Mechanisms of current flow in metal-semiconductor ohmic contacts

Published data on the properties of metal-semiconductor ohmic contacts and mechanisms of current flow in these contacts (thermionic emission, field emission, thermal-field emission, and also current flow through metal shunts) are reviewed. Theoretical dependences of the resistance of an ohmic contact on temperature and the charge-carrier concentration in a semiconductor were compared with experimental data on ohmic contacts to II–VI semiconductors (ZnSe, ZnO), III–V semiconductors (GaN, AlN, InN, GaAs, GaP, InP), Group IV semiconductors (SiC, diamond), and alloys of these semiconductors. In ohmic contacts based on lightly doped semiconductors, the main mechanism of current flow is thermionic emission with the metal-semiconductor potential barrier height equal to 0.1–0.2 eV. In ohmic contacts based on heavily doped semiconductors, the current flow is effected owing to the field emission, while the metal-semiconductor potential barrier height is equal to 0.3–0.5 eV. In alloyed In contacts to GaP and GaN, a mechanism of current flow that is not characteristic of Schottky diodes (current flow through metal shunts formed by deposition of metal atoms onto dislocations or other imperfections in semiconductors) is observed.     

Dual-level transmission line model for current flow in metal-semiconductor contacts

Parasitic resistance in the metal-oxide-semiconductor field-effect transistor becomes increasingly important as design rules shrink. The majority of this resistance arises from the contact resistance of the metal-semiconductor interface and the resistance of the semiconductor source and drain regions. The most popular method for deriving current flow in this region is the transmission line model. Though this model has proven quite useful, the severe restriction of one-dimensional current flow will introduce errors in some situations. We formulate and solve a more sophisticated dual-level transmission line model in which we incorporate to first order the two-dimensional nature of the current flow. We discuss this model in terms of an enhancement to the transmission line model and present a detailed comparison of the two models. We find that the dual-level transmission line model produces significant (12 percent) corrections to the transmission line model with source resistivity and thickness and specific contact resistivity typical of 1-µm design rule technologies.     

Effect of the periphery of metal-semiconductor contacts with Schottky barriers on their static current-voltage characteristic

Kelvin probe atomic-force microscopy of the electrostatic surface potential of gold Schottky contacts on n-GaAs showed that there is an extended transition area (halo) (tens of micrometers) around contacts in which the surface potential varies from the n-GaAs free surface potential to the gold contact surface potential. The contact potential and its distribution in the surrounding halo are controlled by the contact structure. The study of spreading currents showed that there is a high-conductance area (periphery) around the contact perimeter due to strong electric fields of the halo, which causes leakage currents. The conductivity of the main contact area is caused by 100- to 200-nm local areas with higher and lower conducting abilities. Mesa formation around contacts causes a decrease in the work function, a decrease in the halo extent and electric field strength, which is accompanied by spreading and decreasing of the peripheral area conductance. This results in disappearance of leakage currents and a decrease in the ideality index. In contrast, protection of the peripheral area by a SiO₂ insulating film 0.5 μm thick increases the work function, which is accompanied by the formation of potential lobes around the contact in two mutually perpendicular crystallographic directions. A stronger penetration of halo electric fields into the contact area results in an increase in the ideality index and disappearance of high-conductance peripheral area and leakage currents. The difference between the electrical properties of the periphery, gold grains, and their boundaries controls the contact switching mechanism when applying forward or reverse biases.     

On the direct currents through interface states in metal-semiconductor contacts

The direct currents through interface states in metal-semiconductor majority-carrier contacts are calculated analytically. A transition is observed with forward bias voltage from metal-controlled to semiconductor-controlled occupancy of the interface states. Calculations for the Au-n-type silicon system indicate that interface state currents are comparable with band currents for densities ? 10¹¹ cm^?2 eV^?1 (oxide thickness 15 Å) or 10¹³ cm^?2 eV^?1 (10 Å).     

High-frequency barrier capacitance of metal-semiconductor contacts and abrupt p-n junctions

Semiconductors - High-frequency capacitance of a Schottky barrier and an abrupt p-n junction are calculated taking into account the free charge carrier concentration in the space-charge region of a...     

