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Switching from weakly to strongly limited injection in self-aligned, nano-patterned organic transistors

View Article: PubMed Central - PubMed

ABSTRACT

Organic thin-film transistors for high frequency applications require large transconductances in combination with minimal parasitic capacitances. Techniques aiming at eliminating parasitic capacitances are prone to produce a mismatch between electrodes, in particular gaps between the gate and the interlayer electrodes. While such mismatches are typically undesirable, we demonstrate that, in fact, device structures with a small single-sided interlayer electrode gap directly probe the detrimental contact resistance arising from the presence of an injection barrier. By employing a self-alignment nanoimprint lithography technique, asymmetric coplanar organic transistors with an intentional gap of varying size (< 0.2 μm) between gate and one interlayer electrode are fabricated. An electrode overlap exceeding 1 μm with the other interlayer has been kept. Gaps, be them source or drain-sided, do not preclude transistor operation. The operation of the device with a source-gate gap reveals a current reduction up to two orders of magnitude compared to a source-sided overlap. Drift-diffusion based simulations reveal that this marked reduction is a consequence of a weakened gate-induced field at the contact which strongly inhibits injection.

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Simulated drain-source current, iSD, scaled with the inverse mobility as a function of the gate bias in device structure IV operated either with a source-gate gap of Lov,B = −100 nm (open circles) and a source-gate overlap of Lov,A = 1 μm (closed squares) for, (a), an injection barrier of 0.2 eV and μ = 1 cm2V−1s−1 for gap operation; (b) as in (a) for gap and overlap operation with higher injection barrier 0.5 eV corresponding to Au-pentacene; (c), as in (b) with a lower mobility μ = 10−3 cm2V−1s−1. For comparison, the corresponding value derived from the gradual channel approximation is shown (dashed line). The devices were operated at VDS = −14 V.
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f3: Simulated drain-source current, iSD, scaled with the inverse mobility as a function of the gate bias in device structure IV operated either with a source-gate gap of Lov,B = −100 nm (open circles) and a source-gate overlap of Lov,A = 1 μm (closed squares) for, (a), an injection barrier of 0.2 eV and μ = 1 cm2V−1s−1 for gap operation; (b) as in (a) for gap and overlap operation with higher injection barrier 0.5 eV corresponding to Au-pentacene; (c), as in (b) with a lower mobility μ = 10−3 cm2V−1s−1. For comparison, the corresponding value derived from the gradual channel approximation is shown (dashed line). The devices were operated at VDS = −14 V.

Mentions: To explore the first scenario, we performed a simulation assuming perfect injection, i.e., the injection barrier height Φ, defined as the offset between the Fermi level in the metal and the transport level in the OSC, is set to a small value of 0.2 eV. Moreover, a large mobility of 1 cm2V−1s−1 is assumed. Due to the large current demand associated to such large mobilities, the consequences of imperfections near the contacts are expected to be larger pronounced than for small mobilities. It is shown in Fig. 3a that the resulting transfer curve (at VDS = −14 V) for gap operation (circles) essentially coincides with the curve expected from GCA (dashed line). Despite a small gap segment, charges spread and accumulate across the entire channel as in the ideal case.


Switching from weakly to strongly limited injection in self-aligned, nano-patterned organic transistors
Simulated drain-source current, iSD, scaled with the inverse mobility as a function of the gate bias in device structure IV operated either with a source-gate gap of Lov,B = −100 nm (open circles) and a source-gate overlap of Lov,A = 1 μm (closed squares) for, (a), an injection barrier of 0.2 eV and μ = 1 cm2V−1s−1 for gap operation; (b) as in (a) for gap and overlap operation with higher injection barrier 0.5 eV corresponding to Au-pentacene; (c), as in (b) with a lower mobility μ = 10−3 cm2V−1s−1. For comparison, the corresponding value derived from the gradual channel approximation is shown (dashed line). The devices were operated at VDS = −14 V.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5037384&req=5

f3: Simulated drain-source current, iSD, scaled with the inverse mobility as a function of the gate bias in device structure IV operated either with a source-gate gap of Lov,B = −100 nm (open circles) and a source-gate overlap of Lov,A = 1 μm (closed squares) for, (a), an injection barrier of 0.2 eV and μ = 1 cm2V−1s−1 for gap operation; (b) as in (a) for gap and overlap operation with higher injection barrier 0.5 eV corresponding to Au-pentacene; (c), as in (b) with a lower mobility μ = 10−3 cm2V−1s−1. For comparison, the corresponding value derived from the gradual channel approximation is shown (dashed line). The devices were operated at VDS = −14 V.
Mentions: To explore the first scenario, we performed a simulation assuming perfect injection, i.e., the injection barrier height Φ, defined as the offset between the Fermi level in the metal and the transport level in the OSC, is set to a small value of 0.2 eV. Moreover, a large mobility of 1 cm2V−1s−1 is assumed. Due to the large current demand associated to such large mobilities, the consequences of imperfections near the contacts are expected to be larger pronounced than for small mobilities. It is shown in Fig. 3a that the resulting transfer curve (at VDS = −14 V) for gap operation (circles) essentially coincides with the curve expected from GCA (dashed line). Despite a small gap segment, charges spread and accumulate across the entire channel as in the ideal case.

View Article: PubMed Central - PubMed

ABSTRACT

Organic thin-film transistors for high frequency applications require large transconductances in combination with minimal parasitic capacitances. Techniques aiming at eliminating parasitic capacitances are prone to produce a mismatch between electrodes, in particular gaps between the gate and the interlayer electrodes. While such mismatches are typically undesirable, we demonstrate that, in fact, device structures with a small single-sided interlayer electrode gap directly probe the detrimental contact resistance arising from the presence of an injection barrier. By employing a self-alignment nanoimprint lithography technique, asymmetric coplanar organic transistors with an intentional gap of varying size (< 0.2 μm) between gate and one interlayer electrode are fabricated. An electrode overlap exceeding 1 μm with the other interlayer has been kept. Gaps, be them source or drain-sided, do not preclude transistor operation. The operation of the device with a source-gate gap reveals a current reduction up to two orders of magnitude compared to a source-sided overlap. Drift-diffusion based simulations reveal that this marked reduction is a consequence of a weakened gate-induced field at the contact which strongly inhibits injection.

No MeSH data available.


Related in: MedlinePlus