Stanislav Popelka

The Effect of Light Ion Irradiation on 4H-SiC MPS Power Diode Characteristics: Experiment and Simulation

In this article, the effect of local radiation damage on the electrical characteristics of 1700 V 4H-SiC Merged-Pin Schottky (MPS) diode have been investigated. Radiation defects introduced by irradiation with 670 keV protons were placed into the low-doped n-type epi-layer and their influence on diode characteristics were characterized by capacitance DLTS, C-V profiling and I-V measurements. Simulation model accounting for the effect of proton irradiation was developed, calibrated and used for analysis of underlying effects and temperature dependencies. Results show that the forward voltage drop and breakdown voltage is insensitive to the position of the damage region when the defect peak is placed far away from the Schottky metal contact of the MPS diode. However, when the damage region approaches to the p+ regions, forward voltage drop degrades significantly. For fluences higher than 3.3×1010 cm - 2, the acceptor concentration in the peak region achieves donor doping level of the epi-layer and a sharp increase in the diode forward voltage drop is observed. Acceptor centers introduced by proton irradiation also slightly increase the breakdown voltage while decreasing the leakage current at voltages close to the MPS diode breakdown ( > 2000 V).

Impact of Electron Irradiation on the ON-State Characteristics of a 4H–SiC JBS Diode

The ON-state characteristics of a 1.7-kV 4H-SiC junction barrier Schottky diode were studied after 4.5-MeV electron irradiation. Irradiation doses were chosen to cause a light, strong, and full doping compensation of an epitaxial layer. The diodes were characterized using Deep Level Transient Spectroscopy, C-V (T), and I-V measurements without postirradiation annealing. The calibration of model parameters of a device simulator, which reflects the unique defect structure caused by the electron irradiation, was verified up to 2000 kGy. The quantitative agreement between simulation and measurement requires: 1) the Shockley-Read-Hall model with at least two deep levels on the contrary to ion irradiation and 2) a new model for enhanced mobility degradation due to radiation defects. The diode performance at high electron fluences is shown to be limited by the doping compensation at the epitaxial layer.

Effect of Neutron Irradiation on High Voltage 4H-SiC Vertical JFET Characteristics: Characterization and Modeling

The effect of neutron irradiation on commercial vertical high voltage normally-OFF SiC power N-JFETs was investigated. JFETs were irradiated with 1 MeV neutron equivalent fluences up to 4×1014 cm-2. Measurement showed that fast neutrons introduce deep levels acting mostly as deep acceptor centers. These centers gradually compensate lightly doped channel and drift regions of JFETs. As a result, characteristics are deteriorated, the JFET threshold voltage gradually increases and transconductance is lowered. At fluences higher than 4×1014 cm-2, the low doped n-regions are fully compensated and transistor loses its functionality. The 2D physical model of JFET in ATLAS simulator was developed and calibrated including the neutron irradiation effects. Simulation showed a good agreement with experimental data. This confirmed that carrier removal in the channel and drift region by acceptors centers introduced by neutrons is a dominant reason of SiC JFET degradation.

Lifetime control in SiC PiN diodes using radiation defects

Application of radiation defects for lifetime control in contemporary SiC PiN diodes was investigated using the calibrated device simulator ATLAS from Silvaco, Inc. Recombination models accounting for the effect of deep levels introduced by the irradiation were set according to experimental results obtained by C-V and DLTS measurements performed on low-doped n-type SiC epilayers irradiated with 4.5 MeV electrons and 670 keV protons. Global (4.5 MeV electron irradiation) and local (700 keV proton irradiation) lifetime reduction was then applied on the 2A/10kV SiC PiN diode and the ON-state and reverse recovery characteristics were simulated and compared. Results show that the proton irradiation can substantially improve the trade‑off between the diode ON‑state and turn‑OFF losses. Compared to the electron irradiation, the local lifetime killing by protons allows achieving better trade-off and softer recovery curves.

The Effect of Proton and Carbon Irradiation on 4H-SiC 1700V MPS Diode Characteristics

Electronic properties of radiation damage produced in 1700 V 4H-SiC MPS diodes by proton and carbon irradiation were investigated and compared. 4H-SiC epilayers, which formed the lowdoped N-base of MPS power diodes, were irradiated to identical depth with 670 keV protons and 9.6 MeV C4+ ions. Results show that irradiation with both projectiles produces strongly localized damage (deep levels) peaking at ion’s projected range. Compared to protons, heavier carbon ions introduce more defects with deeper levels in the SiC bandgap and more stable damage. Radiation damage act as electron traps and compensates donor doping of the epilayer and decreases electron mobility. Forward voltage drop of irradiated diodes then sharply increases when the peak concentration of introduced acceptor levels donor doping. The effect of both the proton and carbon irradiation can be simulated using a simple model accounting only for one dominant electron trap.

