Dependence Of Contact Resistance Research Articles

Structural and electrical studies on Ti/Al-based Au-free ohmic contact metallization for AlGaN/GaN HEMTs

In order to fabricate the AlGaN/GaN-on-Si HEMTs on existing Si-CMOS production lines, manufacturing processes must meet the rigors of Si-CMOS technology eliminating Au. One of the requirements is formation of Au-free, low resistivity ohmic contacts to AlGaN/GaN heterostructures. In this work we report structural and electrical studies of Ti/Al-based ohmic contacts to AlGaN/GaN HEMTs with TiN/Cu cover layers. Ohmic contacts have been observed after annealing of Ti/Al/TiN/Cu or Ti/Al/Ti/TiN/Cu/TiN multilayers on recessed AlGaN/GaN structure at temperature of 550 °C, specific contact resistance is about 2.3 × 10−4 Ω cm2, however, lower value was obtained after annealing at 750 °C. The XRD and TEM studies reveal formation of additional phases during annealing, namely Al3Ti and Ti2AlN inside the volume of the metallization stack and AlN at metal/GaN interface. Measured dependence of contact resistance on temperature suggests a “metal-like” carrier transport mechanism in partially recessed Ti/Al/Ti/TiN/Cu ohmic contact to 2D electron gas.

Structure and electrical erosion resistance of an electro-explosive coating of the CuO–Ag system

For the first time, electroerosion-resistant coatings of the CuO–Ag system were obtained by electro-explosive spraying. The coating structure was investigated by atomic force microscopy and scanning electron microscopy. The formed coating is a homogeneous composite material consisting of a silver matrix and CuO inclusions located in it. The composite electro-explosive coating of CuO–Ag system is obtained on copper electric contact of contactor KPV-604. Tests of sprayed coatings for electrical erosion resistance under arc erosion conditions were carried out. A formation mechanism of hierarchical levels of the structure of electro-explosive coatings was suggested. The mechanism is based on theoretical concepts of the formation and evolution of fractal aggregates with diffusion-limited and cluster-cluster aggregations of the CuO. Based on the electrical properties of silver and copper oxide, the dependence of electrical contact resistance on the number of on/off cycles in the process of testing for electrical erosion resistance is substantiated.

Current transport mechanism of Mg/Au ohmic contacts to lightly doped n-type β-Ga2O3

The carrier transport mechanism of Mg/Au ohmic contact for lightly doped β-Ga2O3 is investigated. An excellent ohmic contact has been achieved when the sample was annealed at 400 °C and the specific contact resistance is 4.3 × 10−4 Ω·cm2. For the annealed sample, the temperature dependence of specific contact resistance is studied in the range from 300 to 375 K. The specific contact resistance is decreased from 4.3 × 10−4 to 1.59 × 10−4 Ω·cm2 with an increase of test temperature. As combination with the judge of E00, the basic mechanism of current transport is dominant by thermionic emission theory. The effective barrier height between Mg/Au and β-Ga2O3 is evaluated to be 0.1 eV for annealed sample by fitting experimental data with thermionic emission model.

Dynamic variation of carrier transport properties of recessed Au-free ohmic contacts to InAlN/AlN/GaN on Si-wafer

The temperature dependence of specific contact resistivity of recessed ohmic contacts was investigated to examine the carrier conduction mechanism in Ti/Al/W ohmic contacts on InAlN/AlN/GaN on Si wafer. The lowest specific contact resistivity of 2.36 × 10−5 Ω·cm2 was obtained from a sample with a half-etched InAlN layer. The barrier heights and donor concentrations of recessed contacts with various recess depths were calculated using a thermionic field emission model that showed good fitting with the experimental data. It was found that the rate of tunneling in carrier conduction decreased with increasing recess depth.

