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.     

The nature of electrical interaction of Schottky contacts

Electrical interaction between metal-semiconductor contacts combined in a diode matrix with a Schottky barrier manifests itself in an appreciable variation in their surface potentials and static current-volt-characteristics. The necessary condition for appearance of electrical interaction between such contacts consists in the presence of a peripheral electric field (a halo) around them; this field propagates to a fairly large distances (<30 μm). The sufficient condition is the presence of regions where the above halos overlap. It has been shown that variation in the surface potential and the current-voltage characteristics of contacts occurs under the effect of the intrinsic electric field of the contact’s periphery and also under the effect of an electric field at matrix periphery; the latter field is formed as a result of superposition of electric fields of halos which form its contacts. The degree of the corresponding effect is governed by the distance between contacts and by the total charge of the space charge regions for all contacts of the matrix: their number, sizes (diameter D _{i, j} ), concentration of doping impurities in the semiconductor N _D , and physical nature of a metal-semiconductor system with a Schottky barrier (with the barrier height φ _b ). It is established that bringing the contacts closer leads to a relative decrease in the threshold value of the “dead” zone in the forward current-voltage characteristics, an increase in the effective height of the barrier, and an insignificant increase in the nonideality factor. An increase in the total area of contacts (a total electric charge in the space charge region) in the matrix brings about an increase in the threshold value of the “dead” zone, a relative decrease in the effective barrier height, and an insignificant increase in the ideality factor.     

Thin active layer a-Si:H thin-film transistors

We show that hydrogenated amorphous silicon thin-film transistors (TFT's) with active layer thickness less than 50 nm have improved performance for display applications. Using two-dimensional (2-D) modeling, we find previously observed degradation for thin active layers is due to electric field effects in the contact regions of staggered inverted devices and affects only the saturation characteristics; linear region performance actually improves with decreasing thickness. We have fabricated devices with extremely thin active layer (10 nm), and indeed find excellent linear region characteristics. In addition, direct tunneling across the undoped regions at device contacts reduces electric field effects, resulting in excellent saturation region characteristics, and gate-induced channel accumulation reduces the Schottky barrier width at direct metal contacts so that even devices without doped contact regions (i.e., tunneling contacts) are possible     

Reversible Conversion between Schottky and Ohmic Contacts for Highly Sensitive,Multifunctional Biosensors

Schottky and Ohmic contacts–based electronics play an important role in highly sensitive detection of biomolecules and neural electric impulses, respectively. The reversible conversion between these two contacts appears especially important for multifunctional sensing by just one biosensor. Here, Schottky barrier height (SBH) is successfully tuned by triboelectric nanogenerator (TENG) and the same device is made to achieve reversible conversion between Schottky contact and Ohmic contact. In the same Schottky to Ohmic reversible (SOR) biosensor, highly sensitive detections of biomolecule (i.e., neurotransmitter) and neural electric signal are achieved at different contact states. The SOR biosensor reveals the feasibility of using one device to realize multifunctional detection. This work proposes a simple and significant method to achieve reversible tuning between the Schottky contact and Ohmic contact on one device by TENG, which exhibits great potential in developing multifunctional and high‐sensitivity biosensors, rectifiers, and other functional electronic devices.     

Recombination beneath ohmic contacts and adjacent oxide covered regions

By modelling a contact as a Schottky barrier space-charge region it is possible to calculate the effective surface-recombination velocity which it displays. Such a model is of use in analysing the behaviour of thin emitter regions, and of contacts, or oxide covered regions in the extrinsic base of I²L vertical transistors.It is shown necessary to take account of both the doping and electric field dependence of the carrier mobility as well as the exact form of the potential at the boundary between the space-charge region and the substrate.The values of surface recombination velocity predicted by the model are in fair agreement with available data, and a tentative model for recombination beneath an oxide is proposed.     

