Gernot Paasch

Subthreshold characteristics of field effect transistors based on poly(3-dodecylthiophene) and an organic insulator

The properties of field effect transistors with organic insulator and semiconducting regions, fabricated with a top-gate architecture, have been investigated. Thin films (d≈30 nm) of regioregular poly(3-dodecylthiophene) were employed as the active semiconductor and the gate insulator was formed by a 500-nm-thick layer of poly(4-vinylphenol). Both were solution-processed on top of poly(ethylenetherephthalate) films, which were used as substrates. The output characteristics show a pronounced saturation behavior with an unconventional nonquadratic saturation current dependence on the gate voltage. Hence the (hole) mobility of 0.002–0.005 cm2/Vs has been estimated from the linear region of the transfer characteristics. The transistor turn-on occurs at a threshold voltage of approximately Vth=0 V, and the device can be operated with a supply voltage of between 15 and 20 V. As is usually observed for organic transistors, the inverse subthreshold slope (S) is very high, in our case S≈7 V/dec, by contrast with S...

Interdependence of contact properties and field- and density-dependent mobility in organic field-effect transistors

The current characteristics of organic field-effect transistors (OFET) often show a disadvantageous nonlinearity at low drain voltages. It has been shown recently [J. Appl. Phys. 102, 054509 (2007)] that in top contact (TOC) OFETs this effect can be caused by trap recharging if the contacts are of Schottky type. For bottom contact (BOC) OFETs, in spite of controversial discussions, Schottky contacts as origin of the nonlinearity are often stated. At first, it is shown here by a mixed mode simulation that for large ideality factors a Schottky contact only at drain leads to such a nonlinearity. However, with the same Schottky contacts at drain and source the effect is covered by the high resistance of the contact at source. Next, the different influences of Schottky contacts on BOC OFETs and TOC OFETs with varying overlap of the source/drain contacts with the gate are clarified. Further, it is demonstrated with detailed two-dimensional simulations that the combination of the presence of Schottky contacts wi...

Comparative full-band Monte Carlo study of Si and Ge with screened pseudopotential-based phonon scattering rates

In a previous article [J. Appl. Phys. 92, 5359 (2002)], we presented a combination of a full-band Monte Carlo method using an advanced band structure and a variable Brillouin zone discretization, with phonon scattering rates based on the screened pseudopotential considering the positions of the atoms in the elementary cell. To make the method suitable for sufficiently fast applications, such as device simulations, the simplest wave number dependent approximation was introduced. It contains an average of the cell structure factor, and only two fit parameters: The acoustic and the optical deformation potentials. As the pseudopotential, the Ashcroft model potential is chosen, and screening is taken into account using the Lindhard dielectric function. In the present article, based on the study of the influence of the two deformation potentials on the electron and hole drift velocities in Si and Ge, we show how to select the deformation potentials. Depending on the targeted agreement with experimental results,...

Mixing of interface dipole and band bending at organic/metal interfaces in the case of exponentially distributed transport states

The interface dipole for organic adlayers on metal substrates, as determined by photoemission spectroscopy, is often almost as large (>80%) as the built-in potential determined from the ionization potential and the transport gap. Based on our experimental data and the formal description of the band bending in the thin layer, it is shown that the generally accepted view on the interface of thin organic adlayers with a metal substrate must be refined. First, besides band bending one has in the layer also floating of the potential expressed by the finite value of the potential at the outer surface of the layer. Second, for the usually observed large interface dipole, band bending is negligible as long as the electronic states are well defined in energy. It is demonstrated that an exponential distribution of the transport states, with a width of the distribution that is large compared to the thermal energy, leads to a drastic modification of this picture. For CuPc and two fluorinated CuPc’s a band bending of ...

Determination of trap distributions from current characteristics of pentacene field-effect transistors with surface modified gate oxide

It has been demonstrated [K. P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D. J. Gundlach, B. Batlogg, A. N. Rashid, and G. Schitter, J. Appl. Phys. 96, 6431 (2004)] that a controllable shift of the threshold voltage in pentacene thin film transistors is caused by the use of organosilanes with different functional groups forming a self-assembled monolayer (SAM) on the gate oxide. The observed broadening of the subthreshold region indicates that the SAM creates additional trap states. Indeed, it is well known that traps strongly influence the behavior of organic field-effect transistors (OFETs). Therefore, the so-called “amorphous silicon (a-Si) model” has been suggested to be an appropriate model to describe OFETs. The main specifics of this model are transport of carriers above a mobility edge obeying Boltzmann statistics and exponentially distributed tail states and deep trap states. Here, approximate trap distributions are determined by adjusting two-dimensional numerical simulations to the experime...