Bifacial n-PERC solar cell characterization

This paper investigates the outdoor bifacial n-type silicon passivated emitter rear cell (PERC) and the influence of the contact resistance and temperature dependence on the output properties by using Silvaco TCAD tools. PERC is considered as an effective solution to increase the energy yield and reduce the levelized costs of electricity in a real PV system. This paper presents a comparison between bifacial n-PERC and standard n-PERC cell technology. This investigation at 300 K leads to achieve the following results Jsc = 43.93 mA/cm2, Voc = 0.763 V, FF = 80.44%, Pmax = 269.77 W/m−2, and η = 26.97% for AM1.5 spectra and with only 20% albedo which are close to and improved with those found in different research works. This technology provides a better electrical control over the cell and thus leads to valuable improvements in photovoltaic devices performance.

Simulation of Electrical Characteristics of PERC Solar Cells

In recent years, silicon solar cells have reached efficiencies above 24%, which is ensuring rapid progress in the photovoltaic (PV) market. We review herein recent passivated emitter rear cell (PERC) technology and the influence of the contact resistance and temperature dependence on the output properties by using Silvaco technology computer-aided design (TCAD) tools. This numerical simulation indicates efficiency of 24.82% for the n-type monocrystalline PERC cell, with open-circuit voltage of up to 761 mV. PV parameters, such as the open-circuit voltage (Voc), short-circuit current (Jsc), maximum power (Pmax), fill factor, and efficiency (η), are compared with those of other device architectures.

Comparing the Gate Dependence of Contact Resistance and Channel Resistance in Organic Field-Effect Transistors for Understanding the Mobility Overestimation Issue

Contact resistance is a common issue in organic field-effect transistors (OFETs). Although it is well known that both the contact resistance ( ${R}_{C}$ ) and channel resistance ( ${R}_{ {Ch}}$ ) are generally gate-dependent, i.e., the resistance values are reduced by increasing gate voltages, the comparison of the gate dependence of ${R}_{C}$ and ${R}_{ {Ch}}$ has rarely been reported. In this letter, we used scanning Kelvin probe microscopy to experimentally measure and compare the gate dependence of the two resistances. The results provide valuable information regarding the charge injection/extraction physics in OFETs. Moreover, the correlation between the gate dependence of the two resistances and the charge transfer characteristic of OFETs was investigated, which can give us some clues to the understanding of the non-ideal transfer curves (“kink” feature) observed in high-mobility OFETs, an issue that has caused both great interest and concern in the community.

Experimental and Numerical Investigation of Contact Doping Effects in Dinaphthothienothiphene Thin‐Film Transistors

SUMMARYContact doping effects in p‐channel dinaphthothienothiphene (DNTT) thin‐film transistors with a bottom‐gate, top‐contact configuration were investigated with both experimental and numerical approach. Characteristic variation in transistor parameters such as the gate threshold voltage and the field‐effect mobility for devices with various channel lengths was suppressed by the contact doping with tetrafluorotetracyanoquinodimethane (F4TCNQ) as an acceptor dopant. The gate‐voltage dependence of contact resistance and channel resistance was also evaluated separately to examine the contact doping effect in detail. In addition, device simulation considering a Schottky barrier at a metal/semiconductor interface successfully reproduced the experimental current–voltage characteristics by using a hole concentration of the active DNTT layer in the order of 1017 cm−3, which was estimated by capacitance–voltage measurement for a metal/insulator/semiconductor capacitor structure. This study suggests the importance of establishing both the carrier doping and carrier concentration measurements toward realizing practical applications of organic transistors.

Multiscale modeling of electrical conductivity of carbon nanotubes based polymer nanocomposites

In this work, we applied multiscale modeling to investigate electrical conductivity of carbon nanotube (CNT) enhanced polymer composites. The multiscale approach is based on a combination of first-principles calculations of contact resistance between CNTs using Green's functions approach and statistical calculation of CNTs ensemble conductivity using the Monte Carlo percolation model. The results of first-principles calculations show strong dependence of contact resistance between CNTs on the angle φ between nanotubes axes: for (5,5) CNTs contact resistance increases by two orders of magnitude as φ changes from φ=0 to φ=π/4. This angular dependence of contact resistance has strong influence on conductivity of CNTs ensemble, decreasing composite conductivity by about an order of magnitude. We stress that obtained conductivity is the upper theoretical limit for an ideal CNT composite, and experimental data with larger values of conductivity should be attributed to positive influence of some non-idealities in composite.