Optimization of 1700V 4H-SiC JBS Diode Parameters

The in-depth design optimization of the active layer of the 1700V class 4H-SiC JBS/MPS diode structure is discussed. The important design parameters such as junction depth (d), width (w) of p+ areas, and spacing (s) between them were optimized for the best possible trade-off between the unipolar ON-state voltage drop, the OFF-state breakdown voltage, and the bipolar surge current capability. The optimization was performed using a state-of-the-art simulator using device models calibrated on a commercially available JBS rectifier. The results show that the spacing s between the p+ regions is the most decisive parameter which has to be properly designed according to the required voltage class. For the 1700 V voltage class, s should be between 2 to 4 μm and the s/w ratio should be kept low. The depth d of the p+ pattern has a pronounced impact on the ignition of bipolar action such that with decreasing d the surge current capability decreases significantly.

Effect of Electron Irradiation on 1700V 4H-SiC MOSFET Characteristics

The effect of 4.5 MeV electron irradiation on static characteristics of commercially available 5 A/1700 V SiC power MOSFETs is investigated. Results show that in the low dose range (up to 20 kGy) the threshold voltage decreases rapidly with irradiation dose but devices keep full functionality. This effect is caused by embedding of the positive charge into the gate oxide. When electron dose reaches 200 kGy, the threshold voltage moves back close to its original value, however, the ON‑state resistivity increases and transconductance is lowered. This is caused by introduction of deep acceptor centers into the low doped drift region of MOSFET. This effect can be considered as a cause of the final failure of the device (the lost of the ON-state capability).

Radiation Damage in 4H-SiC and its Effect on Power Device Characteristics

The effect of neutron, electron and ion irradiation on electrical characteristics of unipolar 1700V SiC power devices (JBS diodes, JFETs and MESFETs) was investigated. DLTS investigation showed that above mentioned projectiles introduce similar deep acceptor levels (electron traps) in the SiC bandgap which compensate nitrogen shallow donors and cause majority carrier (electron) removal. The key degradation effect occurring in irradiated devices is the increase of the ON-state resistance which is caused by compensation of the low doped n-type epilayer and simultaneous lowering of electron mobility. In the case of SiC power switches (JFET, MOSFET), these effects are accompanied by the shift of the threshold voltage. Radiation defects introduced in SiC power devices is unstable and some defects anneal out already at operation temperatures (below 175°C). However, this does not have significant effect on device characteristics.

The Influence of Neutron Irradiation on Electrical Characteristics of 4H-SiC Power Devices

Commercial 1200V and 1700V MPS diodes and 1700V vertical JFETs produced on 4H-SiC n-type epilayers were neutron irradiated with fluences up to 4x1014 cm-2 (1 MeV neutron equivalent Si). Radiation defects and their effect on carrier removal were investigated by capacitance deep-level transient spectroscopy, I-V and C-V measurement. Results show that neutron irradiation introduces different point defects giving rise to deep acceptor levels which compensate nitrogen doping of the epilayer. The carrier removal rate increases linearly with nitrogen doping. Introduced defects deteriorate ON-state characteristics of irradiated devices while their effect on blocking characteristics is negligible. The effect of neutron irradiation can be simulated by TCAD tools using a simple model accounting for introduction of one dominant deep level (Z1/Z2 centre).

Local Lifetime Control in 4H-SiC by Proton Irradiation

The effect of local lifetime control by proton irradiation on the OCVD response of a 10 kV SiC PiN diode was investigated. Carrier lifetime was reduced locally by irradiation with 800 keV protons at fluences up to 1x1011 cm-2. Radiation defects were characterized by DLTS and C-V profiling; excess carrier dynamics were measured by the OCVD and analyzed using the calibrated device simulator ATLAS from Silvaco, Inc. Results show that proton implantation followed by low temperature annealing can be used for controllable local lifetime reduction in SiC devices. The dominant recombination centre is the Z1/2 defect, whose distribution can be set by irradiation energy and fluence. The local lifetime reduction, which improves diode recovery, can be monitored by OCVD response and simulated using the SRH model accounting for the Z1/2 defect.