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

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

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

Extracting parameters from the current-voltage characteristics of polycrystalline octithiophene thin film field-effect transistors

Organic field-effect transistors (OFETs) in which the active semiconductor is made of polycrystalline octithiophene (8T) were fabricated and characterized. Several methods have been used to extract the parameters of the polycrystalline (8T) transistors as function of gate voltage at room and at low temperatures. These parameters such as the mobility, the threshold voltage, contact resistance and density of traps are extracted from the current-voltage characteristics of OFETs. The first method consists of deriving the drain current as function of gate voltage (transconductance), leading to the so-colled field effect mobility. It appears that the data must be corrected for the substantial source and drain contact resistance. In the second method, the carrier mobility has been corrected for the contact resistance. It is found to increase quasi linearly with gate voltage. Therefore, the contact resistances have been directly extracted from the experimental channel conductance. In the third method, data are interpreted according the multiple thermal trapping and release model using a power law dependence of the mobility with gate voltage together with a constant resistance. Finally, using a model where charge transport is limited by trapping and thermal release at localized states located at grain boundaries, the barrier height at grain boundaries in polycrystalline octhithiophene FETs has been estimated. It is shown that is gate bias dependent