A method for direct contact resistance evaluation in low voltage coplanar organic field-effect transistors

In this paper, a method for the extrapolation of contact resistance in organic field-effect transistors (OFETs) from a single transfer characteristic curve in the linear regime is proposed. The method, namely DIrect Contact Resistance Extrapolation (DICRE), is based on the idea of making the current dependent only on contact resistance by setting the device in large over-threshold conditions. Constant contact resistance with respect to gate-to-source voltage is considered as an acceptable approximation, as confirmed by other examples in the literature. The effectiveness of the method is demonstrated by extrapolating the contact resistance of two different OFET structures (self-aligned and not self-aligned) and comparing the results with standard reference techniques, namely the Modified Transmission Line Method (M-TLM) and the Y function method. The results demonstrate that the DICRE method can be applied to low voltage devices without any damage to the gate insulator, even if the applied gate-to-source voltage drop is well beyond the values normally employed for transistor operation. The proposed method allows extrapolating a value of contact resistance comparable with the ones derived by TLM, with restrained variability. Moreover, the capability of properly recognizing the differences in contact resistance values between OFET structures with different features in terms of source/drain-gate overlap is reported. Finally, the possibility of correctly deriving the contact resistance dependence on drain-to-source voltage using DICRE is discussed.

An analytical solution for contact resistance of staggered organic field-effect transistors

We have developed analytical models for bias dependent contact resistance (RC) and output characteristics of staggered organic field-effect transistors (OFETS) based on a bulk resistance-approximated and mobility-modified current-crowding method. Numerical evaluations of RC and its resistive components show that the bias dependency of the bulk resistance is negligible. Consequently, the properties of the active layer interfaces determine RC and its characteristics. Effective parameters include a normally constant charge injection barrier at the organic-metal interface (Eb) and a gate induced surface carrier-concentration (PS0) at the organic-insulator boundary. The energy barrier pertains to the fabrication process, and its related resistance (rc) can be determined as the fitting parameter of the theoretical model. However, PS0 is strongly gate bias dependent and the results of the numerical model indicate that the resulting component (rch) is dominant and has a considerable effect on RC and its characteristics. More importantly, PS0 as the key parameter of the contact resistance is analytically expressible and by using a proposed mobility-modified current-crowding model, the contact resistance can be analytically formulated. Accordingly, the output characteristics of the OFETs in the triode region can be also analytically modeled using the developed relation of RC.

Thermal-assisted band to band tunneling at the electron donor/acceptor energy harvesting assembly

Understanding interfacial electronic properties at the electron donor/acceptor energy harvesting assembly in photovoltaic devices is important in that it can provide insights into revealing charge carrier recombination mechanism at the interface. From temperature dependent contact resistance measurements at the aluminum-doped zinc oxide (AZO)/poly-hexylthiophene (P3HT)/aluminum-doped zinc oxide (AZO) structure in which AZO (electron acceptor) acts as highly conductive source and drain electrodes, charge transfer mechanism at the electron donor/acceptor interface is elucidated. In the mechanism, charge transfer process is initiated by band to band tunneling (BTBT) of hole carriers from AZO to P3HT through localized electronic states populated near the highest occupied molecular orbital (HOMO) level of the P3HT close to the AZO favored by energetic alignment between the conduction band edge of AZO and the HOMO of P3HT. Following BTBT, charge transfer process is completed by thermal activated hopping in the P3HT (electron donor) close to the AZO. It is found that thermally activated transport in the P3HT close to the AZO dominates the magnitude of the contact resistance, providing insights into the origin of carrier recombination mechanism at the donor/acceptor interface in photovoltaic devices.

