Small Molecule Contact-Controlled Transistors with Reduced Saturation Voltage via Vacuum Deposition

When and Where

May 12, 2022
9:30am - 9:45am

Hawai'i Convention Center, Level 3, 328

Presenter

Eva Bestelink

Ute Zschieschang

Hagen Klauk

Radu Sporea

Co-Author(s)

Eva Bestelink¹,Ute Zschieschang²,Hagen Klauk²,Radu Sporea¹

University of Surrey¹,Max Planck Institute for Solid State Research²

Abstract

Eva Bestelink¹,Ute Zschieschang²,Hagen Klauk²,Radu Sporea¹

University of Surrey¹,Max Planck Institute for Solid State Research²

Contact effects represent a long-standing bottleneck for improving the operating frequency of thin-film transistors, particularly as material performance is improved [1]. There are, however, numerous low-frequency applications, including biosensing and quasi-d.c. environmental sensors, where energy efficiency and amplification of the electronic system may take precedence. By deliberately utilizing contact barriers, source-gated transistors (SGTs) rely on the contact area as the main control mechanism and have been demonstrated in numerous material systems using varied deposition techniques [2,3]. Here, we demonstrate the initial findings of a systematic study of contact materials aimed at creating optimal SGTs with small molecule organic semiconductors (OSC). SGTs were fabricated in a bottom-gate, top-contact architecture on silicon substrates with 100-nm-thick SiO2 gate dielectric, ~25-nm-thick vacuum-deposited layer of DNTT (dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene) OSC [4], and 40-nm-thick source/drain contacts with various metals (Ag, Al, Au, Cr, Cu, Ti) capped with 30-nm-thick Au. The contacts were patterned using a shadow mask. The SGTs have a channel length of 100 µm and a source-gate overlap of 200 µm. To achieve low-voltage saturation characteristics of SGTs, a low-k, thick gate dielectric was preferred [5,6]. The measured SGT behavior is in line with the variation of the contacts’ work function. Transistors with Au or Cu contacts show Ohmic characteristics, with effective carrier mobility up to 4 cm2/(V s) and subthreshold swing of 0.5 V/decade. In the transistors with Al contacts, drain currents are below 100 pA, due to the large injection barrier. With Ti or Cr contacts, the barrier height is sufficient to operate the devices as SGTs, with reduced effective mobility (0.7 cm2/(V s)) and a fourfold diminution of subthreshold swing. These suboptimal metrics are offset by reduced saturation voltage and improved gain characteristics. The thin semiconductor layer is easily depleted [7], leading to low-voltage saturation, with a dVDS(sat) / dVGS ratio below 0.2 V/V. This corresponds to the expected value of 0.17 V/V, as calculated from the electrostatic properties of the various constituent layers. Saturation is sharp, and intrinsic gain is on the order of 100 V/V. The relatively long channel cannot be assumed to be responsible for this performance, as the saturation characteristics confirm operation firmly into the contact-dominated mode [8]. From an application perspective, such devices can operate as effective active loads for energy efficient amplifiers, due to their low output conductance [9]. By choosing different contact metals or different source geometries, the temperature dependence of drain current can be modified. These transistors could also bring temperature compensation to more complex circuits. [1] U. Zschieschang, J.W. Borchert, M. Giorgio, M. Caironi, F. Letzkus, J.N. Burghartz, U. Waizmann, J. Weis, S. Ludwigs, and H. Klauk, Adv. Funct. Mater., 30, 1903812 (2020). [2] J.M. Shannon and E.G. Gerstner, IEEE Electron Device Lett., 24, 405–407 (2003). [3] G. Wang, X. Zhuang, W. Huang, J. Yu, H. Zhang, A. Facchetti, and T.J. Marks, Adv. Sci., 2101473, 1–23 (2021). [4] U. Kraft, K. Takimiya, M.J. Kang, R. Rödel, F. Letzkus, J.N. Burghartz, E. Weber, and H. Klauk, Org. Electron., 35, 33–40 (2016). [5] J.M. Shannon, R.A. Sporea, S. Georgakopoulos, M. Shkunov, and S.R.P. Silva, IEEE Trans. Electron Devices, 60, 2444–2449 (2013). [6] A. Valletta, L. Mariucci, M. Rapisarda, and G. Fortunato, J. Appl. Phys., 114, 064501 (2013). [7] R.A. Sporea, M.J. Trainor, N.D. Young, J.M. Shannon, and S.R.P. Silva, IEEE Trans. Electron Devices, 57, 2434–2439 (2010). [8] R.A. Sporea, X. Guo, J.M. Shannon, and S.R.P. Silva, Proc. Int. Semicond. Conf. CAS, 2, 413–416 (2009). [9] E. Bestelink, K.M. Niang, G. Bairaktaris, L. Maiolo, F. Maita, K. Ali, A.J. Flewitt, S.R.P. Silva, and R.A. Sporea, IEEE Sens. J., 20, 1–11 (2020).