4H-silicon carbide Schottky barrier diodes for microwave applications

In this paper, physical models for vertical 4H-silicon carbide (4H-SiC) Schottky diodes are used to develop a design method, where a maximum cutoff frequency for a given punch-through is achieved. The models presented are also used to extract microwave simulator computer-aided design (CAD) models for the devices. A device process was developed and Schottky diodes were fabricated in-house. Characterization of the devices was performed and compared to the theoretical models with good agreement. A demonstrator singly balanced diode mixer was simulated using the developed models. The mixer was fabricated using the in-house developed diodes, and measurements on the mixer show good agreement with the CAD simulations. A conversion loss of 5.2 dB was achieved at 850 MHz, and an excellent IIP/sub 3/ of 31 dBm at 850-MHz RF was measured, at 30-dBm P/sub LO/. These results verify the enhanced properties of the SiC Schottky diode compared to other nonwide bandgap diodes.     

Forward transient behavior of P-N junction diodes at high injection levels

By use of the finite-difference method of numerical analysis, a detailed numerical solution is obtained for the complete one-dimensional continuity equation, to study the transient and steady-state behavior of p-n junction diodes at moderate-to-high injection levels. The solution of the continuity equation is obtained for minority carriers in the base of a diode under the influence of drift, diffusion, and recombination. From the calculated distribution of minority carriers in the base, minority-carrier and majority-carrier currents and their components are obtained. The electric field and its components in the base are determined, and the total voltage across the diode and its components are calculated. All results are obtained for transient and steady-state conditions. The effect of variations of certain physical parameters on the diode behavior is investigated. All calculations were performed on the IBM 709 digital computer.     

Dopant activation and surface morphology of ion implanted 4H- and 6H-silicon carbide

The activation of ion-implanted B into 4H-SiC, and B, and Al into 6H-SiC is investigated. Complete activation of B implants into 4H-SiC is achieved by annealing at 1750°C for 40 min in an Ar environment. Significant activation (>10%) is not achieved unless the annealing temperature is 1600°C or greater. Sheet resistances of Al-implanted 6H-SiC annealed at 1800°C are 32.2 kΩ/□, indicating high activation of Al at this temperature. Annealing conditions which result in good acceptor activation are shown to be damaging to the surface of either 4H- or 6H-SiC. Atomic force microscopy and Nomarski differential interference contrast optical microscopy are applied to characterize the surfaces of these polytypes. Roughening of the surfaces is observed following annealing in Ar, with measured roughnesses as large as 10.1 nm for B-implanted 4H-SiC annealed at 1700°C for 40 min. Based on data obtained from these techniques, a model is proposed to describe the roughening phenomenon. The premise of the model is that SiC sublimation and mobile molecules enable the surface to reconfigure itself into an equilibrium form.     

Device processing and characterisation of high temperature silicon carbide Schottky diodes

High temperature silicon carbide diodes with nickel silicide Schottky contacts were fabricated by deposition of titanium-nickel metal film on 4H-SiC epitaxial wafer followed by annealing at 550 °C in vacuum. Room temperature boron implantation have been used to form single zone junction termination extension. 4H-SiC epitaxial structures designed to have theoretical parallel-plain breakdown voltages of 1900 and 3600 V have been used for this research. The diodes revealed soft recoverable avalanche breakdown at voltages of 1450 and 3400 V, respectively, which are about 80% and 95% of theoretical values. I-V characteristics of fabricated 4H-SiC Schottky diodes have been measured at temperatures from room temperature up to 400 °C. The diodes revealed unchangeable barrier heights and ideality factors as well as positive coefficients of breakdown voltage.     

Thermal effects on the forward characteristics of silicon p-i-n diodes at high pulse currents

When applying high pulse currents, the resulting temperature increase in the base regions of p-i-n diodes and thyristors leads to notable changes in the forward characteristics. (I) Decrease of carrier mobility noticeably increases the voltage drop across the diode and tends to limit the current density. (II) When the temperatures are sufficiently high to supply high intrinsic carrier concentrations, the temperature coefficient of the resistivity becomes negative. This can lead to current localization and destruction of the diodes.