Electric-field enhanced thermionic emission model for carrier injection mechanism of organic field-effect transistors: understanding of contact resistance

We developed an electric-field enhanced thermionic emission model combined with an equivalent circuit for a three-terminal organic transistor structure to interpret the gate-voltage dependent contact resistance. In the model the contact resistance is composed of two components: (i) the interfacial resistance not only influenced by interfacial energy barrier but also strongly dependent on active layer thickness, and (ii) the bulk resistance that is affected only by active layer itself. The model having physical meaning in the fitting parameters, different from the previous with simple power functions, can well fit the voltage dependence for a series of independent data. In addition, the bulk resistance component can be extracted and is estimated reasonable for the first time, which is demonstrated not to be neglected even for the devices with high effective mobility. The developed model will be helpful for understanding of contact resistance and charge carrier injection behavior in the organic thin film transistors.

Temperature dependence of contact resistance at metal/MWNT interface

Although contact resistance of carbon nanotube (CNT) is one of the most important factors for practical application of electronic devices, a study regarding temperature dependence on contact resistance of CNTs with metal electrodes has not been found. Here, we report an investigation of contact resistance at multiwalled nanotube (MWNT)/Ag interface as a function of temperature, using MWNT/polydimethylsiloxane (PDMS) composite. Electrical resistance of MWNT/PDMS composite revealed negative temperature coefficient (NTC). Excluding the contact resistance with Ag electrode, the NTC effect became less pronounced, showing lower intrinsic resistivity with the activation energy of 0.019 eV. Activation energy of the contact resistance of MWNT/Ag interface was determined to be 0.04 eV, two times larger than that of MWNT-MWNT network. The increase in the thermal fluctuation assisted electron tunneling is attributed to conductivity enhancement at both MWNT/MWNT and MWNT/Ag interfaces with increasing temperature.

Dependence of ohmic contact properties on AlGaN layer thickness for AlGaN/GaN high-electron-mobility transistors

The dependence of ohmic contact resistance on the AlGaN layer thickness was evaluated for AlGaN/GaN high-electron-mobility transistor (HEMT) structures. Mo/Al/Ti contacts were formed on AlGaN layers with various thicknesses. The observed resistance characteristics are discussed on the basis of a model in which the overall contact resistance is composed of a series of three resistance components. Different dependences on the AlGaN layer thickness was observed after annealing at low temperatures (800–850 °C) and at high temperatures (900–950 °C). It was determined that lowering the resistance at the metal/AlGaN interface and that of the AlGaN layer is important for obtaining low-resistance ohmic contacts.

Modeling the gate-bias dependence of contact resistance in staggered organic field effect transistors based on carrier-concentration dependent mobility

In this paper, using a mobility modified current crowding formalism, we have modeled the gate-bias dependence of contact resistance in staggered organic field effect transistors. For this purpose, the gate modulation of the potential and carrier-concentration in the organic semiconductor layer has been investigated by solving the 1-D Poisson's equation. Due to this effect, the charge hopping mobility (μ) and therefore the conductivity of this layer which are carrier-concentration dependent are also modulated. Using an analytical carrier-concentration dependent model for μ with current crowding formalism, the gate-bias dependence of the contact resistance has been exactly simulated. Based on the obtained mobility relation, bulk and channel resistive components of the current crowding model are calculated. This method has been used for simulation of experimental data, and results confirm the reliability of the presented approach.

