Ying-Hsuan Chou

Polymeric charge storage electrets for non-volatile organic field effect transistor memory devices

In this review, we present the effects of the chemical structure and composition of polymer or composite electrets on tuning the memory characteristics of the non-volatile organic field effect transistor (OFET) memory devices, including surface polarity, π-conjugation length, architecture, donor–acceptor strength, and interface energy barrier. The recent progress in developing polymer-based charge storage electrets is highlighted in order to provide insights into understanding the operation mechanism and the molecular design–memory properties relationship, as well as improving the overall performance of OFET memory devices.

Nonvolatile organic field-effect transistor memory devices using polymer electrets with different thiophene chain lengths

We report the synthesis of poly(5-hexyl-2-vinylthiophene) (PVT) and poly(5-hexyl-5′′-vinyl-2,2′:5,2′′-terthiophene) (PVTT) as charge storage electrets for nonvolatile organic field effect transistor (OFET) memory devices of n-type semiconducting N,N′-bis(2-phenylethyl)perylene-3,4,9,10-bis(dicarboximide) (BPE-PTCDI). The effects of the conjugated thiophene chain length on the morphology, OFET mobility and memory characteristics are explored and compared to those of the styrene or fluorene side chain. The mobility of the OFET memory device using PVTT as an electret is significantly smaller compared with that of PVT because its large torsional angle hinders the molecular packing of BPE-PTCDI. However, the OFET memory device using the PVTT electret has the largest hysteresis window of 81 V, compared to PVT, polystyrene (PS), and poly(styrene) para-substituted with fluorene (PSt-Fl). The highest HOMO energy level of PVTT facilitates the charge transfer from BPE-PTCDI and leads to the largest memory window. The backbone non-coplanarity prevents the back transfer of the charge for the nonvolatile memory characteristics. The device shows excellent nonvolatile behavior for bistable switching and the write–read–erase–read (WRER) cycles are operated over 100 cycles. The shifted threshold voltages of the OFET memory devices using PVTT are stable over 104 s, and the ON and OFF states could maintain 104 s with the Ion/Ioff current ratios of 103. This study suggests that the pendent conjugation length and the backbone coplanarity of polymer electrets significantly affect the charge mobility and electrical characteristics of OFET memory devices.

Nonvolatile transistor memory devices using high dielectric constant polyimide electrets

We report on the nonvolatile memory characteristics of pentacene-based organic field-effect transistors (OFETs) using polyimides, PI(6FDA-TPA-CN), PI(DSDA-TPA-CN), and PI(BTDA-TPA-CN), consisting of electron-donating 4,4′-diamino-4′′-cyanotriphenylamine (TPA-CN) and different electron-accepting dianhydrides as polymer electrets. The dielectric constants of PI(BTDA-TPA-CN), PI(DSDA-TPA-CN), and PI(6FDA-TPA-CN) are 3.44, 3.52, and 3.70, respectively, higher than those (∼3) of common polyimides. Among the polymer electrets, the OFET memory device based on PI(6FDA-TPA-CN) exhibits the highest OFET mobility of 0.5 cm2 V−1 s−1 due to the formation of a pentacene film of large grain size by the hydrophobic surface. The OFET memory devices with the configuration of n+Si/SiO2/PI/pentacene/Au show excellent nonvolatile memory behaviors for bistable switching. The stability for ON and OFF states can be maintained for 104 s with a Ion/Ioff current ratio of 104 for PI(6FDA-TPA-CN). Moreover, the higher dipole moment and larger torsion angle result in the more stable charge transfer complex, accompanied by the largest memory window of 84 V for the fabricated device. The write–read–erase–read (WRER) cycles can be operated over 100 cycles. The present study suggests that the high dielectric constant polyimide electrets with the enhanced capabilities for storing the charges have great potential applications for advanced OFET memory devices.

Nonvolatile organic field effect transistor memory devices using one-dimensional aligned electrospun nanofiber channels of semiconducting polymers

We report the nonvolatile memory characteristics of organic field effect transistors (OFETs) using one-dimensional aligned electrospun (ES) nanofibers of semiconducting poly(9,9-dioctyl-fluorene-co-bithiophene) (F8T2). The effects of the nanofiber diameter associated with the polymer chain orientation on the charge transport and storage ability were explored. The OFET devices using the F8T2 ES nanofibers exhibited a large average memory window of ∼30 V, an on–off ratio of 102–103 and a hole mobility of 10−4 to 10−2 cm2 V−1 s−1. The write, erase and read processes of the memory device were performed by voltages across the nanofiber channels for at least 100 cycles and could be stabilized for at least 1000 s. The reversible hysteresis characteristics were attributed to the shallow trapping in the ordered/disordered domains of the F8T2 nanofibers from the in situ grazing incidence wide angle X-ray scattering (GIWAXS) analysis. Our results suggest that the semiconducting ES nanofibers could have potential applications for high performance OFET-based nonvolatile organic memory devices.

Nonvolatile transistor memory devices based on high-k electrets of polyimide/TiO2 hybrids

Novel nonvolatile memory behaviors of n-type N,N′-bis(2-phenylethyl)-perylene-3,4,9,10-tetracarboxylic diimide (BPE-PTCDI) based organic field-effect transistors (OFET) using the new polyimides (PIs), (poly[9,9-bis(4-(4-amino-3-hydroxyphenoxy)phenyl)fluorene-oxydiphthalimide]) PI(F-ODPA) and (poly[4,4′-bis(4-amino-3-hydroxyphenylthio)diphenyl sulfide-oxydiphthalimide]) PI(3S-ODPA), and their PI/TiO2 hybrids as electrets are reported. The OFET memory devices derived from PI(F-ODPA) with π-conjugated fluorene moieties exhibited a larger memory window (8.6 V), and could be further enhanced by introducing TiO2 (up to 20 wt%) into the PIs. In addition, PI/TiO2 hybrids as electrets for memory devices also could effectively reduce the operating voltage, indicating that the charge transfer capability of TiO2 plays an important role in transferring and storing the charge for OFET memory devices.

CHAPTER 10:Organic Floating Gate Transistor Memory Devices

Floating gate charge storage devices are one of the largest families of organic transistor-type memory electronics. The quantity of charge carriers stored in a specific trapping site can be precisely controlled in floating gate memory, breaking through the limitations of device size and meeting the requirement for high density data storage. In this chapter, we briefly introduce common charge storage materials, mostly metallic nanoparticles, used as charge storage elements. Then floating gate materials with various fabrication processes and chemical structures are discussed. In addition, the operating mechanism and future flexible digital memory electronic devices using floating gate charge storage layers are presented.