To study these effects, the temperature of the base regions was monitored during high pulse currents using thermal i.r. emission. Indication for current limit occurs above 200°C, when the diodes are heated from room temperature. The negative temperature coefficient of the resistivity occurs at temperatures at which the intrinsic carrier concentration has reached the same order of magnitude as the injected carrier density. Simple theoretical treatment yields reasonably good agreement with the experimental results.     

High power 4H–SiC pin diodes (10 kV class) with record high carrier lifetime

The hole lifetime τ_p in the n-base and isothermal (pulse) current–voltage characteristics have been measured in 4H–SiC diodes with a 10 kV blocking voltage (100 μm base width). The τ_p value found from open circuit voltage decay (OCVD) measurements is 3.7 μs at room temperature. To the best of the authors’ knowledge, the above value of τ_p is the highest reported for 4H–SiC. The forward voltage drops V_F are 3.44 V at current density j = 100 A/cm² and 5.45 V at j = 1000 A/cm². A very deep modulation of the blocking base by injected non-equilibrium carriers has been demonstrated. Calculations in term of a simple semi-analytical model describe well the experimental results obtained.     

Experimental characterization and numerical analysis of the 4H-SiC p-i-n diodes static and transient behaviour

Steady-state and turn-off switching characteristics of aluminium-implanted 4H-SiC p-i-n diodes designed for high-current density operation, are investigated experimentally and by mean of numerical simulations in the 298-523 K temperature range. The diodes present circular structure with a diameter of 350 μm and employ an anode region with an aluminium depth profile peaking at 6×10¹⁹ cm⁻³ at the surface. The profile edge and the junction depth are located at 0.2 and 1.35 μm, respectively. At room temperature the measured forward current density is close to 370 A/cm² at 5 V with an ideality factor always less than 2 before high-current injection or device-series resistance became dominant. The transient analysis reveals a strong potential of this diodes for use in high-speed, high-power applications, especially at high temperature, with a very low turn-off recovery time (<80 ns) in the whole range of test conditions. The simulated results match the experimental data, showing that the switching performance is mainly due to the poor minority charge carrier lifetime estimated to be 15 ns for these implanted devices.     

Influence of operation conditions on true-static DC characteristics and on electro-thermal transient states in silicon carbide Merged PiN Schottky diodes.

In this paper, measured and calculated non-isothermal DC characteristics of silicon carbide MPS devices are investigated, with special attention paid on critical parameters, such as maximum current and junction temperature at which a thermal runaway may occur. Electro-thermal transient states in single MPS devices (forward surge current tests) and in the simple Greatz rectifier are simulated and compared to measurements. Various electro-thermal models of SiC SBDs, with a simplified, effective procedure for calculations of junction temperature are proposed.     

Theory and experiment for silicon Schottky barrier diodes at high current density

Metal-silicon Schottky barrier diodes exhibit n values which theoretically vary as a function of doping and applied voltage. The expected variation depends on which theoretical model is used to describe the current transport.Titanium n-type silicon barriers were prepared. At a doping level of 3 × 10¹⁵ cm^?3 the barrier height and n-value measured at 100 mV were 0.485±0.005 V and 1.02±0.01 whereas for a doping level of 2 × 10¹⁴ cm^?3 the corresponding values were 0.500±0.005 V and 1.18±0.05.The experimental variation of the diode n value as a function of semiconductor band bending showed good agreement with the thermionic-diffusion model of Crowell and Beguwala: n values increased rapidly as the band bending β → 2, and n values were highest at a given β for diodes with the lowest doping concentration. Similar results were obtained by measurements on magnesium and aluminium barriers on n-type silicon.An analysis of the results has shown that the variation of the diode saturation current I_s follows the predictions of the thermionic-diffusion theory, although there were some anomalies at high current densities. The anomalies did not result from variation of the width of the undepleted region of the epitaxial silicon layer or from diode self-heating effects.     

