made on a representative device (Fig. 2b, comparable metal-ferroelectric-semiconductor (MFS) structure in Fig. S1). According to the TEM profile image, all interfaces between every two layers are flat and clear, validating the integrity of the MFIS gate dielectric core structure. While it is admittedly challenging to identify the orthorhombic phase³⁷ in FFT of the TEM image showcased in Fig. 2b, it is convinced that the analysis via XRD, in conjunction with the electrical polarization P versus electric field E measurements, is adequate to ascertain the ferroelectric properties of HZO.

Ferroelectric properties embodied in, for example, the ferroelectric P-V hysteresis loop and the NC effect, are of key importance in the electrical characterizations of NC FeFETs. In this work, it is observed that a robust ferroelectric polarization response is from metal-ferroelectric HZO film-metal P-V measurement. Fig. 2c depicts the P-V hysteresis loop with a remnant polarization (P_r) of 25.6 μC/cm². Fig. S2 demonstrates an additional ferroelectric hysteresis loop of 10 nm HZO with switching current.

Fig. 3a presents the output curves of a typical NC FeFET device with a 5 μm gate length (L_G). Notably, the device exhibits a negative differential resistance (NDR) effect in the whole subthreshold V_DS region ranging from −0.4 V to −2 V (Fig. S3). One step further, we measured the transfer characteristics and depicted the corresponding SS, as shown in Fig. 3b. Strikingly, augmented by virtue of the NC effect from HZO ferroelectricity, the commendable FeFET achieved sub-60 mV/dec below the thermionic limit. That is, Fig. 3b demonstrates 3 data points below the Boltzmann limit in the subthreshold region with the lowest value of $S S=\frac{d V_{\mathrm{G}}}{d \lg I_{\mathrm{DS}}}=40$ mV/dec, where the potential-sensing response will suppress that of conventional MOSFET devices. The suppressed gate leakage current due to MFIS structure was presented in Fig. S4. It is noted here that the fluctuations in SS (Fig. 3b) may be attributed to the switching of the multi-domains in the ferroelectric HZO thin film, as reported in our previous publications^36,38,39. Such fluctuations are reproducible to a large extent, as illustrated in Fig. S5a. Such significant SS fluctuations, which attribute to non-smooth transfer curves, were also observed in NC FinFETs⁴⁰. To promote further development of highly sensitive biochemical sensors with low SS value and memory functions, the synergetic performance of the ferroelectric memory effect and charge-trapping effect in the subthreshold region of NC FeFET devices will be examined in the following parts.

Fig. 3c depicts the dual sweepings of gate voltage (V_GS) with various ranges (±2 V to ±4 V), which leads to either a counter-clockwise ferroelectric loop or a clockwise hysteresis in an n-NC FeFET. Calculated SS values are shown in Fig. S6. Biased at a sufficiently large V_GS sweeping range of ±4 V, initial/remnant polarization can be flipped/switched, and the device exhibits a significant counter-clockwise ferroelectric loop (with a memory window of 1.3 V). We also observed clockwise hysteresis due to charge-trap states if biased at a small V_GS sweeping range of ±2 V, whereas at a moderate V_GS sweeping range (±3 V), a counter-clockwise loop with a relatively small ferroelectric memory window was also observed. Such changes in the direction, as well as the width of the memory windows, can be ascribed to an underlying synergetic effect by taking into account both the ferroelectric polarization effect and the interfacial charge-trapping effect⁴¹.

