Study and modeling of the transport mechanism in a Schottky diode on the basis of a GaAs semiinsulator

The current through a metal-semiconductor junction is mainly due to the majority carriers.Three distinctly different mechanisms exist in a Schottky diode:diffusion of the semiconductor carriers in metal,thermionic emission-diffusion（TED） of carriers through a Schottky gate,and a mechanical quantum that pierces a tunnel through the gate.The system was solved by using a coupled Poisson-Boltzmann algorithm.Schottky BH is defined as the difference in energy between the Fermi level and the metal band carrier majority of the metal-semiconductor junction to the semiconductor contacts.The insulating layer converts the MS device in an MIS device and has a strong influence on its current-voltage（I-V） and the parameters of a Schottky barrier from 3.7 to 15 eV.There are several possible reasons for the error that causes a deviation of the ideal behaviour of Schottky diodes with and without an interfacial insulator layer.These include the particular distribution of interface states,the series resistance, bias voltage and temperature.The GaAs and its large concentration values of trap centers will participate in an increase in the process of thermionic electrons and holes,which will in turn act on the I-V characteristic of the diode,and an overflow maximum value[NT = 3×10²⁰]is obtained.The I-V characteristics of Schottky diodes are in the hypothesis of a parabolic summit.     

Minority carrier effects upon the small signal and steady-state properties of Schottky diodes

Schottky diode theory is re-evaluated by applying the combined thermionic emission-diffusion theory to both the majority and minority carrier flows across the metal-semiconductor contact. Under both steady-state d.c. and small signal a.c. conditions, numerical solutions to the semiconductor transport equations subject to boundary conditions determined from the combined theory are used to investigate the effects of minority carriers upon the properties of uniformly doped Schottky diodes. High injection effects and contact limitations are shown to influence the minority carrier injection ratio and the total stored minority carrier charge. It is further shown that the small signal impedance of a large class of Schottky diodes becomes inductive under moderate forward bias.     

Accurate surface potential determination in Schottky diodes by use of correlated current and capacitance voltage measurements. Application to n–InP

Based on current voltage (I-V_g) and capacitance voltage (C-V_g) measurements, a reliable procedure is proposed to determine the effective surface potential V_d.V_g/ in Schottky diodes. In the framework of thermionic emission, our analysis includes both the effect of the series resistance and the ideality factor, even voltage dependent. This technique is applied to n-type indium phosphide (n-InP) Schottky diodes with and without an interfacial layer and allows us to provide an interpretation of the observed peak on the C-V_g measurements. The study clearly shows that the depletion width and the flat band barrier height deduced from C-V_g, which are important parameters directly related to the surface potential in the semiconductor, should be estimated within our approach to obtain more reliable information.     

Inversion layer at the interface of Schottky diodes

An inversion layer can be present at the metal-semiconductor inteface of Schottky diodes with a high barrier and a lightly doped semiconductor. Its influence on the potential distribution and on the electric field distribution (especially on its maximum) is quite important and may be analyzed by means of an analytical model. The current characteristics calculated by the usual models are modified if one takes the inversion layer into account. In particular, the theoretical n of the Schottky diode is smaller than the value obtained from the usual depletion hypothesis, while the barrier height deduced from the experimental saturation current becomes larger. Excellent agreement between the experimental current characteristics of an PtSiSi diode and the combined model of thermionic emission-diffusion is obtained if the inversion layer is considered.     

Numerical simulation of the inhomogeneous barrier height of an Au/n-InP Schottky diodeKamel Zeghdar1, Lakhdar Dehimi1, 2, Achour Saadoune1, Nouredine Sengouga1,*