Characteristics of metal-semiconductor contacts fabricated by the electroless deposition method

Metal-semiconductor contacts have been fabricated by electroless deposition of Cu on chemically cleaned n-type silicon and their characteristics studied. The values of barrier height and the ideality factor are found to be comparable to those of vacuum evaporated contacts. A non-linearity in the 1/C² vs V plot has been observed and the same has been satisfactorily explained by taking surface state capacitance into consideration.     

Mechanisms of current flow in Au-CdTe contacts with a modified surface

The reverse current-voltage characteristics of surface-barrier diodes based on n-CdTe with a surface modified by treating in an aqueous solution of alkali metal salts have been investigated. The reverse current is found to have a tunneling nature at low biases and be caused by avalanche multiplication of carriers as a result of impact ionization at high biases.     

the effect of the periphery of metal-semiconductor Schottky-barrier contacts on their electrical characteristics

During the formation of the metal-semiconductor contact with a Schottky barrier (as a gold film on the p- or n-type gallium arsenide surface), an electric field E _l built into the electric contact is induced, which propagates around the contact to the distance l (halo) tens of times larger than space-charge region sizes. This field reduces the electrostatic potential of the φ_Au contact by a significant value φ*. In the general case, the halo size l and the decrease φ* in the electrostatic potential are controlled by the charge value and sign in the space-charge region, which depend on the contact diameter D, semiconductor concentration and conductivity type. For Au/n-GaAs Schottky-barrier contacts, a decrease in D results in the increasing role of periphery, which manifests itself in increasing φ* and decreasing φ_Au and l. For Au/p-GaAs contacts, a decrease in D results in the decreasing effect of periphery, which appears in decreasing φ* and increasing φ_Au and l. The absence of the space-charge region in metal-insulator-semiconductor contacts results in the fact that the halo size l and φ* are independent of their diameters.     

Properties of metal-semiconductor contacts—II: Numerical solutions for intrinsic material

Part I of this paper was concerned mainly with an explicit solution to the problem of contacts on intrinsic material. The principal result was the prediction of a linear log j vs V_a dependence in the forward direction, with a slope of 2qV_a/kT, instead of the more familiar qV_a/kT expected for the simplest extrinsic case, V_a being the voltage applied to the contact barrier itself. The present paper deals with numerical solutions of the same transport equations, free from the approximations which are necessary in the course of analytic treatments to make the problem soluble. Some of the evaluated relationships refer to semi-infinite models, and some to models of severely restricted size. Solutions in the form of one-dimensional contours of potential, field and carrier concentration are provided, as well as the relationship between the injection ratio and current density. Inasmuch as the computations concern the injection region, they confirm (qualitatively and quantitatively) the previous calculations made by linearized analytic methods; inasmuch as they include the entire barrier region, they represent new solutions to a complementary aspect of the total problem.     

Interphase interactions and the mechanism of current flow in Au-TiB x -AuGe-n-GaP ohmic contacts

Au-TiB _x -AuGe-n-GaP ohmic contacts have been investigated before and after rapid thermal annealing at T = 723, 773, and 873 K for 60 s in a hydrogen atmosphere. It is shown that the contact resistivity decreases with an increase in temperature in the range 77–232 K due to the thermionic nature of current flow in inhomogeneous ohmic contacts, while in the range 232–386 K the contact resistivity increases, which can be related to the conduction through metal shunts.     

Mechanisms of anomalous photovoltage Effect in CdTe films

The method for determining the contributions of asymmetry of illumination and a difference in parameters of p-n and n-p junctions of the p-n-p structure in cadmium-telluride films to the anomalous photovoltage effect is suggested.     

Amorphous metal-semiconductor contacts for high temperature electronics—II Thermal stability of Schottky barrier characteristics

We report results on the thermal stability of the Schottky barrier formed by each of two amorphous metal alloys (from the NiNb and TaIr systems) on Si and GaAs. We have found the barrier height to be stable to within 0.05 eV after treatment for 2.5 hr at 500°C in the case of TaIr/n/n⁺ GaAs, and within 0.06 eV after treatment for 40 hr at 350°C in the case of TaIr/n/n⁺ Si.     

