Experimental study and analytical modeling of the channel length influence on the electrical characteristics of small-molecule thin-film transistors

Bottom-contact p-type small-molecule copper phthalocyanine (CuPc) thin film transistors (TFTs) with different channel lengths have been fabricated by thermal evaporation. The influence of the channel length on the current-voltage characteristics of the fabricated transistors were investigated in the linear and saturation regimes. The devices exhibit excellent p-type operation characteristics. Results show that devices with smaller channel length (L = 2.5 mu m and 5 mu m) present the best electrical performance, in terms of drain current value, field effect mobility and subthreshold slope. Saturation field-effect mobilities of 1.7 x 10(-3) cm(2) V-1 s(-1) and 1 x 10(-3) cm(2) V-1 s(-1) were obtained for TFTs with channel lengths of L = 2.5 mu m and L = 5 mu m, respectively. Transmission line method was used to study the dependence of the contact resistance with the channel length. Contact resistance becomes dominant with respect to the channel resistance only in the case of short channel devices (L = 2.5 mu m and 5 mu m). It was also found that the field effect mobility is extremely dependent on the channel length dimension. Finally, an analytical model has been developed to reproduce the dependence of the transfer characteristics with the channel length and the obtained data are in good agreement with the experimental results for all fabricated devices.Peer ReviewedPostprint (author's final draft

Transport properties of copper phthalocyanine based organic electronic devices

Ambipolar charge carrier transport in Copper phthalocyanine (CuPc) is studied experimentally in field-effect transistors and metal-insulator-semiconductor diodes at various temperatures. The electronic structure and the transport properties of CuPc attached to leads are calculated using density functional theory and scattering theory at the non-equilibrium Green's function level. We discuss, in particular, the electronic structure of CuPc molecules attached to gold chains in different geometries to mimic the different experimental setups. The combined experimental and theoretical analysis explains the dependence of the mobilityand the transmission coefficient on the charge carrier type (electrons or holes) and on the contact geometry. We demonstrate the correspondence between our experimental results on thick films and our theoretical studies of single molecule contacts. Preliminary results for fluorinated CuPc are discussed.Comment: 18 pages, 16 figures; to be published in Eur. Phys. J. Special Topic

Analytical model for charge transport in organic thin-film transistors: application to polythiophene

The electronic charge transport in active layer of organic thin-film transistor (OTFT) based on conductor polythiophene (sexithiophene (6T) and octithiophene (8T)) was studied. A mathematical model is presented based on the variable range hopping (VRH) transport theory. Using the VRH model the expression of source-drain current is established for two regimes: linear regime for drain bias VD = −2 V and saturation regime for VD = −30 V at low temperature and at room temperature. All electrical key parameters of OTFTs based on polythiophene have been extracted. A good agreement between theoretical model and experimental measurement of electrical characteristics is obtained for both temperature ranges: low temperature and room temperature

Investigation on base surface recombination in Self Passivated GaAlAs/GaInP/GaAs Heterojunction Bipolar Transistor

The dependence of the current gain with the size of the emitter-base junction of double mesa Self Passivated Heterojunction Bipolar Transistors (SP-HBT) has been investigated, the extrinsic base layer being passivated with a n-type GaInP layer. The current gain is widely improved, due to a 18-fold reduction of the surface recombination in the extrinsic base region with respect to unpassivated HBT. The surface recombination current ideality factor has been found to be 1.13

The effect of Si impurities on the transport properties and the electron-surface phonon interaction in single layer graphene deposited on polar substrates