We report the current-voltage(I-V) characteristics of the Schottky diode(Au/n-InP) as a function of temperature. The SILVACO-TCAD numerical simulator is used to calculate the I-V characteristic in the temperature range of 280-400 K. This is to study the effect of temperature on the I-V curves and assess the main parameters that characterize the Schottky diode such as the ideality factor, the height of the barrier and the series resistance. The I-V characteristics are analyzed on the basis of standard thermionic emission(TE) theory and the inhomogeneous barrier heights(BHs) assuming a Gaussian distribution. It is shown that the ideality factor decreases while the barrier height increases with increasing temperature, on the basis of TE theory. Furthermore, the homogeneous BH value of approximately 0.524 eV for the device has been obtained from the linear relationship between the temperature-dependent experimentally effective BHs and ideality factors. The modified Richardson plot, according to the inhomogeneity of the BHs, has a good linearity over the temperature range. The evaluated Richardson constant A^* was 10.32 A·cm^-2·K^-2, which is close to the theoretical value of 9.4 A·cm^-2·K^-2 for n-InP. The temperature dependence of the I-V characteristics of the Au/n-InP Schottky diode have been successfully explained on the basis of the thermionic emission(TE) mechanism with a Gaussian distribution of the Schottky barrier heights(SBHs). Simulated I-V characteristics are in good agreement with the measurements[Korucu D, Mammadov T S. J Optoelectronics Advanced Materials, 2012, 14:41]. The barrier height obtained using Gaussian Schottky barrier distribution is 0.52 eV, which is about half the band gap of InP.     

Power Schottky diode design and comparison with the junction diode

Because the Schottky diode is a one-carrier device, it has both advantages and disadvantages with respect to the junction diode which is a two-carrier device. The advantage is that there are practically no excess minority carriers which must be swept out before the diode blocks current in the reverse direction. The disadvantage of the Schottky diode is that for a high voltage device it is not possible to use conductivity modulation as in the pin diode; since charge carriers are of one sign, no charge cancellation can occur and current becomes space charge limited. The Schottky diode design is developed in Section 2 and the characteristics of an optimally designed silicon Schottky diode are summarized in Fig. 9. Design criteria and quantitative comparison of junction and Schottky diodes is given in Table 1 and Fig. 10. Although somewhat approximate, the treatment allows a systematic quantitative comparison of the devices for any given application.     

Effect of the design and technology factors on electrical characteristics of the Au/Ti-n-GaAs Schottky diodes

Electrical properties of the Au/Ti-n-GaAs Schottky diodes are studied in relation to the production technology. The forward and reverse current-voltage characteristics of the diodes at low electric fields are analyzed on the basis of the mechanism of thermionic emission through the metal-semiconductor barrier. It is assumed that an increase in the reverse currents in the voltage range from 20 to 60 V can be accounted for by the Pool-Frenkel effect. The excess reverse currents at voltages higher than 60 V are caused by the phonon-assisted tunneling via deep states in the depletion region of the semiconductor.     

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.     

The stability of electrical characteristics of Ti/n-Si/Ag,Ti/n-Si/Cu and Ti/n-Si/AgCu diodes prepared under the same conditions with respect to increasing aging time

The purpose of this study is to fabricate Ti/n-Si/Ag, Ti/n-Si/Cu and Ti/n-Si/AgCu Schottky type diodes and to investigate the effects of aging time on the diode parameters such as ideality factor, barrier height, series resistance, interface state density and rectification ratio. High purity titanium (Ti) metal was deposited on the back side of the n-Si semiconductor and then the Ti/n-Si junction was annealed at 420 °C in nitrogen atmosphere. This junction showed ohmic behavior. To fabricate rectifier contacts, Ag, Cu metals and AgCu alloy have been evaporated on the other polished surface of n-Si with Ti ohmic contact. Ag and Cu ratios in the AgCu alloy which are used in the process of preparing the Schottky contact were taken in equal weights. Thus, Ti/n-Si/Ag, Ti/n-Si/Cu and Ti/n-Si/AgCu Schottky type diodes were prepared under the same conditions. The current-voltage (I-V) characterization of Ti/n-Si/Ag, Ti/n-Si/Cu and Ti/n-Si/AgCu diodes were immediately made at room temperature in dark conditions. To investigate the effect of aging time, the I-V measurements of the diodes have been repeated after 1, 7, 15, 30 and 90 days. Characteristic parameters of the diode were calculated from the I-V measurements which are taken with respect to aging time. The results were compared. From these results, it can clearly be seen that the electrical characteristics of diode which is made from AgCu alloy are more stable than other two diodes.     