High current pressure contacts to Ag pads on thin filmsuperconductors

High current, low resistance, nonmagnetic, and nondestructive pressure contacts to Ag pads on YBa₂Cu₃O_7-δ (YBCO) thin film superconductors were developed in this study. The contact resistance reported here includes the resistance of the current lead/Ag pad interface, the Ag pad/YBCO interface, and the bulk resistance of the contact material. This total contact resistance is the relevant parameter which determines power dissipation during critical-current measurements. It was found that regardless of the optimization of the Ag pad/YBCO interface through annealing, a pressure contact can yield a lower total resistance than a soldered contact. The lowest resistance obtained with pressure contacts was 3 μΩ (for a 2×4 mm ² contact). These contacts may be useful for many different high temperature superconductor (HTS) studies where high-current contacts with low heating are needed     

Displacement current of Au/p-diamond Schottky contacts

In this study, the displacement current of Au/p-diamond Schottky contacts was studied by comparing them with low-Mg-doped p-GaN Schottky contacts. In the current–voltage (I–V) characteristics, the current was proportional to the voltage sweep speed at V >−1.5 V. The differential output waveform was obtained through AC operation. Therefore, the displacement current was dominant in the low voltage region where in the true current was extremely small due to the large Schottky barrier height of 1.57 eV. The memory effect, due to the charge and discharge of localized acceptor-type deep-level defects, was negligible in the I–V curve. A clear AC differential signal was confirmed. This suggested that the interface defect density of the p-diamond contacts was very small compared to the p-GaN contacts. Hence, p-diamond Schottky contacts were expected to be a good candidate for a key device such as a phase modulator in a wireless transmitter.     

气体吸附对SnO2／Si光电压的影响

测量了ＳｎＯ２／Ｓｉ表面吸附Ｈ２、液化石油气等还原性气体前后光电压的变化；分析了ＳｎＯ２／Ｓｉ表面的吸附性和机理。     

Inversion-layer induced body current in SOI MOSFETs with body contacts

This letter investigates the body current of thin silicon-on-insulator MOSFETs with body contacts using H-gate and T-gate structures. Due to tunneling between the inversion layer and body contacts, the extra body current was measured and confirmed by the floating-source measurement technique. The drain current at saturation is increased due to the extra body current, which may result in smaller output resistance. A measurement example is also demonstrated.     

Dark current and photovoltage models on the formation of depletion region in C60/NPB organic heterojunctions

We establish quantitative models on the formation of depletion regions in organic photodiodes (OPD) based on fullerene/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (C₆₀/NPB) heterojunctions. The models describe the relation of dark current and open-circuit voltage to the deposited thickness of C₆₀ or NPB. Interfacial electronic structures, such as built-in potential, the charge density, the minimized thicknesses of completely developed depletion regions and the energy level bending on each side of the heterojunction were derived from the fitting model. Also, we observed a shift of depletion region from NPB to C₆₀ due to the relative change of charge density under illumination. The device performance proved the reasonability of the models. This paper provides a universally applicable method to probe the interfacial information of organic semiconductors.     

Numerical study on the short-circuit current in single layer organic solar cells with Schottkey contacts

The effects of the cathode operation voltage,the absorption coefficient,the carrier mobility,the temperature and the thickness of organic active layer on the short-circuit current density of single layer organic solar cells(OSCs) with Schottkey contacts are numerically studied.Quantitative dependences of the short-circuit current density on the respective parameters are obtained.The results show that a larger operation-voltage difference between the anode and the cathode,a higher carrier mobility as well as...     

Effect of electrode materials and annealing on metal-semiconductor contact of Ga2O3 with metal

Gallium oxide(Ga2O3) thin films were prepared on Si substrate by magnetron sputtering. The obtained samples were comprehensively characterized by X-ray photoelectron spectroscopy(XPS) and scanning electron microscope(SEM). Ti, Pt, Ni and AZO were deposited on the Ga2O3 thin films as electrodes. This paper mainly studies the metal-semiconductor contact formed by these four materials on the films and the influence of annealing at 500℃ on the metal-semiconductor contact. The I-V characteristics show a good linear relationship, which indicates ohmic contact between Ga2O3 and other electrodes.     

Increase in barrier height of Al/n-GaAs contacts induced by high current

High forward-gate current (HFGC) density induces an increase of the Al/n-GaAs barrier height from 0.8 to 0.96 eV, thus suggesting the formation of an GaxAl1-xAs layer at the interface. Results show that this interaction is more enhanced by the electron current from the semiconductor to the metal than by thermal treatments. The intense electron flow is believed to contribute to the breaking of the interfacial oxide layer present at the metal-semiconductor interface, thus promoting Al/GaAs interdiffusion. Data were obtained on power MESFET's with Al metallized gate.