SiC Schottky Diodes Research Articles

A SiC Varactor With Large Effective Tuning Range for Microwave Power Applications

SiC Schottky diode varactors have been fabricated for use in microwave power applications, specifically the dynamic load modulation of power amplifiers. A custom doping profile has been employed to spread the C(V) over a large bias voltage range, thereby increasing the effective tuning range under large voltage swing conditions. The small-signal tuning range is approximately six, and punch through is reached at a bias voltage of -60 V, while the breakdown voltage is on the order of -160 V. An interdigitated layout is utilized together with a self-aligned Schottky anode etch process to improve the Q-factor at 2 GHz, which is 20 at zero bias and approximately 160 at punch through.

Electrical characteristics of Mo/4H-SiC Schottky diodes having ion-implanted guard rings: temperature and implant-dose dependence

The electrical characteristics of ion-implanted guard rings for molybdenum (Mo) Schottky diodes on 4H-SiC are analyzed on the basis of the standard thermionic emission model and the assumption of a Gaussian distribution of the barrier height. For edge termination, high-resistivity guard rings manufactured by carbon and aluminum ion-implanted areas were used. Extractions of barrier heights of molybdenum on silicon carbide (4H-SiC) Schottky diodes have been performed on structures with various gate metallization, using both current–voltage–temperature (I–V–T) and capacitance–voltage (C–V) measurements. Characteristic features of the Schottky barrier height (SBH) are considered in relation to the specific dose of the carbon- or aluminum-implanted guard ring. Contacts showed excellent Schottky behavior ideality factors between 1.02 and 1.24 in the range of 303–473 K. The measured SBHs were between 0.92 and 1.17 eV in the same temperature range from I–V–T characteristics. The variations in the barrier height, which is significantly temperature- and implantation-dose-dependent, are well fitted to a single Gaussian distribution function. Experimental results agree reasonably well by using this approach, particularly for carbon implantation dose of 1.75 × 1014 cm−2, and a mean barrier height () of 1.22 eV and zero bias standard deviation σ0 = 0.067 V have been obtained. Furthermore, the modified Richardson plot according to the Gaussian distribution model resulted in a mean barrier height () and a Richardson constant (A*) of 1.22 eV and 148 A cm−2 K−2, respectively. The A* value obtained from this plot is in very close agreement with the theoretical value of 146 A cm−2 K−2 for n-type 4H-SiC. Therefore, it has been concluded that the temperature dependence of the forward (I–V) characteristics of the Mo/4H-SiC contacts can be successfully explained on the basis of a thermionic emission conduction mechanism with Guassianly distributed barriers.

Thermal Management versus Full Isolation: Trade Off in Packaging Technologies of Modern SiC Diodes

In this paper we compare the thermal behavior of identical SiC Schottky diodes mounted in i) a standard TO220 package (TO220) with non-isolated backside applying standard soft solder and diffusion solder die attach with ii) a so called FULLPAK TO220 package (TO220FP, only diffusion soldering). Depending on the solder technique the heat transport from the junction area of the SiC Schottky diode to the heat sink or to the package backside is improved for the diodes mounted via diffusion solder. For small chips this holds even for TO220FP in comparison to TO220 with standard solder. Simulations of the vertical temperature distribution after electrically heating with a half sine wave for 10ms up to 190W show a decrease of the maximal junction temperature of the SiC Schottky diode from TJ=260 °C to TJ=180 °C if the diffusion solder is used independent from the package type.

Low Switching Energy 1200V Normally-Off SiC VJFET Power Modules

This work presents the progress in developing an all SiC based power module for use in high frequency and high efficiency applications. Using parallel combinations of 1200V enhancement mode SiC VJFETs (36mm2) and Schottky diodes (23mm2), a total on-resistance of only 10mOhm (2.7m-cm2) was achieved at ID=100A in a commercially available standard module configured as a half-bridge circuit. Careful attention to module layout, gate driver design, and the addition of optimized snubbers resulted in excellent switching waveforms with low total switching losses of 1.25mJ when switching 100A at 150oC.