Hysteresis in organic field-effect devices: Simulated effects due to trap recharging

Current organic field-effect transistors feature predominantly undesirable hysteresis effects which appear also in the capacitance-voltage (CV) characteristics of organic metal-oxide-semiconductor (MOS) capacitors. So far, these effects are not sufficiently characterized experimentally and their origin is even less understood. In the literature one finds presumptions that they are caused by trap recharging or by mobile ions (in the organic semiconductor or in the insulator). In order to check the first of these presumptions, detailed numerical simulations have been carried out. Hysteresis in the field effect is studied at best in the MOS capacitor without the additional influence of the source/drain contacts of the transistor. Although there are differences between quasistatic and dynamic measurements, our simulations are done for the quasistatic CV characteristics since they already give the desired information for the understanding of the hysteresis effects. Organic semiconductors with different types of traps of different energetic positions, concentrations, and energetic and spatial distributions are considered and their parameters are varied in a wide range. Trap recharging is connected with the transport of the emitted (captured) carriers from (to) the traps during the voltage sweep. It is demonstrated that rather different types of hysteresis can occur thereby. All of the simulated shapes of the hysteresis are qualitatively different from that one which is typically observed experimentally. Therefore, it should be ruled out that the hysteresis observed in organic MOS devices is solely caused by trap recharging. The observed changes of the CV characteristics for a variation of different measuring conditions and parameters indicate that the hysteresis is caused by at least two different processes. We suppose that the origin of the hysteresis in organic devices is a combination of slow transport (polarons or mobile ions) with a reaction other than trap recharging, e.g., the direct polaron-bipolaron reaction or a complex formation reaction of polarons/bipolarons with counterions.Current organic field-effect transistors feature predominantly undesirable hysteresis effects which appear also in the capacitance-voltage (CV) characteristics of organic metal-oxide-semiconductor (MOS) capacitors. So far, these effects are not sufficiently characterized experimentally and their origin is even less understood. In the literature one finds presumptions that they are caused by trap recharging or by mobile ions (in the organic semiconductor or in the insulator). In order to check the first of these presumptions, detailed numerical simulations have been carried out. Hysteresis in the field effect is studied at best in the MOS capacitor without the additional influence of the source/drain contacts of the transistor. Although there are differences between quasistatic and dynamic measurements, our simulations are done for the quasistatic CV characteristics since they already give the desired information for the understanding of the hysteresis effects. Organic semiconductors with different types o...

Charge carrier density of organics with Gaussian density of states: Analytical approximation for the Gauss–Fermi integral

Active layers in organic devices prepared in solution based preparation routes are usually disordered. Their highest occupied molecular orbitals and lowest unoccupied molecular orbitals, or the valence and conduction band states, show an energetic distribution which can be approximated by a Gaussian density of states (DOS). The resulting dependency of the (electron and hole) mobility on temperature, carrier density, and field can be easily implemented into advanced device simulation programs. However, in addition the charge carrier density is needed as the integral over the DOS multiplied with the Fermi–Dirac distribution. We denote this normalized quantity as the Gauss–Fermi integral. Since it cannot be evaluated analytically, similarly as in the case of the Fermi–Dirac integral F1/2, an analytical approximation is needed for efficient device simulation. In the present article, such an approximation is proposed with different expressions in the nondegenerate and degenerate regions with a continuous and d...

Organic field-effect transistors with nonlithographically defined submicrometer channel length

We developed an underetching technique to define submicrometer channel length polymer field-effect transistors. Short-channel effects are avoided by using thin silicon dioxide as gate insulator. The transistors with 1 and 0.74 mum channel length operate at a voltage as low as 5 V with a low inverse subthreshold slope of 0.4-0.5 V/dec, on-off ratio of 10(4), and without short-channel effects. The poly(3-alcylthiophene)s still suffer from a low mobility and hysteresis does occur, but it is negligible for the drain voltage variation. With our underetching technique also device structures with self-aligned buried gate and channel length below 0.4 mum are fabricated on polymer substrates.

Space charge layers in organic field-effect transistors with Gaussian or exponential semiconductor density of states

Space charge layers (SCLs) in metal-insulator-semiconductor (MIS) structures are critical for the operation of field-effect transistors (FETs). For many organic semiconductors, transport takes place as hopping in Gaussian or exponentially distributed states. However, existing theoretical descriptions of a SCL and advanced device simulation programs suppose a density of states other than a Gaussian or an exponential, employing often the nondegenerate limit for the concentrations. We present results of a simulation study for the MIS structure as the basic module of the FET and for a thin semiconducting layer on a metal substrate. The second system was extensively investigated by photoelectron spectroscopy to characterize the metal-organics interface occurring at the source/drain contact of FETs and as anode and cathode in organic light emitting diodes. For broader distributions, the densities deviate strongly from the nondegenerate limit which leads indeed in a MIS structure to a strong deviation of the dep...

Influence of distributed trap states on the characteristics of top and bottom contact organic field-effect transistors

Numerical simulations of organic field-effect transistors (OFET) of bottom and top contact (BOC, TOC) design with different source/drain contacts were carried out considering an exponential distribution of trap states in the gap of the active layer (a-Si model). For ohmic contacts, the current-voltage characteristics are similar to the trap-free case and there is not much difference between the two designs. However, the currents are lower due to immobile trapped charges, the threshold voltage is shifted, and the inverse subthreshold slope increases due to trap recharging. An analytical approximation for the effective mobility deviates from the simulation up to 20%. For low source/drain work function, there occur particular dependencies of the current on the gate voltage for the two designs, which are explained with the internal concentration and field profiles. A series resistance between source and channel causes in the TOC structure an abrupt transition from the gate voltage independent active region into saturation. In the BOC case, the reverse-biased Schottky-type source contact dominates the current. Through simulation of measured characteristics of prepared OFETs based on a modified poly-(phenylene-vinylene), the observed hysteresis is analyzed.