Top‐Split‐Gate Ambipolar Organic Thin‐Film Transistors

Abstract

easily degrades the electrical characteristics of the p-type or vice versa as optimal process conditions, [12,13] appropriate insulator surface, [14,15] and metal-semiconductor interfaces [16,17] are typically different for each semiconductor type.In contrast, ambipolar semiconducting polymers, in which both holes and electrons can be modulated simultaneously, provide a simple approach toward complementary OTFTs. [18][19][20][21] Fabricating ambipolar OTFTs on a substrate dramatically reduce the complexity of the fabrication process. The ambipolar transistor however exhibits a clear issue limiting its widespread use. The main disadvantage is the lack of well-defined off-state in ambipolar OTFTs. This results in poor circuit noise margin, gain, and high power consumption, thus making it difficult to implement practical integrated circuits (ICs) based on ambipolar organic semiconductors. To selectively make the ambipolar device unipolar, several methods have been proposed including adding different selfassembled monolayers between the ambipolar semiconductor and the dielectric layer. [22,23] Another design approach is implementing a split-gate ambipolar OTFT. [24][25][26][27][28][29] In a split gate device, the polarity of the transistor can be controlled using an additional gate electrode. Depending on the voltage bias of the additional control gate, a unipolar type OTFT (p-or n-) can be obtained exhibiting a large on/off current ratio. Since the polarity of the thin-film transistor (TFT) depends on the control gate voltage bias, a device can be reconfigured to n-type metal-oxide-semiconductor or p-type metal-oxide-semiconductor electrically even after chip is fabricated. This opens up a new design topology for relevant applications including programmable logic circuits or field programmable gate arrays.An underexposed aspect of ambipolar as well as split gate transistors, is their strong susceptibility to gate bias stress. Gate bias stress, a shift in threshold voltage under operation, should be negligible in order to realize robust circuits. Unfortunately, previous works on split-gate ambipolar OTFTs either showed large hysteresis in I-V characteristics [26,27] or reported only the forward voltage sweep characteristics without investigating the I-V hysteresis [24,25,29] (Table 1). The main reason for the observed gate bias stress in previously reported split-gate ambipolar OTFTs is that previous devices used the bottom-gate device geometry based on inorganic oxide dielectric layers such as SiO 2 or Al 2 O 3 . These dielectric materials have good dielectric characteristics, but hydroxyl groups at the interface result Split-gate ambipolar organic transistor technology is gaining interests as a practical solution for the implementation of complementary transistors. It is known that conventional ambipolar transistors suffer from poor DC gain, noise margin, and high power consumption, as they do not have a well-defined off-state region. A split-gate device structure enables ambipolar transistors operating in a contro...

Identifying Key Transport Mechanisms in Organic Antiambipolar Transistors

“…Despite the direct contact with the Au source/drain electrodes available to only one semiconductor (α-6T), both electron-and holeaccumulation modes were clearly observed in these devices, resembling today's widely investigated ambipolar organic fieldeffect transistors (OFETs). [2][3][4][5] Interestingly, when a slight lateral shift occurs between the two overlapping organic layers and each of the p-and n-type molecules forms a contact to source or drain electrode, a strikingly opposite trend is observed in a transistor's transfer characteristics; the drain current (I D ) peaks at an intermediate gate voltage (V G ) with rapid falls at both higher and lower V G values. [6] This so-called "antiambipolar" behavior has been experimentally observed in various combinations between organic, metal-oxide, and atomically thin two-dimensional semiconductors, [7] and such a distinct current-regulating feature has recently been widely exploited to build multi-functional device platforms in electronics and optoelectronics.…”

Negative Transconductance Heterojunction Organic Transistors and their Application to Full‐Swing Ternary Circuits

“…First, the ternary inverters using H‐TRs tend to show incomplete circuit operation in which the output voltage ( V OUT ) does not fully swing between V DD and G ND (Figure S1, Supporting Information) . Because H‐TRs exhibit anti‐ambipolar characteristics (i.e., an inverted ambipolar transfer characteristic) such that either the p‐ or n‐type semiconductor is completely depleted as V G approaches V D (i.e., V G ≈ V D ) due to a high energy barrier (Figure S2, Supporting Information), H‐TRs have relatively lower on currents ( I ON ) than do typical n‐ or p‐type transistors. Hence, the I ON / I OFF ratio of H‐TRs is relatively low.…”

“…The reduction of I – V hysteresis and the high bias stress stability are due to the use of poly‐(perfluorobutenylvinylether) (Cytop) film. Cytop surface is hydrophobic enough to minimize charge trap sites such as hydroxyl group (OH − ) at the dielectric‐semiconductor interface …”

Overlapping‐Gate Organic Light‐Emitting Transistors

“…The overlapping‐gates OLET is a dual‐gate device in which the two gates, separated by a dielectric layer of good quality, have a partial vertical overlap over part of the channel . A fabrication scheme for the OG‐OLET is presented in Figure a and detailed in the Experimental Section.…”