The effects of the temperature and annealing on current-voltage characteristics of Ni/n-type 6H-SiC Schottky diode

The temperature dependence of current-voltage (I-V) characteristics of as-fabricated and annealed Ni/n-type 6H-SiC Schottky diode has been investigated in the temperature range of 100-500 K. The forward I-V characteristics have been analysed on the basis of standard thermionic emission theory. It has been shown that the ideality factor (n) decreases while the barrier height (Φ_b) increases with increasing temperature. The values of Φ_b and n are obtained between 0.65-1.25 eV and 1.70-1.16 for as-fabricated and 0.74-1.70 eV and 1.84-1.19 for annealed diode in the temperature range of 100-500 K, respectively. The I-V characteristics of the diode showed an increase in the Schottky barrier height, along with a reduction of the device leakage current by annealing the diode at 973 K for 2 min.     

A simple model for computer simulation of Schottky-barrier diodes

To simulate the electrical characteristics of metal-semiconductor Schottky barrier diodes, a numerical analysis program based on the Shockley's semiconductor equations has been established. The thermionic emissions of electrons and holes from semiconductor to metal as well as the electric field in the interfacial layer are taken as the derivative boundary conditions of the nonlinear equations. The forward and reverse current-voltage characteristics of various metal-silicon and metal-silicide-silicon Schottky barrier diodes can be simulated by properly choosing the zero-field barrier height and the interfacial-layer capacitance. The barrier height variation as a function of applied voltage is related to the space-charge density and the interfacial-layer capacitance. The nonideality of forward characteristics is attributed to the bending of majority carrier imref and the raising of barrier height. The soft behavior of reverse characteristics can be modeled in terms of the interfacial-layer capacitance.     

The effect of series resistance on calculation of the interface state density distribution in Schottky diodes

This work presents an attempt related to the importance of the fact that the series resistance value is considered in calculating the inter-face state density distribution from the non-ideal forward bias current–voltage (I-V) characteristics of Au/n-Si Schottky barrier diodes (SBDs). To examine the consistency of this approach, Au/n-Si SBDs with Si bulk thicknesses of 200 and 400μm have been prepared. Both diodes showed non-ideal I-V behaviour with ideality factors of 1.14 and 1.12, respectively, and thus it has been thought that the diodes have a metal–interface layer–semiconductor configuration. At the same energy position near the bottom of the conduction band, the interface state density (N _SS) values, without taking into account the series resistance value of the devices, are almost one order of magnitude larger than the N _SS values obtained taking into account the series resistance value.     

Gate controlled 2-DEG varactor for VCO applications in microwave circuits

A novel gate controlled Schottky diode varactor is introduced. The three-terminal varactor is a modulation-doped heterostructure of AlGaAs/GaAs with two Schottky contacts, similar to a metal–semiconductor–metal (MSM) diode. Schottky metal contacts are made to a two-dimensional electron gas (2-DEG). The third contact, the gate contact is formed from highly doped n⁺ GaAs material to allow an open optical window that can be used for optical gating and mixing. Structure capacitance is less than 1 PF and a change of more than 30% from the zero bias capacitance is observed with the applied gate voltage. On the basis of our quasi two-dimensional C–V model, the layer structure and device dimensions can be optimized and scaled to cover a wide range of operations in the microwave and millimeter wave regimes.     

碳化硅器件发展概述

概要介绍了第三代半导体材料碳化硅(SiC)在高温、高频、大功率器件应用方面的优势,结合国际上SiC肖特基势垒二极管,PiN二极管和结势垒肖特基二极管的发展历史,介绍了SiC功率二极管的最新进展,同时对我国宽禁带半导体SiC器件的研究现状及发展方向做了概述及展望。     