Nature of forward and reverse saturation currents in metal—semiconductor contacts with the Schottky barrier

The heavy dependence of the saturation currents for the forward and reverse I–V characteristics of high-barrier (>0.6 V) metal-semiconductor cont acts with the Schottky barrier on their diameter D is determined by an additional electric field formed under the effect of the contact periphery; this field is built into and codirected with the intrinsic electric field of the contact. It prevents the motion of electrons through the contact when a forward bias is applied across it. An increase in contact diameters from 5 to 700 μm results in decreasing the difference in forward and reverse saturation currents from five orders of magnitude to almost zero. The increase in the contact diameter, thus, results in decreasing periphery effect and absolute value of the built4n electric field. Adecrease in the barrier height( ≤0.6 V for D = 5 μm) also results in almost complete coincidence of forward and reverse saturation currents. At the reverse portions of the I–V characteristics, the effect of the built-in field manifests itself in a significant decrease in the effective height of the potential barrier due to a decrease in its width near the top and the substantial increase in the field electron emission through the barrier for lower energies. At the forward portions, it manifests itself in almost complete absence of the forward currents at low biases.     

Analysis of contact effects in fully printed p-channel organic thin film transistors

Contact effects have been analyzed in fully printed p-channel OTFTs based on a pentacene derivative as organic semiconductor and with Au source–drain contacts. In these devices, contact effects lead to an apparent decrease of the field effect mobility with decreasing L and to a failure of the gradual channel approximation (GCA) in reproducing the output characteristics. Experimental data have been reproduced by two-dimensional numerical simulations that included a Schottky barrier (Φ_b = 0.46 eV) at both source and drain contacts and the effects of field-induced barrier lowering. The barrier lowering was found to be controlled by the Schottky effect for an electric field E < 10⁵ V/cm, while for higher electric fields we found a stronger barrier lowering presumably due to other field-enhanced mechanisms. The analysis of numerical simulation results showed that three different operating regimes of the device can be identified: (1) low |V_ds|, where the channel and the Schottky diodes at both source and drain behave as gate voltage dependent resistors and the partition between channel resistance and contact resistance depends upon the gate bias; (2) intermediate V_ds, where the device characteristics are dominated by the reverse biased diode at the source contact, and (3) high |V_ds|, where pinch-off of the channel occurs at the drain end and the transistor takes control of the current. We show that these three regimes are a general feature of the device characteristics when Schottky source and drain contacts are present, and therefore the same analysis could be extended to TFTs with different semiconductor active layers.     

Modeling the gate bias dependence of contact resistance in staggered polycrystalline organic thin film transistors

We have modeled the dependence on the gate voltage of the bulk contact resistance and interface contact resistance in staggered polycrystalline organic thin film transistors. In the specific, we have investigated how traps, at the grain boundaries of an organic semiconductor thin film layer placed between the metal electrode and the active layer, can contribute to the bulk contact resistance. In order to the take into account this contribution, within the frame of the grain boundary trapping model (GBTM), a model of the energy barrier E_B, which emerges between the accumulation layer at the organic semiconductor/insulator interface and injecting contact, has been proposed. Moreover, the lowering of the energy barrier at the contacts interface region has been included by considering the influence of the electric field generated by the accumulation layer on the injection of carriers at the source and on the collection of charges from the accumulation layer to the drain contact. This work outlines both a Schottky barrier lowering, determined by the accumulation layer opposite the source electrode, as well as a Poole-Frenkel mechanism determined by the electric field of the accumulation layer active at the drain contact region. Finally it is provided and tested an analytical equation of our model for the contact resistance, summarizing the Poole-Frenkel and Schottky barrier lowering contribution with the grain boundary trapping model.     

Intrinsic difference in Schottky barrier effect for device configuration of organic thin-film transistors

Schottky barrier effect for n-channel organic thin-film transistors (OTFTs) with bottom-gate, top-contact (TC) and bottom-gate, bottom-contact (BC) configuration was examined by using device simulation with a thin-film organic transistor advanced simulator (TOTAS). A thermionic field emission (TFE) model which addresses tunneling of thermally excited electrons was applied as a carrier injection model of OTFTs. Simulation results reveal that the BC configuration is affected by Schottky barrier more severely than the TC configuration under the same condition for device parameters, and that this discrepancy in device characteristics can be completely alleviated by contact-area-limited doping, where highly-doped semiconducting layers are prepared in the neighborhood of contact electrodes. Moreover, the existence of an intrinsic Schottky barrier is indicated even though an ohmic-contact condition is assumed, which becomes more prominent for lower bulk carrier concentration in organic semiconductor. This work suggests the availability of the TFE model for simulating realistic OTFT devices with Schottky contacts. From the simulation results, intrinsic differences in device performance for the TC and BC configurations are discussed.     

