Papers by Author: Pavel Hazdra

Abstract: Compact simulation models of two key silicon carbide power components, the Junction Barrier Schottky diode and the power MOSFET, which are taking into account the effect of irradiation by highenergy electrons, were developed. Two 1.7 kV class devices: the 14 A JBS diode C3D10170H and the 5 A SiC power MOSFETs C2M1000170D produced by Wolfspeed were irradiated by 4.5 MeV electrons in the dose range up to 2000 kGy. Electrical characteristics were measured prior to and after irradiation. Radiation defects were studied by deep level transient spectroscopy and the effect of irradiation on device characteristics was established. SPICE models taking into account the irradiation fluence were proposed and calibrated using the parameters extracted from experiment. Simulated characteristics show a very good agreement with reality.

Abstract: 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 Z_1/2 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 Z_1/2 defect.

Abstract: 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.

Abstract: In this contribution, we report on the electrical characterization of point defects in 4H-SiC p⁺in diodes. Ten electrically active levels have been detected in the base region of the devices, by employing Deep Level Transient Spectroscopy (DLTS) and Minority Carrier Transient Spectroscopy (MCTS). Of these ten levels, six are majority carrier traps, in the 0.1-1.7 eV energy range below the conduction band edge, and four were minority carrier traps located in the 0.13-0.4 eV energy range above the valence band edge. We found that, during DLTS measurements, both majority and minority carrier traps can be detected and we explain this by considering the behavior of the quasi-Fermi levels. At last, we studied the impact of proton irradiation on the minority charge carrier lifetime.

Abstract: 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).

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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).

Abstract: The effect of radiation damage produced by fast neutrons on characteristics of JBS diodes produced on 4HSiC epilayers was investigated. 1200V JBS diodes from Cree were irradiated in nuclear reactor by fast neutrons with fluences ranging from 1.3x10¹³ to 4x10¹⁴ cm^-2 (1MeV NIEL equivalent in Si). Evolution of radiation damage was studied by deep level transient spectroscopy. New characterization method based on dynamic measurement of current to voltage characteristics in the kV range then allowed precise analysis of blocking characteristics and observation of free charge carrier removal with increasing neutron fluence. Results show that irradiation with fast neutrons introduces different point defect giving rise to acceptor like deep levels in SiC bandgap. These levels have a negligible effect on dynamic and blocking characteristics of irradiated JBS diodes, however, acceptor character of introduced deep levels accompanied by deactivation of donor dopants deteriorates diodes ON-state resistance already at low irradiation fluences. This degradation is then manifested by increasing values of the series resistance and the emission coefficient in the SPICE model of the JBS diode.