Thermal stability of SiC Schottky diode anode and cathode metalisations after 1000 h at 350 °C

The thermal stability of two commercially available silicon carbide Schottky diode types has been evaluated following a 1000 h non-biased storage test under vacuum at 350 °C. The Ti-based Schottky (Anode) contact shows excellent stability over the duration of the test with less than 5% change in either extracted Schottky barrier height or ideality values. The Al die attach metalisation on the anode also shows no evidence of degradation after the test. However, a considerable change in series resistance was observed for both diode types, with up to a factor of 100 measured for one of the diodes. The primary early failure mode is related to degradation of the NiAg Ohmic (cathode) die attach metalisation. Demixing of the NiAg alloy, leading to Ag agglomeration is proposed to be the underlying degradation mechanism involved resulting in delamination of the die attach metalisation and the corresponding series resistance increase.

DC characteristics of the SiC Schottky diodes

The isothermal and non-isothermal characteristics of silicon carbide Schottky diodes in the wide range of currents and ambient temperatures are investigated in this paper. The measurements of the diodes characteristics have been performed with the use of a pulse method, with fast registration of measurement points after the diode current turning on, or with the use of a fully static method, in which the self-heating phenomenon is taken into account. Apart from the measurements, the series of numerical experiments, giving the isothermal and non-isothermal characteristics as a result, were executed. The complex, accurate numerical procedures as well as simplified analytical calculations were implemented. A good conformity of all calculation and measurement results have been obtained. In the presented investigations, for relatively high currents and ambient temperatures, the influence of self-heating on the SiC Schottky diodes static characteristics is significant. The large (even 4 V for the ambient temperature 300◦C ) values of voltages corresponding to the nominal diode currents have been observed.

SiC X-ray detectors for harsh environments

We have characterised a number of SiC Schottky Diodes as soft X-ray photon counting detectors over the temperature range -30°C to +80°C. We present the spectroscopic performance, as measured over the energy range ∼ 6 keV–30 keV and correlate the data with measurements of the temperature dependence of the device leakage current. The results show that these detectors can be used for X-ray photon counting spectroscopy over a wide temperature range. Measurement of the radiation tolerance of Semi Transparent SiC Schottky Diodes (STSSD) has shown that these devices can still operate as photon counting X-ray spectrometers after proton irradiation (total dose of 1013 cm−2, 50 MeV). We present measurements on proton irradiated STSSDs that indicate that radiation induced traps, located in the upper half of the bandgap, have reduced the mobility and concentration of charge carriers. X-ray spectra predicted using a Monte Carlo model for SiC diodes are compared with measured spectra.

Investigation of barrier inhomogeneities in Mo/4H–SiC Schottky diodes

Using current–voltage measurements, we have investigated the electrical behavior of molybdenum on 4H–SiC Schottky diodes of various areas and having different edge terminations consisting of high resistivity guard rings manufactured by carbon ion-implantation. Both forward and reverse electrical characteristics of Schottky contacts indicated a presence of inhomogeneities. The forward I–V characteristics have been primarily analyzed within the framework of a standard thermionic emission theory. Schottky-barrier heights and ideality factors are found to appreciably vary from diode to diode. A more general model which takes into account the inhomogeneity of the Schottky barrier has been then used to extract the parameters pertinent to the barrier height distribution. The description of the experimental results using Tung’s model allowed us to determine the value of the average laterally homogeneous SBH barrier height between 1.2 and 1.39 eV for Mo/4H–SiC Schottky diodes. The patch’s properties (the number of patches, the patch strength and the local series resistance) were also obtained from the fit to the experimental I–V characteristics of the current through “patchy” diodes. The obtained results are best described with this extended “pinch off” model. With respect to the reverse characteristics, the remarked absence of a non-saturating behavior as a function of bias in the experimental reverse-bias branch may well be attributed to the presence of defects and/or inhomogeneous Schottky barrier heights, associated with the non-ideal contacts.

