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We developed an underetching technique to define submicrometer channel length polymer field-effect transistors. Short-channel effects are avoided by using thin silicon dioxide as gate insulator. The transistors with 1 and 0.74 m channel length operate at a voltage as low as 5 V with a low inverse subthreshold slope of 0.4-0.5 V/dec, on-off ratio of 10 4 , and without short-channel effects. The poly͑3-alcylthiophene͒'s still suffer from a low mobility and hysteresis does occur, but it is negligible for the drain voltage variation. With our underetching technique also device structures with self-aligned buried gate and channel length below 0.4 m are fabricated on polymer substrates. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1758775͔Organic field-effect transistors ͑OFETs͒ are of interest for many applications including organic displays, complementary circuits, and all-polymer integrated circuits. 1-3Low-cost fabrication requires the use of solution-processible polymers instead of vacuum deposited low-molecular weight organics. Although mobilities of up to 0.1 cm 2 /V s have been demonstrated for soluble regioregular poly͑3-alkylthiophene͒, 3,4 poly-͑9,9-dioctylfluorene-bithiophene͒, 5 and pentacene from soluble precursors, 6 OFETs needed for high-performance circuits still require extremely small gate length to be modulated at competitive frequencies. The cutoff frequency f 0 ϭg m /(2C GS ) is determined by the maximum transconductance g m and the gate-source capacitance C GS . Neglecting parasitic capacitances, the latter is approximated by the gate oxide capacitance. Using the simple Shockley current characteristics one obtains as an estimation f 0 ϽU GS,eff /(2L 2 ), where U GS,eff is the gate-source voltage relative to the threshold voltage. With a voltage of at most 10 V, a realistic upper value of 0.01 cm 2 /V s for the mobility and a lower limit for the cut-off frequency of 100 kHz, one obtains as an upper limit for the channel length L Ͻ4 m. 10 used embossing to fabricate vertical-channel fieldeffect transistors with submicrometer channel lengths, but could not observe saturation in the measured output characteristics. These authors also presented a self-aligned gate structure with reduced overlap capacitance. We demonstrate the fabrication of submicrometer field-effect transistor channels by employing an underetching technique for submicrometer patterning, to enable the use of low-resolution photolithographic steps and standard microelectronic processes. Transistor design and dimensions are closely related to the material properties. At first, in order to reach the off-state and saturation, the active layer thickness must be less than the depletion length. 11 We use poly͑3-octylthiophene͒ ͑P3OT͒ and poly͑3-hexylthiophene͒ ͑P3HT͒, which are both unintentionally highly doped ͑of the order 10 17 cm Ϫ3 Refs. 11-13͒. To successfully deplete the layers their thickness must be of the order of 30 nm or less, which can be achieved by controlling the solution concentration and the spincoating process.11...

Hysteresis effects do occur usually in polymer field‐effect devices. It has been suggested that trap recharging or mobile ions, or formation/dissociation of bipolarons (BP) in the accumulation layer can cause this effect. Here, at first a literature survey on the hysteresis in field‐effect transistors is given. Then examples of measured hysteresis in field‐effect transistors and in metal‐insulator‐semiconductor (MIS) capacitors are presented. The typical peculiarity is the dependency of the flat band voltage on the sweep direction of the gate voltage. Our recent numerical simulations on the trap recharging mechanism in MIS capacitors are continued and extended to transistors. Energetically distributed traps can lead to hysteresis. But the form of the hysteresis deviates from the observed one and extreme parameter values are needed. Thus it is more likely that trap recharging can modify a hysteresis caused by another mechanism. For discussing the bipolaron mechanism the equilibrium between polarons and doubly charged states of the polymer chains was analyzed anew. With a rate constant for the bipolaron formation determined recently by Salleo and Street relaxation times for formation/dissociation are determined. They indicate that these processes can cause the hysteresis. A further possible mechanism influencing the hysteresis is connected with complexes between polarons, bipolarons and mobile counter ions. The estimated rate constants indicate processes on the time scale of the measurements. Numerical simulations of these processes as well as of the role of mobile ions and their reactions are required for a full description of the hysteresis. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the investigations of the differences in the measured current characteristics of source/drain top contact (TOC) and bottom contact (BOC) organic field-effect transistors (OFETs). The active layer was made from poly[1,4-phenylene-(4-methylphenyl)imino-4,48-diphenylene-(4-methylphenyl)imino-1,4-phenylenevinylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] and a highly doped silicon wafer with a thermal oxide was used for the gate and the insulator, respectively. Both transistors show a good subthreshold behavior characterized by a threshold voltage of Vth≈−5 V and an inverse subthreshold increase of S≈0.25 V∕dec. The estimated mobility is in the range of 10−3 cm2∕V s. The main difference between the TOC and the BOC transistors is a nonlinear increase of the drain current at small drain voltages in the output characteristics of the TOC OFETs. Numerical two-dimensional simulations show that a high concentration of donor-like traps with a Gaussian or exponential distribution are the reason for this peculiarity. The investigation of the influence of air confirms the presence of such traps.

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