A unified simulation of Schottky and ohmic contacts

The Schottky contact is an important consideration in the development of semiconductor devices. This paper shows that a practical Schottky contact model is available for a unified device simulation of Schottky and ohmic contacts. The present model includes the thermionic emission at the metal/semiconductor interface and the spatially distributed tunneling calculated at each semiconductor around the interface. Simulation results of rectifying characteristics of Schottky barrier diodes (SBD's) and resistances under high impurity concentration conditions are reasonable, compared with measurements. As examples of application to actual devices, the influence of the contact resistance on salicided MOSFETs with source/drain extension and the immunity of Schottky barrier tunnel transistors (SBTTs) from the short-channel effect (SCE) are demonstrated     

Carrier transport mechanism in La-incorporated high-k dielectric/metal gate stack MOSFETs

In this paper, carrier transport mechanism of MOSFETs with HfLaSiON was analyzed. It was shown that gate current is consisted of Schottky emission, Frenkel-Poole (F-P) emission and Fowler-Nordheim (F-N) tunneling components. Schottky barrier height is calculated to be 0.829 eV from Schottky emission model. Fowler-Nordheim tunneling barrier height was 0.941 eV at high electric field regions and the trap energy level extracted using Frenkel-Poole emission model was 0.907 eV. From the deviation of weak temperature dependence for gate leakage current at low electric field region, TAT mechanism is also considered.     

Metal-semiconductor-metal photodetectors with InAlGaP capping and buffer layers

To improve the Schottky contact performance and carrier confinement of GaAs metal-semiconductor-metal photodetectors (MSM-PDs), we employed the wide bandgap material, In/sub 0.5/(Al/sub 0.66/Ga/sub 0.34/)/sub 0.5/P, for the capping and buffer layers. We directly evaluated the Schottky contact parameters on the MSM-PD structure. The reverse characteristics of the Schottky contacts were examined by taking into account the Schottky barrier height depended on the electric field in the depletion region, and hence on the applied bias. The ideality factor and Schottky barrier height of Ti-Pt-Au contacts to In/sub 0.5/(Al/sub 0.66/Ga/sub 0.34/)/sub 0.5/P are 1.02 and 1.05 eV, respectively. Extremely low dark currents of 70 and 620 pA were obtained for these MSM-PDs when they were operated at a reverse bias of -10 V at room temperature and at 70/spl deg/C, respectively.     

Formation of potential barriers in undoped disordered semiconductors

The formation of potential barriers in undoped disordered semiconductors is considered. A generalized model of the potential barrier formation in such structures is examined using the example of a metal-amorphous hydrogenated silicon contact. It is shown that the properties of barriers in disordered semiconductors are determined by the energy distribution of the localized states in the mobility gap. An analytical expression for the electric field and potential in the space-charge region of a disordered semiconductor is obtained and a new method for the formation of surface quasi-ohmic contacts is suggested.     

A 2-dimensional fully analytical model for design of high voltage junction barrier Schottky (JBS) diodes

A physics-based closed form analytical model for the reverse leakage current of a high voltage junction barrier Schottky (JBS) diode is developed and shown to agree with experimental results. Maximum electric field “seen” by the Schottky contact is calculated from first principles by a 2-dimensional method as a function of JBS diode design parameters and confirmed by numerical simulations. Considering thermionic emission under image force barrier lowering and quantum mechanical tunneling, electric field at the Schottky contact is then related to reverse current. In combination with previously reported forward current and resistance models, this gives a complete I-V relationship for the JBS diode. A layout of interdigitated stripes of P-N and Schottky contacts at the anode is compared theoretically with a honeycomb layout and the 2-D model is extended to the 3-D honeycomb structure. Although simulation and experimental results from 4H-Silicon Carbide (SiC) diodes are used to validate it, the model itself is applicable to all JBS diodes.     

Conductance simulation in an a-Si:H thin-film transistor with Schottky barriers

It is shown by numerical simulation that the drain-source Schottky contacts substantially control the conductance of a thin-film transistor in the above-barrier region. At a barrier height in excess of 0.75 eV, the effect of crowding manifests itself; this effect is caused by an increase in electric field at the edge of the source electrode as the pulling voltage is increased, which brings about a local lowering of the barrier and an increase in the current through the reverse-biased Schottky barrier. The effective mobility in the thin-film transistor is controlled by the film and is independent of the barrier height.     