Ruggedness analysis of 3.3 kV high voltage diodes considering various buffer structures and edge terminations

Buffer structures and edge termination have a decisive influence on the static and dynamic characteristics of free-wheeling diodes. In this paper the influence of buffer structures at the cathode side, the influence of the design of the edge termination and of a resistive zone at the anode side are analysed with respect to the ruggedness of free-wheeling diodes. Therefore, we investigated the device behaviour by means of numerical device simulation concerning the formation of current filamentation and the correlated shape of the electrical field distribution. The considered edge termination of the diodes was planar junction termination extensions and a beveled edge. Various buffer structures, a Gaussian buffer and a buried n-doped layer of increased doping called epitaxy level buffer are compared with a reference diode without any buffer structure.     

Low noise wide bandgap SiC based IMPATT diodes at sub-millimeter wave frequencies and at high temperature

We have presented a comparative account of the high frequency prospective as well as noise behaviors of wide-bandgap 4H-SiC and 6H-SiC based on different structures of IMPATT diodes at sub-millimeter-wave frequencies up to 2.18 THz. The computer simulation study establishes the feasibility of the SiC based IMPATT diode as a high power density terahertz source. The most significant feature lies in the noise behavior of the SiC IMPATT diodes. It is noticed that the 6H-SiC DDR diode shows the least noise measure of 26.1 dB as compared to that of other structures. Further, it is noticed that the noise measure of the SiC IMPATT diode is less at a higher operating frequency compared to that at a lower operating frequency.     

Observation of minority-carrier traps in Schottky diodes with a high barrier and a compensated near-contact region using deep-level transient spectroscopy

Schottky diodes based on the single-crystal n-ZnSe and fabricated by nitrogen-ion implantation with subsequent postimplantation treatment employing radical-beam epitaxy in atomic oxygen were studied using deep-level transient spectroscopy. On the assumption that the Schottky barrier is high and the near-contact region is compensated, the processes resulting in the occurrence of traps of minority charge carriers under negative biases were analyzed. The procedure for determining the compensated region thickness and the concentration of minority charge carriers in this region is described. The mechanisms of defect formation in zinc selenide crystals under annealing in atomic oxygen are described on the basis of the deep-level transient spectroscopy results.     

Dielectric properties and ac electrical conductivity studies of MIS type Schottky diodes at high temperatures

Dielectric properties and ac electrical conductivity of the Al/SiO₂/p-Si (MIS) Schottky diodes were studied in the frequency and temperature range of 10 kHz-1 MHz and 300-400 K, respectively. Experimental results show that the dielectric constant (ε′), dielectric loss (ε″), loss tangent (tan δ), ac electrical conductivity (σ_ac) and the electric modulus were found a strong function of frequency and temperature. The values of the ε′, ε″ and tan δ decrease with increasing frequencies due to the interface states capacitance and a decrease in conductance with increasing frequency. Also, these values increase with increasing temperature. The σ_ac is found to increase with increasing frequency and increasing temperature. The variation of conductivity as a function of temperature and frequency reveals non-adiabatic hopping of charge carriers between impurities localized states. In addition, the experimental dielectric data have been analyzed by considering electric modulus formalism.     

Vertical-cavity surface-emitting laser diodes at 1.55 μm withlarge output power and high operation temperature

Improved 1.55 μm InGaAlAs/InP vertical-cavity surface-emitting lasers were fabricated in the buried tunnel junction technology yielding sub-mA threshold currents, 0.9 V threshold voltage, 10-40 Ω series resistance, output power up to 7 mW (20°C, CW) and CW operation up to >110°C     

Electron transient response and impact-ionized plasma instability in GaAs at very high electric fields

Dynamic behavior of electrons in GaAs under impact ionization conditions is studied by the Monte Carlo technique. The relaxation time of impact ionization generation rate is estimated. The linear stage of instability caused by the generation rate lagging the electric field is investigated. The nonlinear regime of this instability is numerically simulated. The electric field and plasma density oscillations with a frequency of the order of 50–1000 GHz are obtained.     

