Self-aligned organic field-effect transistors on plastic with picofarad overlap capacitances and megahertz operating frequencies

Abstract

Using a combination of nanoimprint lithography, gate-source/drain self-alignment, and gravure and inkjet printing, we fabricate organic field-effect transistors on flexible plastic substrates with gate-source and gate-drain electrode overlap capacitances of COL < 1 pF, equivalent to channel-width normalised capacitances of C*OL = 0.15–0.23 pF mm−1. We compare photopatterned and nanoimprint lithography patterned channels of L ≈ 3.8 μm and L ≈ 800 nm, respectively. The reduction in L was found on average to result in order of magnitude greater switching frequencies. Gravure printing the dielectric (versus photo-patterning) was found to yield an order of magnitude lower overlap capacitance C*OL = 0.03 pF mm−1, at the expense of greater processing variation. Inkjet printed p- and n-type polymeric organic semiconductors were used to fabricate organic-field effect transistors with a peak cutoff frequencies of fS = 9.0 ± 0.3 MHz at VGS = 30 V, and transition frequencies of fT = 3.3 ± 0.2 MHz at VGS = 30 V.

Accessing MHz Operation at 2 V with Field‐Effect Transistors Based on Printed Polymers on Plastic

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Nonetheless, several among the most desirable applications, such as radio-frequency (RF) tags and smart labels, driving circuitry for large-area high-resolution flexible displays, and real-time sensor arrays, along with wireless sensors and sensors networks, require high-speed and low-voltage operation of the basic components of the circuits, in particular the transistor. [14][15][16][17][18][19] Alternative flexible electronics technologies with higher carrier mobilities are being developed to achieve highfrequency circuits, such as metal-oxide semiconductors, carbon nanotubes, and 2D materials, reaching in some cases very high "transition frequency" f t,[20] the highest operational frequency of a transistor, in the order of the GHz or tens of GHz. [14][15][16][17][18][19] Alternative flexible electronics technologies with higher carrier mobilities are being developed to achieve highfrequency circuits, such as metal-oxide semiconductors, carbon nanotubes, and 2D materials, reaching in some cases very high "transition frequency" f t,

[20] the highest operational frequency of a transistor, in the order of the GHz or tens of GHz.

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“…While the possibility to obtain GHz organic transistors has only recently become argument of discussion, [30] progresses are being made in the range of near-field wireless communication. [15,17,18] Moreover, since f t is proportional to the bias voltage, the requirement of low-voltage operation, at least below 10 V as necessary for portable, self-powered wireless electronics, further complicates the achievement of high operational frequency. The maximum frequency drastically decreases for organic transistors on flexible substrates.…”

Direct-written polymer field-effect transistors operating at 20 MHz

“…To date, one order-of-magnitude lower operating frequencies have been demonstrated when mask-less, scalable techniques were used, with a maximum of 3.3 MHz25353637.…”

Roadmap to Gigahertz Organic Transistors

“…At these channel lengths, the transit frequency is essentially unaffected by the charge‐carrier mobility and is instead determined mainly by the contact resistance, which will need to be smaller than ≈10 Ω cm. 2)In the event that the transit frequency is limited not by the ratio between the transconductance and the gate capacitance, but by the saturation of the charge‐carrier velocity in the semiconductor ( f T = v sat /(2 πL )), the channel length may have to be even smaller than indicated above. For example, if the carrier velocity were to saturate at 10 5 cm s −1 , a transit frequency of 1 GHz might require a channel length below 100 nm. 3)Regarding the question of how to fabricate nanoscale organic TFTs on flexible, large‐area substrates with sufficient yield and uniformity in a scalable and cost‐effective manner, a number of techniques have been developed over the past few years; one of these is nanoimprint lithography, which has been used to demonstrate functional organic TFTs with channel lengths as small as 70 nm and which can be combined with self‐alignment techniques to define nanoscale gate‐to‐contact overlaps …”