To evaluate the sensing performance of the NC FeFETs, further chemical-sensing experiments were performed in aqueous solutions with various K⁺ concentrations as testbeds. Fig. S7a depicts the transfer characteristics of the FeFET and the calculated SS spectra, respectively. It is important to note that the HZO ferroelectric thin film was polarized in P_down state as the gate voltage (from −0.8 to 0.8 V) was swept within the negative coercive voltage (< −2 V, Fig. 4b, see also the inset of Fig. 3c) of the HZO ferroelectric thin film. Thanks to the ferroelectricity of the nanoscale ferroelectric HZO thin film, comparably lower SS (Fig. S7b), as well as amplified voltage-gating efficiency due to NC effect can be achieved advantageously⁴² for ion sensing with enhanced responsivity and sensitivity. It is noted that the FeFET was operated under the same gate step of 20 mV and constant output S/D voltage V_DS (V_DS = 0.1 V for n-type FETs and V_DS = −0.1 V for p-type FETs) for excluding a possible impact of gate voltage step and S/D applied voltage on the electrical behaviors^43-45. A Ag/AgCl reference electrode was adopted as the extended liquid gate to define the electrostatic potential in a home-made liquid chamber and was then connected with the on-chip gate electrode of NC FeFETs as illustrated in Figs. 1a and S8a. Fig. 4a depicts the drain current (I_DS) plotted against the reference voltage (V_ref) in KCl solutions with increased concentrations from 1 mM to 1 M. The NC FeFET sensor yielded a stable and evident response towards positive V_th. We plotted the deduced V_th shifts against the KCl concentrations in the inset of Fig. 4a, which exhibited a positive V_th shift of 62 (±2) mV/dec, corresponding to an overall 76 times change in current at V_ref = −0.5 V, when the concentration varies from 1 mM to 1 M. The relative sensing response can be deduced as $S_{\text {ConI }}=\frac{\left(I_{\mathrm{ConI}}-I_{\mathrm{Con}, \mathrm{ref}}\right)}{I_{\mathrm{Con}, \mathrm{ref}}}$. Repeating the measurements gave stable and reproducible results. In addition, Fig. S8b illustrates that the calculated SS values here are consistent with those obtained in ambient air conditions. The observed potassium sensitivity of 62 mV/dec even slightly exceeds the theoretical value according to the thermodynamic Nernst limit (60 mV/dec at room temperature). The superior sensitivity beyond the Nernst limit can be ascribed to a capacitive effect, which can be referred to the gate voltage amplification due to ferroelectric NC effect⁴². Remarkably, unless the baseline MOSFET has ideal SS at 60 mV/dec limit, it is almost impossible to obtain sub-60 mV/dec SS. Nevertheless, better SS can be obtained in NC FET over the control device no matter the SS is steep or not, i.e., the NC effect might not always lead to the sub-kT/q SS, but it must contribute to the improved performance (SS) compared to the baseline device^46,47. When comparing its performance with its counterparts without ferroelectric NC effect (see Fig. S7b), an improvement in the SS values can be clearly identified, suggesting the effectiveness of the NC effect from HZO.

Furthermore, smart sensing is expected based on the ferroelectric memory effect of the FeFETs. The polarization state of ferroelectric layer could be modulated by the voltage on the extended gate, as illustrated in Fig. 1b. In principle, downward and upward polarization states of P_down and P_up occur under respective gate biases separated by the ferroelectric window, which results in a low-resistance state (LRS) and a high-resistance state (HRS), corresponding to logic bits ‘1’ and ‘0’ as indicated by the arrows in Fig. 4b, respectively. Fig. 4b denotes the transfer curves of an n-NC FeFET with a relatively large counter-clockwise ferroelectric window (1.3 V), tested in ambient air as the background (black line) and against 1 mM K⁺ solution (blue line) as the analyte. Here, it is conceived that chemical-sensing information could be stored by with the adoption of ion-modulated polarization states. The appreciable V_th shift upon introducing 1 mM K⁺ (270 mV) in Fig. 4b suggests the possibility of switching the FeFET sensor from its HRS (P_up state) to LRS (P_down state) when assuming an initial bias EG voltage of −0.7 V (see the red line in Fig. 4b). This polarization state persists even when the sensor returns to its ambient background condition due to the LRS along the reverse transfer curve. A coercive voltage of −1.9 V for upward polarization state (erasing operation) is required to reprogram information, which is far away from the assumed V_ref = −0.7 V. It is also noted here that the aforementioned non-volatile sensing can be achieved, without compromising any prospects of the CMOS compatibility and computing capability of the NC FeFET.

In summary, this work presented the demonstration of an HZO NC FeFET and its promising applications for sensing purposes in the form of EG ISFETs. The potassium ion-sensitive FeFETs exhibit a slightly supra-Nernst sensitivity of 62 (±2) mV/dec towards potassium ions, indicating their potential for highly sensitive ion detection. Furthermore, the incorporation of a configurable non-volatile ferroelectric memory effect adds valuable memory retention and switching capabilities to the FeFETs, which enhances their versatility in sensing applications. This opens up exciting possibilities for the future development of diverse HZO NC FeFET biochemical sensor platforms in IoSMC applications.

Supplementary materials Supplementary data to this article can be found online at https://doi.org/10.1016/j.chip.2023.100074.

Declaration of competing interest The authors declare no competing interests.

Funding This study received financial support from the National Natural Science Foundation of China No. 52073160, the National Key Research and Development Program of China No. 2020YFF01014706, Beijing Municipal Science and Technology Commission (Z211100002421012), and Key Laboratory of Advanced Materials (MOE).