The effect of capacitive mixing and detection in Schottky-barrier diodes at high, intermediate, or modulation frequencies

At high IF or modulation frequencies a Schottky-barrier diode not only operates as a conductance mixer or detector due to its nonlinear conductance but also as a capacitance mixer or detector due to its nonlinear capacitance. The effect is calculated for a heavily doped Schottky-barrier diode in which the pump amplitude and the signal amplitude are relatively small so that short Taylor expansions of the current and charge characteristics can be used. Beyond an upper corner frequency f1the effect gives rise to a considerable increase in the output signal and to a considerable increase in signal-to-noise ratio. Due to the electronic feedback between input and output, the output conductance g_{out} can be negative foromega_{i} = omega_{P} - omega_{0}when capacitive mixing predominates. At the limit of stability (g_{out} = 0) the noise figureF = 1 + n_{1}omega_{i}/omega_{0}, where n1lies between ½ and 1.     

High speed turn-on reverse conducting 4 kV static induction thyristors based on the buried gate type p-base n-emitter soft contact structure and anti-parallel diodes for solid-state power supplies in high energy accelerators

Buried gate type p-base n-emitter soft contact (PNSC) structure 4 kV static induction thyristors (SIThys) have been fabricated in order to make thyristors for high-speed turn-on applications to such as high energy accelerators. It was found that these static induction thyristors could be fabricated under the test pilot line with a good device production yield. The static characteristics and pulse switching characteristics of these static induction thyristors are examined. Several application examples of the present 4 kV static induction thyristors to power supplies for high energy accelerators are shown.     

The influence of high energy electron irradiation on the Schottky barrier height and the Richardson constant of Ni/4H-SiC Schottky diodes

The influence of high energy electron (HEE) irradiation from a Sr-90 radio-nuclide on n-type Ni/4H–SiC samples of doping density 7.1×10¹⁵ cm⁻³ has been investigated over the temperature range 40–300 K. Current–voltage (I–V), capacitance–voltage (C–V) and deep level transient spectroscopy (DLTS) were used to characterize the devices before and after irradiation at a fluence of 6×10¹⁴ electrons-cm⁻². For both devices, the I–V characteristics were well described by thermionic emission (TE) in the temperature range 120–300 K, but deviated from TE theory at temperature below 120 K. The current flowing through the interface at a bias of 2.0 V from pure thermionic emission to thermionic field emission within the depletion region with the free carrier concentrations of the devices decreased from 7.8×10¹⁵ to 6.8×10¹⁵ cm⁻³ after HEE irradiation. The modified Richardson constants were determined from the Gaussian distribution of the barrier height across the contact and found to be 133 and 163 A cm⁻² K⁻² for as-deposited and irradiated diodes, respectively. Three new defects with energies 0.22, 0.40 and 0.71 eV appeared after HEE irradiation. Richardson constants were significantly less than the theoretical value which was ascribed to a small active device area.     

A 4.15 kV 9.07-m/spl Omega//spl middot/cm/sup 2/ 4H-SiC Schottky-barrier diode using Mo contact annealed at high temperature

In this letter, we report the fabrication of high-voltage and low-loss 4H-SiC Schottky-barrier diodes (SBDs) with a performance close to the theoretical limit using a Mo contact annealed at high-temperature. High-temperature annealing for the Mo contact was found to be effective in controlling the Schottky-barrier height at 1.2-1.3 eV without degradation of n-factor and reverse characteristics. We successfully obtained a 1-mm/sup 2/ Mo-4H-SiC SBD with a breakdown voltage (V/sub b/) of 4.15 kV and a specific on resistance (R/sub on/) of 9.07 m/spl Omega//spl middot/cm/sup 2/, achieving a best V/sub b//sup 2//R/sub on/ value of 1898 MW/cm/sup 2/. We also obtained a 9-mm/sup 2/ Mo-4H-SiC SBD with V/sub b/ of 4.40 kV and R/sub on/ of 12.20 m/spl Omega//spl middot/cm/sup 2/.     

Computer studies on the widening of the avalanche zone and the decrease in efficiency of silicon X-band symmetrical double-drift impatt diodes at high current densities

A computer study of the field and current profiles of silicon X-band symmetrical d.d.r. impatt diodes is presented. The results show that there is considerable widening of the avalanche zone at high values of current density. This causes a sharp fall in the efficiency of the device at high current density after reaching a maximum.