Analysis of the contact resistance in amorphous InGaZnO thin film transistors

Contact resistance has great impact on the performance of oxide thin film transistors (TFTs) and their applications. In this letter, temperature, gate voltage, and electrode dependences of the contact resistance were investigated in amorphous InGaZnO (a-IGZO) TFTs. We found that gate voltage dependent contact resistance made a large contribution to or even dominated the “field effect” of oxide TFTs. After separating the influence of contact resistance, the intrinsic temperature dependent field effect mobility of the a-IGZO TFTs was obtained. Furthermore, the experimental data of the contact resistance can be well described by an optimized transmission line model, and the height of the Schottky barrier in the interface between the metal electrode and a-IGZO semiconductor was found to be related to the gate voltage and account for the contact resistance's dependence on the gate voltage.

Impact of Contact on the Operation and Performance of Back-Gated Monolayer MoS2 Field-Effect-Transistors

Metal contacts to atomically thin two-dimensional (2D) crystal based FETs play a decisive role in determining their operation and performance. However, the effects of contacts on the switching behavior, field-effect mobility, and current saturation of monolayer MoS2 FETs have not been well explored and, hence, is the focus of this work. The dependence of contact resistance on the drain current is revealed by four-terminal-measurements. Without high-κ dielectric boosting, an electron mobility of 44 cm(2)/(V·s) has been achieved in a monolayer MoS2 FET on SiO2 substrate at room temperature. Velocity saturation is identified as the main mechanism responsible for the current saturation in back-gated monolayer MoS2 FETs at relatively higher carrier densities. Furthermore, for the first time, electron saturation velocity of monolayer MoS2 is extracted at high-field condition.

An analytical approach for parameter extraction in linear and saturation regions of top and bottom contact organic transistors

This paper analyzes the behaviour of top contact (TC) and bottom contact (BC) OTFTs through charge drift model modifying in terms of device series resistance. Subsequently, the drain and the gate voltages take into account the potential drop across the respective contacts. The mobility is modified in terms of overdrive voltage $$(V_{GS}-V_{T})$$(VGS-VT) and mobility enhancement factor as well. Additionally, the mathematical models are employed to extract the contact resistance including other parameters, such as, field dependent mobility, threshold voltage, mobility enhancement factor and drain current, separately for linear and saturation regimes. The model includes straightforward differential mathematics accounting of gate bias dependent contact resistance to evaluate the parameters analytically that too with single device only This resolves the complexity for realizing several devices with exactly same dimensions (except L) and moreover with same physical fabrication parameters. Besides this, the simulation is performed for four different TC and BC OTFTs using Atlas-Silvaco to comprehensively understand the device physics. Finally, the model is validated in terms of output characteristics and performance parameters against the experiment and simulation results. Analytically extracted mobility along with current for all four OTFTs is in a close harmony to the simulation results with an average error of 4 and 2.7 % in linear and saturation regions, respectively; whereas $$V_{T}$$VT is reasonably matched with 2 and 3 % average deviation.

Comparison of mobility extraction methods based on field-effect measurements for graphene

Carrier mobility extraction methods for graphene based on field-effect measurements are explored and compared according to theoretical analysis and experimental results. A group of graphene devices with different channel lengths were fabricated and measured, and carrier mobility is extracted from those electrical transfer curves using three different methods. Accuracy and applicability of those methods were compared. Transfer length method (TLM) can obtain accurate density dependent mobility and contact resistance at relative high carrier density based on data from a group of devices, and then can act as a standard method to verify other methods. As two of the most popular methods, direct transconductance method (DTM) and fitting method (FTM) can extract mobility easily based on transfer curve of a sole graphene device. DTM offers an underestimated mobility at any carrier density owing to the neglect of contact resistances, and the accuracy can be improved through fabricating field-effect transistors with long channel and good contacts. FTM assumes a constant mobility independent on carrier density, and then can obtain mobility, contact resistance and residual density stimulations through fitting a transfer curve. However, FTM tends to obtain a mobility value near Dirac point and then overestimates carrier mobility of graphene. Comparing with the DTM and FTM, TLM could offer a much more accurate and carrier density dependent mobility, that reflects the complete properties of graphene carrier mobility.