Electrical and optical characteristics of Au/PbS/n-6H–SiC structures prepared by electrodeposition of PbS thin film on n-type 6H–SiC substrate

To realize Schottky barrier height (SBH) modification in the Au/n-6H–SiC Schottky diodes, lead sulfide (PbS) thin films were grown on n-6H–SiC by electrodeposition method. At first, XRD experiments were performed to investigate the crystal structure of the PbS film electrodeposited on n-6H–SiC. It has been deduced from the diffraction profile that the PbS thin film has a crystal structure more strongly oriented along the [2 0 0] direction. An optical energy band gap value of 1.42 eV for the PbS film was obtained from its optical absorption spectra. Then, we have prepared Au/PbS/n-6H–SiC Schottky barrier diodes (SBDs) with interface layer and reference Au/n-6H–SiC/Ni SBDs. The SBH enhancement has been succeeded by the PbS interlayer, influencing the space charge region of the SiC. The SBH values of 1.03 and 0.97 eV for the samples with and without the interfacial PbS layer were obtained from the forward bias current–voltage ( I– V) characteristics. The SBH increase in the Au/PbS/n-6H–SiC SBD with the interfacial PbS layer has been attributed to the fact that the interface states contain a net negative interface charge in metal/n-type semiconductor contact due to the presence of the interfacial PbS layer.

Analysis of Displacement Damage Dose and Low Annealing Temperatures on the I-V Characteristics of SiC Schottky Diodes Using ANOVA Method

Silicon carbide (SiC) is a promising semiconductor material for use in solid-state radiation detectors. SiC’s wide bandgap makes it an appropriate semiconductor for high-temperature applications. Because of the annealing process that occurs at temperatures above 150°C for SiC, SiC semiconductors may function in a radiation environment for longer periods of time at elevated temperatures than at room temperature. Unlike thermal annealing effects that can act to improve the electrical characteristics of SiC, fast neutrons create displacement damage defects in SiC Schottky diodes through scattering and thus rapidly degrade the electrical properties of the SiC diodes.We irradiated SiC Schottky diodes at the Ohio State University Research Reactor at room temperature with neutrons for displacement damage doses (Dd’s) ranging from 7.6 × 1010 to 3.8 × 1011 MeV/g. After irradiation, we annealed the diodes, at either 175 or 300°C. We measured the SiC diodes’ forward bias resistances at different steps of the experiments. To perform the experiments and study the results meaningfully, we performed a full factorial design of experiments with two factors: Dd and annealing temperature. The Dd factor had five levels of treatment, and the temperature had three levels of treatment. We did one-way and two-way analysis of variance to understand which factor is more dominant and whether or not the interaction effects are significant. It was determined that for Dd up to 2.3 × 1011 MeV/g the fractional damage recovery decreases with increasing Dd, but that Dd is not a significant factor affecting further changes in damage recovery for Dd’s ranging from 2.3 × 1011 to 3.8 × 1011 MeV/g when the annealing temperature varies between 175 and 300°C. For high Dd (greater than 2.3 × 1011 MeV/g) neutron irradiations, the annealing temperature significantly affects the damage recovery.

A High-Power-Density Converter

This article presents the development and experimental performance of a 10-kW high-power-density three-phase ac–dc–ac converter. The converter consists of a Vienna-type rectifier front end and a two-level voltage source inverter (VSI). To reduce the switching loss and achieve a high operating junction temperature, the SiC JFET and SiC Schottky diode are used. Design considerations for the phase-leg units, gate drivers, integrated input filter—combining electromagnetic interference (EMI) and boost inductor stages—and the system protection are described in full detail. Experiments are carried out under different operating conditions, and the results obtained verify the performance and feasibility of the proposed converter system.