We investigated theoretically the effect of introducing Si impurities in a single layer graphene (1LG) that had been deposited on a polar substrate on the transport properties of the graphene layer. We consider in our analysis the scattering effects due to the surface optical (SO) phonons located at the interface of the 1LG with various polar substrates such asSiC, hexagonal BN,SiO2andHfO2. Our results demonstrate a reduction of SO phonon-limited (SOPL) mobility, and SOPL conductivity as well as an increase of the SOPL resistivity and of the scattering rate in the presence of Si impurities in the 1LG. Further, we studied the effect of Si impurities on the electron-surface phonon interaction. For our analysis we used the eigenenergies aquired from the tight-binding Hamiltonian in 1LG. Indeed the presence of the Si impurities induces a decrement in the resonant coupling between the electronic sub-levels and the surface vibration modes in monolayer graphene deposited on polar substrates. Finally, we investigated the effect of Si impurities on the Auger scattering process which affects the carriers relaxation. Our results show an enhancement of the Auger scattering rate in the case of the Si-doped 1LG compared to the undoped 1LG. © 2018 Elsevier B.V

Surface passivation of composition graded base in GaAlAs/GaInP/GaAs heterojunction bipolar transistor

A comparative study on the emitter-base recombination current in different GaAlAs/GaInP/GaAs HBT structures including uniform base HBTs, and composition graded base HBTs, has been carried out. We have demonstrated that a graded base is not sufficient to prevent recombination on the base surface and that a thin GaInP ledge on the base surface reMayns necessary to retain a high enough current gain for small emitter high-frequency devices

Reduction of Space Charge Recombination Current with a Self Passivated GaAlAs/GaInP/GaAs HBT Structure

A Passivated HBT structure which includes a thin GaInP layer between the GaAlAS emitter and the GaAs base layer has been proposed in order to reduce surface and space charge recombination current, while keeping a low p-type ohmic contact resistivity. The optimization of the GaInP layer thickness has been carried out leading to a value of 30 nm

Charge transport limited by grain boundaries in polycrystalline octithiophene thin film transistors

Organic filed-effect transistor (OFETs) based on polycrystalline “octithiophene” has been realized. The current-voltage characteristics at low drain voltage has been used to derive the mobility of organic field effects transistors (OFETs). It appears that the data must be corrected for the substantial source and drain contact resistance. The carrier mobility is found to increase quasi linearly with gate voltage at room temperature. The temperature dependent measurements show that the mobility is thermally activated and becomes practically temperature independent at low temperatures. A model based on trapping mechanism, in which it is assumed that charge transport is limited by grain boundaries, has been used to describe the carrier mobility in polycrystalline “octithiophene” thin film transistors measured at temperatures ranging from 10 K to 300 K

Effect of leakage current induced by B

The excess leakage current due to the ion implantation isolation process used in the fabrication of double mesa Self Passivated GaAlAs/GaInP/GaAs Heterojunction Bipolar Transistors (SP-HBT) has been investigated. This ion implantation process, used to limit the active emitter length results in a drastic reduction of the current gain. The ideality factor of the recombination current associated with the ion implantation has been found to be 1.8, close to the conventional value of 2

Influence of the polymer matrix on the efficiency of hybrid solar cells based on silicon nanowires

International audiencePoly (N-vinylcarbazole) (PVK):SiNWs and poly (2-methoxy, 5-(2-ethyl-hexyloxy)-p-phenyl vinylene) (MEH-PPV):SiNWs bulk-heterojunctions (BHJ) have been elaborated from blends of SiNWs and the polymer in solution from a common solvent. Optical properties of these nanocomposites have been investigated by UV-vis absorption and photoluminescence (PL) spectral measurements. We have studied the charge transfer between SiNWs and the two polymers using the photoluminescence quenching of PVK and MEH-PPV which is a convenient signature of the reduced radiative recombination of the generated charge pairs upon exciton dissociation. We found that PVK and SiNWs constitutes the better donor-acceptor system. In order to understand the difference between PVK:SiNWs or MEH-PPV:SiNWs behaviours, photoluminescence responses were correlated with the topography (SEM) of the thin films. The photovoltaic effect of ITO/PEDOT:PSS/SiNWs:PVK/Al and ITO/PEDOT:PSS/SiNWs:MEH-PPV/Al structures was studied by current-voltage (I-V) measurements in dark and under illumination and interpreted on the basis of the charge transfer differences resulting from the morphologies