Letter pubs.acs.org/NanoLett
One-Volt Operation of High-Current Vertical Channel Polymer Semiconductor Field-Effect Transistors Danvers E. Johnston,† Kevin G. Yager,† Chang-Yong Nam,† Benjamin M. Ocko,‡ and Charles T. Black*,† †
Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, United States
‡
ABSTRACT: We realize a vertical channel polymer semiconductor field effect transistor architecture by confining the organic material within gratings of interdigitated trenches. The geometric space savings of a perpendicular channel orientation results in devices sourcing areal current densities in excess of 40 mA/cm2, using a onevolt supply voltage, and maintaining near-ideal device operating characteristics. Vertical channel transistors have a similar electronic mobility to that of planar devices using the same polymer semiconductor, consistent with a molecular reorientation within confining trenches we understand through X-ray scattering measurements. KEYWORDS: Organic semiconductors, field-effect transistor, electronic mobility, semiconducting polymers, wide-angle x-ray scattering, polythiophene
F
ield effect transistors (FETs) made from organic semiconductors having both high current output and using low power supply voltages may find more widespread use in existing and exciting new applications. Efforts to improve organic FET performance include the synthesis of new organic semiconducting materials, and modification of dielectric and electrode interfaces.1,2 Control of molecular ordering can optimize the organic material electrical performance, for example, through chemical surface functionalization to induce favorable molecular orientation.3,4 Further, the device architecture can be engineered to promote a desired performance aspect, such as current density per unit area.5−7 Here, we demonstrate simultaneous use of device architecture and control of molecular ordering to yield exceptional performance in a polymer semiconductor FET that sources current densities in excess of 40 mA/cm2 using only a one-volt supply voltage. The reported device architecture maintains near-ideal FET operating characteristics, with a low off-state current level, high on- to off-current ratio, and desirable on-state current saturation. The current output of an organic FET is limited fundamentally by the semiconductor electronic charge mobility (μ, typically 60 nm (for n < 1015 cm−3) is close to the confining channel width (70 nm). In the fully on-state, increased n shrinks LD to less than a few nanometers. The vertical P3HT FET has an areal footprint A = 400 μm × 100 μm and areal current density of 17 μA/(0.04 cm2) = 43 mA/cm2 at VDS = 1 V. The longer channel planar interdigitated P3HT FET (L = 4 μm) drives 5 mA/cm2 at VDS = 10 V. Previous reports of P3HT-based planar FETs with areal current densities comparable to our vertical device operate at VDS ≈ 80 V.23,28 Scaling the planar P3HT FET gate length to nanometerscale dimensions can significantly increase areal current density at VDS = 1 V, while also introducing nonideal FET operation due to short channel effects.18 Amorphous silicon FETs with L
(1)
and transfer characteristics (ID−VG) (solid line in Figure 3b): ∂ID W = μCox VDS ∂VG L
known to have lower mobility than those with contacts deposited on top.4 Finally, the P3HT electronic mobility magnitude is influenced by details of the film molecular structure, with highest reported values occurring in highly crystalline films.4,13 The FET device on- to off-current ratios are 2 × 104 (vertical) and 9 × 105 (planar) (Figures 3b,d), and the threshold voltages for turn-on are VT ∼ 0.3 V (vertical) and 1.7 V (planar). Despite differences in tox (17 nm for the vertical FET; 40 nm for planar), the devices exhibit nearly identical gate control of the P3HT channel, evidenced by similar inverse slopes in the device transfer characteristic (ID−VG), 210 mV/ decade and 190 mV/decade for the vertical and planar devices, respectively (dashed lines in Figures 3b,d). The slope (S) depends on gate oxide capacitance and the semiconductor depletion layer capacitance as:22
22
(2)
For the vertical channel P3HT FET (Figures 3a,b) we calculate a vertical direction μ of 0.9 × 10−4 cm2 V−1 s−1 and 1.3 × 10−4 cm2 V−1 s−1 using eqs 1 and 2, similar to the in-plane μ of the planar P3HT FET (1.5 × 10−4 cm2 V−1 s−1 and 2.9 × 10−4 cm2 V−1 s−1, Figures 3c,d). These values are lower than some previously reported P3HT field-effect mobilities,13,23 but in-line with other literature values.24,25 The low dopant concentration in our P3HT films, a consequence of annealing and measuring in vacuum, is known to reduce P3HT mobility.26 We determine the intrinsic carrier concentration of