Barrier height diminution in Schottky diodes due to electrostatic screening

Electrostatic screening in the metal contact of a Schottky (metal-semiconductor) diode is shown to influence the calculated electrical characteristics of the diode. A thin space-charge layer is formed at the surface of the metal contact by capacitively induced free charges, This results in a voltage dependent diminution of the barrier height of the diode that increases in magnitude with increasing semiconductor dielectric constant and carrier concentration. Predicted values of the barrier height diminution exceed those attributed to image forces or tunneling effects for materials with dielectric constants greater than about 20. In diodes using semiconducting ferroelectric or piezoelectric materials, an additional diminution of the barrier height results from free charges induced in the metal contact by a remanent polarization field or an externally applied mechanical stress. Current-voltage characteristics of a metal-semiconductor diode are shown to be significantly influenced by the electrostatic screening effect. A soft breakdown current as opposed to saturation current is predicted for reverse biases while an exponential forward current with an η coefficient exceeding unity is predicted for forward biases. Photoemission characteristics are also affected. A voltage-dependent diminution of the threshold energy for photoresponse is predicted. Capacitance-voltage characteristics, on the other hand, differ only slightly from those of an ideal Schottky diode except in the case of a ferroelectric diode where excessively large screening effects are possible.     

Gate‐Controlled Energy Barrier at a Graphene/Molecular Semiconductor Junction

The formation of an energy‐barrier at a metal/molecular semiconductor junction is a universal phenomenon which limits the performance of many molecular semiconductor‐based electronic devices, from field‐effect transistors to light‐emitting diodes. In general, a specific metal/molecular semiconductor combination of materials leads to a fixed energy‐barrier. However, in this work, a graphene/C₆₀ vertical field‐effect transistor is presented in which control of the interfacial energy‐barrier is demonstrated, such that the junction switches from a highly rectifying diode at negative gate voltages to a highly conductive nonrectifying behavior at positive gate voltages and at room temperature. From the experimental data, an energy‐barrier modulation of up to 660 meV, a transconductance of up to five orders of magnitude, and a gate‐modulated photocurrent are extracted. The ability to tune the graphene/molecular semiconductor energy‐barrier provides a promising route toward novel, high performance molecular devices.     

New approach to the design and the fabrication of THz Schottkybarrier diodes

GaAs Schottky barrier diodes with near-ideal electrical and noise characteristics for mixing applications in the terahertz frequency range are described. The conventional formulas describing these characteristics are valid only in a limited forward bias range, corresponding to currents much smaller than the operating currents under submillimeter mixing conditions. Therefore, generalized analytical expressions for the I-V and C-V characteristics of the metal-semiconductor junction in the full bias range are given. A new numerical diode model is presented which takes into account not only the phenomena occurring at the junction, such as current dependent recombination and drift/diffusion velocities, but also the variations of electron mobility and electron temperature in the undepleted epi-layer. A diode fabrication process based on the electrolytic pulse etching of GaAs in combination with an in situ platinum plating for the formation of the Schottky contacts is described. Schottky barrier diodes with a diameter of 1 μm fabricated by this process have already shown excellent results in a 650-GHz waveguide mixer at room temperature     

Theoretical analysis of the quantum photoelectric yield in Schottky diodes

A detailed analytical calculation of the photoelectric quantum yield in Schottky diodes is presented. The transport of carriers in the surface space charge region is treated explicitly, taking account of photogeneration, diffusion and drift in the non-uniform electric field. Boundary conditions at the interface are expressed in terms of surface recombination velocity and emission velocity of excess carriers into the metal.It is shown that the metal-semiconductor interface strongly affects the collection efficiency of short wavelength generated electron-hole pairs. This effect basically originates in the emission flux of majority carriers into the metal.Current, charge carriers distributions and quantum yields are computed using the data of AuCdTe Schottky barriers.     

Flicker noise in metal semiconductor Schottky barrier diodes due to multistep tunneling processes

In a practical metal semiconductor Schottky barrier diode there is a certain amount of current flow by indirect tunneling through the barrier. Although this component of current is negligibly small compared to the thermionic emission or thermionic field emission current, a large low-frequency 1/f noise is associated with this multistep tunneling process. The multistep tunneling current introduces a random fluctuation of charge density at the trap states, which trap current carriers during the indirect tunneling process, in the space-charge region of the diode. The field intensity at the metal semiconductor interface is therefore modulated, which in turn modulates the Schottky effect and produces a random fluctuation of the diode current. The spectral intensity of noise due to this mechanism is calculated. Large flicker noise is expected at low frequencies.