Schottky barrier height modification in Au/ n-type 6H–SiC structures by PbS interfacial layer

We have prepared the Au/PbS/ n-6H–SiC Schottky diodes with interface layer and the reference Au/ n-6H–SiC/Ni Schottky diodes without interface layer to realize Schottky barrier height (SBH) modification in the Au/SiC Schottky diodes. The BH reduction has been succeeded by the PbS interlayer to modify the effective BH by influencing the space charge region of the SiC. The PbS thin layer on the SiC was formed by the vacuum evaporation. The SBH values of 0.97 and 0.89 eV for the samples with and without the interfacial PbS layer were obtained from the forward bias current–voltage ( I– V) characteristics. X-ray diffraction (XRD) study was carried out to determine the structural formation of the PbS on SiC. The reduction of the BH in the Au/PbS/ n-6H–SiC Schottky diodes has been attributed to the fact that the interface states have a net positive interface charge in metal/ n-type semiconductor contact, and thus the positive space charge Q sc in the Au/PbS/ n-6H–SiC Schottky diodes becomes smaller than if the interface state charges Q ss were absent. The experimental carrier concentration value of 4.73 × 10 17 cm −3 obtained from the forward and reverse bias capacitance–voltage characteristics for the Au/PbS/ n-6H–SiC contacts is lower than the value of 5.52 × 10 17 cm −3 obtained for the reference diode, and this is an evidence of the reduction of the BH by the modification of the space charge density of the SiC.

Electrical characterization of Au/3C-SiC/ n-Si/Al Schottky junction

High temperature silicon carbide Schottky diode was fabricated with Au deposited on poly 3C-SiC thin film grown on n-type Si(1 0 0) using atmospheric pressure chemical vapor deposition. The charge transport mechanism of the diode was studied in the temperature range of 300–550 K. The forward and reverse bias currents of the diode increase strongly with temperature and the diode shows a non-ideal behavior due to the series resistance and the interface states associated with 3C-SiC. The charge transport mechanism is a temperature activated process, in which, the electrons passes over of the low barriers and in turn, the diode has a large ideality factor. The electrical parameters of the Au/3C-SiC/ n-Si/Al Schottky diode were explained on the basis of the thermionic emission theory with double Gaussian distribution of the barrier heights due to the barrier height inhomogeneities at metal/semiconductor interface. The interface state density of the diode was determined by the conductance–frequency method and it was of order of 9.18 × 10 10 eV −1 cm −2.

A Novel High-Temperature Planar Package for SiC Multichip Phase-Leg Power Module

This paper presents the design, development, and testing of a phase-leg power module packaged by a novel planar packaging technique for high-temperature (250°C) operation. The nanosilver paste is chosen as the die-attach material as well as playing the key functions of electrically connecting the devices' pads. The electrical characteristics of the SiC-based power semiconductors, SiC JFETs, and SiC Schottky diodes have been measured and compared before and after packaging. No significant changes (<;5%) are found in the characteristics of all the devices. Prototype module is fabricated and operated up to 400 V, 1.4 kW at junction temperature of 250°C in the continuous power test. Thermomechanical robustness has also been investigated by passive thermal cycling of the module from -55°C to 250°C. Electrical and mechanical performances of the packaged module are characterized and considered to be reliable for at least 200 cycles.

Minority carrier injection limited current in Re/4H‐SiC Schottky diodes

AbstractCurrent–voltage–temperature, capacitance–voltage, and Fourier transform‐deep level transient spectroscopy (FT‐DLTS) measurements have been employed to gain insights into the conduction mechanism in Re/4H n‐silicon carbide (SiC) Schottky diodes. A power law dependence of current on voltage has been observed in the Re/4H‐SiC Schottky diodes at large forward bias. DLTS studies of the diodes reveal the presence of a deep trap at an energy $E_{\rm c} - E_{\rm t} = 0.6\,{\rm eV}$. The electron trap center is tentatively associated with the prominent Z1 defect level, and the obtained trap density and capture cross‐section values correlate well with available literature values. Excess capacitance has been observed under forward bias suggesting that minority carrier injection is the mechanism responsible for the observed characteristics.

