Doped organic transistors operating in the inversion and depletion regime

Abstract

The inversion field-effect transistor is the basic device of modern microelectronics and is nowadays used more than a billion times on every state-of-the-art computer chip. In the future, this rigid technology will be complemented by flexible electronics produced at extremely low cost. Organic field-effect transistors have the potential to be the basic device for flexible electronics, but still need much improvement. In particular, despite more than 20 years of research, organic inversion mode transistors have not been reported so far. Here we discuss the first realization of organic inversion transistors and the optimization of organic depletion transistors by our organic doping technology. We show that the transistor parameters—in particular, the threshold voltage and the ON/OFF ratio—can be controlled by the doping concentration and the thickness of the transistor channel. Injection of minority carriers into the doped transistor channel is achieved by doped contacts, which allows forming an inversion layer.

Investigation of the High Electron Affinity Molecular Dopant F6‐TCNNQ for Hole‐Transport Materials

“…[13] In this early work, the molecule was used to p-dope N,N′-di-1-naphthyl-N,N′-diphenyl-1,1′-biphenyl-4,4′diamine (α-NPD, IE ≈ 5.5 eV) and shown to significantly increase the current density sustained by these films. It was then introduced by Tietze et al [14] and Lüssem et al [15] in electronic structure and device investigations, and found to efficiently p-dope MeO-TPD or pentacene (IE ≈ 4.95 eV). Yet, the EA of the dopant and its doping limit were not fully established.…”

Top-gate organic depletion and inversion transistors with doped channel and injection contact

“…This linear dependence of the threshold voltage shift upon the thickness of the doped channel proves that the shift of the threshold voltage is due to real doping and thus generation of free charge carriers in the channel region. 6 In contrast to the depletion transistors presented in Fig. 2, top-gate organic inversion transistors have not been realized before.…”

“…10,17 L€ ussem et al have previously investigated the Fermi level shift upon doping by ultraviolet photoelectron spectroscopy (UPS) in order to distinguish between the trap filling by doping and the increase of free charge carriers in the bulk of pentacene. 6 They found that the Fermi level shifts significantly towards the transport states, which proves that doping can greatly increase the charge carrier density in the channel region. However, to fully prove that a trap filling at the pentacene/Cytop interface has a negligible effect on the threshold voltage, we study the dependence of the shift of threshold voltage on the thickness of the doped channel.…”

“…The comparatively small mobility of doped-channel transistors is related to the structural phase transition of the pentacene film from crystalline to amorphous at increased doping concentration. 6,15 The inset of Fig. 1(c) presents an example of the leakage current between source and gate electrodes during the gate voltage sweep.…”

“…5 Alternatively, doped OFETs in bottom-gate configuration operated both in depletion and inversion regimes exhibiting high R on-off , stable charge carrier mobility as well as tunable threshold voltage have recently been realized, promising to facilitate the circuit design. 6 OFETs with top-gate architecture have several advantages over bottomgate devices, such as better charge carrier injection and easier gate line design. 7 However, organic inversion transistors in top-gate configuration have up to now not been realized.…”