Steep switching devices for low power applications: negative differential capacitance/resistance field effect transistors

Abstract

Simply including either single ferroelectric oxide layer or threshold selector, we can make conventional field effect transistor to have super steep switching characteristic, i.e., sub-60-mV/decade of subthreshold slope. One of the representative is negative capacitance FET (NCFET), in which a ferroelectric layer is added within its gate stack. The other is phase FET (i.e., negative resistance FET), in which a threshold selector is added to an electrode (e.g., source or drain) of conventional field effect transistor. Although the concept of the aforementioned two devices was presented more or less recently, numerous studies have been published. In this review paper, by reviewing the published studies over the last decade, we shall de-brief and discuss the history and the future perspectives of NCFET/phase FET, respectively. The background, experimental investigation, and future direction for developing the aforementioned two representative steep switching devices (i.e., NCFET and phase FET/negative resistance FET) are to be discussed in detail.

Keywords: Field effect transistor; Low power application; Negative capacitance; Phase FET; Steep switching device.

Fig. 1

Atomic structure of typical ferroelectric material, Pb(Zr_xTi_1−x)O₃ (PZT). The atomic structure of ferroelectric material (herein, PZT) is shown. The polarization direction is set by externally applied bias. When the externally applied electric field is stronger than the coercive electric field of the ferroelectric material, the atom at the center of unit cell can move upward or downward, resulting in the switching of polarization state

Fig. 2

Charge (Q) vs. voltage (V) characteristic of ferroelectric capacitor. The capacitance is defined as the variation of charge over the variation of applied voltage. The red-colored marks represent the coercive voltages of ferroelectric layer. As the externally applied bias becomes higher than the coercive voltages for forward/reverse voltage sweep, the polarization switching occurs. This renders sudden charge reduction in ferroelectric capacitor. This is illustrated as the “S-shaped” curve in the Q vs. V plot

Fig. 3

Energy (U) vs. charge (Q) characteristics of ferroelectric capacitor. The navy, green, and red-colored curve represents the U vs. Q curve of dielectric capacitor, ferroelectric capacitor, and capacitance-matched capacitor (i.e., the capacitor which is composed as the series connection of dielectric and ferroelectric capacitor), respectively. By capacitance-matching, the total U vs. Q curve has single energy minimum region, and thereby the hysteresis of ferroelectric capacitor no longer exists

Fig. 4

Current–voltage characteristics of TS device and phase FET. a Current–voltage (I–V) characteristic of TS device. The TS device is turned on at threshold voltage, and tuned off at hold voltage. b Drain current vs. gate voltage (I_D–V_G) characteristic of phase FET. Both the suppression of off-state leakage current and steep switching characteristic can be achieved, simply by connecting the TS device in series to baseline transistor

Fig. 5

Filament-based threshold selector (TS) device. The Filament of CBRAM can be unstable by modulating compliance current, and therefore, the weak filament is dissolved at hold voltage