A New Generation of SiC Schottky Diodes with Improved Thermal Management and Reduced Capacitive Losses

With the help of an improved die attach the Rth,jc of SiC Schottky diodes can be reduced by 40-50% at a given chip size. This enables a significant higher power density for these SiC diode chips, resulting in a chip shrink of ~ 35% for a given nominal current. This has a significant impact not only on the cost position of the device but also on the switching performance of the diodes, as their capacitive charge directly scales with the chip area. Of course these advantages are accompanied by a small penalty in static losses as the Vf of the diodes at nominal current also slightly increases by the chip shrink. However, the reduction of switching losses dominates upon the marginally increased static losses besides full load operation conditions (which are pretty exceptional in today’s SiC Schottky diode applications) combined with frequencies below 130 kHz. This allows a better competitive positioning against fast Si-based diodes and improved system efficiency at the same time.

Demonstration of High Temperature Bandgap Voltage Reference Feasibility on SiC Material

This work demonstrates that a stable voltage reference with temperature, in the 25°C-300°C range is possible with SiC. This paper reports the simulated and experimental results as well and a practical and simplified vision on the principles of thermally compensated voltage reference circuits, usually named bandgap references. For our demonstration, we have used SiC Schottky diodes. The influence of the barrier height and the ideality factor on the voltage reference and a comparison between simulated and experimental results are also presented.

SiC JFETs for Power Module Applications

Starting with the production of Infineon´s first silicon carbide (SiC) Schottky diodes in 2001, a lot of progress was achieved during recent years. Currently, a 3rd generation of MPS (merged pn Schottky) diodes is commercially available combining tremendous improvements with respect to surge current capability and reduced thermal resistance. In this work we present the implementation of SiC switches in power modules and a comparison of these units with the corresponding Si-based power modules. Also the frequency dependence of the total losses of the 1200V configurations using Si-IGBTs or SiC-JFETs as active device is shown, indicating that modules solution with a state of the art SiC JFET outperforms all other options for switching frequencies of 20 kHz and beyond. Additionally a total loss vs. frequency study will be presented. Furthermore, it is show that the switching losses of JFET based modules can be further reduced by reducing the internal distributed gate resistivity.

Investigations on Surge Current Capability of SiC Schottky Diodes by Implementation of New Pad Metallizations

In this paper we describe how a merged pn Schottky diode (MPS diode) is capable to drive surge current levels far beyond the normal current of the diode and how to improve the device in order to achieve even higher surge current levels. For a sine half wave of 10 µs an 8A MPS diode (size: 2.52mm2) with conventional Al pad metallization shows surge current levels of greater than 500A, using Cu it can be increased to ~900A. For 10ms pulse length a different behaviour was observed, here diodes with Al pad metallization show a higher surge current level (80A) compared to Cu pad metallization (~ 40A). The root cause for this negative result at longer pulse time is based in a chemical interaction between Cu and the Schottky metal (Ti). Additionally, an outlook is given how Cu can contribute to improved surge current capability also at longer pulse lengths.

Germanium – Silicon Carbide Heterojunction Diodes – A Study in Device Characteristics with Increasing Layer Thickness and Deposition Temperature

SiC schottky diodes take advantage of the material's superior reverse breakdown voltage when compared to Silicon (Si) [1]. However, when considered for MOSFET applications, the high concentration of interface traps at the SiC/SiO2 interface reduce the material's already low channel mobility [2]. Therefore, a Ge/SiC heterojunction solution becomes an attractive prospect, whereby the Ge forms the control region after being epitaxially grown on the SiC. With a well established Ge-High K dielectric technology [3], a carbon-free oxide would exist, leaving a channel-region with a mobility approximately four times that of SiC.