Effect of the electric field in the space-charge layer on the efficiency of short-wave photoelectric conversion in GaAs Schottky diodes

A study is reported of the quantum efficiency of the short-wave photoelectric effect as function of the reverse bias applied to GaAs Schottky diodes when the light absorption length is much shorter than the width of the space charge region. The quantum efficiency of photoelectric conversion is found to depend strongly on the contact electric field and the photon energy. The field independence of the quantum efficiency is interpreted in terms of a fluctuational trap model. The model can also be used to determine the loss factor for hot photocarriers, which is found to increase in a stepwise manner with increasing photon energy. This effect is explained in terms of the formation of excitons in X-and L-valleys of the semiconductor. Fiz. Tekh. Poluprovodn. 31, 1225–1229 (October 1997)     

An analysis of the charge-transport mechanisms defining the reverse current-voltage characteristics of the metal-GaAs barriers

Reverse current-voltage characteristics of metal-GaAs contacts with a Schottky barrier were measured. Linear portions of the reverse-current dependence on the squared electric-field strength in the space-charge region of diodes were obtained. Such a dependence is related to electron interaction with the lattice vibrations. The reverse current of the Mo-GaAs:Si contacts is analyzed at different temperatures. Results of the analysis showed that measured current-voltage characteristics are controlled by the phonon-assisted electron tunneling from metal into semiconductor with the involvement of a deep center attributed to the EL2 trap. A similar mechanism governs the reverse current-voltage characteristics of the Ni-GaAs:S Schottky diodes.     

Current-voltage characteristics and X- ray diffraction study of Pd/Si1- xGex schottky contacts

The electrical characteristics of Pd/p-Si_1-xGe_x Schottky contacts have been investigated. The Schottky contacts were formed by depositing Pd metal on substrates at room temperature (RT = 300K) and at low temperature (LT = 77K). Post annealing was performed in nitrogen atmosphere at 450 and 550°C, respectively, to study the effect of silicide formation on contact characteristics. The current-voltage measurements showed that the barrier height, ϕ_B, decreased with the increase of the gemanium composition. The contact postannealed at 550°C showed a current transport mechanism obviously different from the as-deposited Schottky contacts. Nearly identical characteristics were observed for the low temperature deposited contacts and the room temperature deposited contacts with 550°C post-annealing. They both showed thermionic emission dominated transport mechanism. X-ray diffraction technique was used to characterize the effect of different temperature treatments on the crystal structure. The full width at half maximum of Si_1-xGe_x(400) phase decreased at low temperature deposited sample, while it increased at room temperature deposition.     

PECVD diamond-based high performance power diodes

In this study, we have designed, fabricated, characterized, and analyzed plasma-enhanced chemical vapor deposition (PECVD) diamond-based Schottky diodes for high power electronics applications. We have elaborated four critical issues in the synthetic-diamond semiconductor technology: 1) growth, 2) doping, 3) Schottky contact, and 4) different device structures in order to achieve better performance parameters. We have obtained 500 V of breakdown voltage on one device and 100 A/cm/sup 2/ of current density on another device, optimized for different applications. These values are among the highest reported with the polycrystalline diamond-based devices. We have utilized different fabrication techniques for the growth of PECVD-diamond, different metals as a Schottky contact on diamond film and also optimized structural parameters such as diamond film thickness and doping concentration in order to achieve a high-performance power diodes. Analysis of the current conduction mechanisms of these devices in this study revealed a space-charge-limited current conduction mechanism in the forward bias region while thermionic field emission controlled current conduction mechanism in the reverse bias region. Performance parameters such as forward voltage drop, barrier height, and current density were analyzed as a function of temperature and type of metal Schottky contacts.     

A review of the theory and technology for ohmic contacts to group III–V compound semiconductors

The technology for ohmic contacts to group III–V compound semiconductors is reviewed in this paper. The basic principles of current transport in metal-semiconductor (Schottky barrier) contacts are presented first. The modes of current transport considered are thermionic emission over the barrier, and tunneling through the barrier due to thermionic-field or field emission. Special attention is devoted to the parameters of temperature and doping concentration which determine the dominant mode of conduction. As the primary mode of conduction changes from thermionic emission dominated to tunneling dominated, the current-voltage behavior of the contact changes from rectifying to ohmic in character. The experimental techniques for fabricating ohmic contacts to III–V compound semiconductors are then described. Contact problems as they pertain to specific device applications are considered. Finally, present difficulties with contacts to mixed III–V crystals are discussed.