V. Záhlava

Open circuit voltage decay lifetime of ion irradiated devices

Open circuit voltage decay method for measuring of excess carrier lifetime is shown to be effective for in-process checking of ion irradiated power diodes. 2.5 kV/100 A P-i-N diodes irradiated by helium ions with different irradiation energies and doses were used for presentation of capabilities of this method. Differences in carrier dynamics during the OCVD process between unirradiated and irradiated devices were studied by use of the device simulation in ATLAS.

Helium irradiated high-power P–i–N diode with low ON-state voltage drop

Abstract The application of a 300 nm thick platinum silicide (PtSi) layer at the place of the anode contact layer of a soft recovery 2.5 kV/100 A high-power P–i–N diode brought a reduction of the forward voltage drop at several tens percent (for the rating current of 100 A) compared to that of the conventional aluminum and Ti–Ni–Ag layers. This enabled us to greatly improve the trade-off curve between the ON-state and turn-OFF losses of the diode subjected to helium irradiation into the anode and anode junction region. The application of PtSi layers thus opens a new way for the improvement of power devices.

Impact of the electron, proton and helium irradiation on the forward I–V characteristics of high-power P–i–N diode

Abstract 2.5 kV/100 A high-power P–i–N diode was electron, proton and helium irradiated in a wide range of irradiation doses with irradiation energies in the MeV range. The resulting forward I – V curves were registered in the temperature range 30–125 °C to investigate the magnitude of the crossing point current of the I – V curves–– I XING . I XING was found to decrease with increasing irradiation dose and to disappear at high doses for all three irradiation treatments with exception of ion irradiated diodes with defect peaks placed deeply into the anode region. Using a simple model based on the thermal and injection dependence of the carrier lifetime, the explanation of this effect is presented with the support of the non-isothermal 2-D device simulation of helium irradiated devices.

Radiation Defects Produced in 4H-SiC Epilayers by Proton and Alpha-Particle Irradiation

Electronic properties of radiation damage produced in 4H-SiC epilayer by proton and alpha particle irradiation were investigated and compared. 4H-SiC epilayers, which formed the low doped n-base of Schottky barrier power diodes, were irradiated to identical depth with 550 keV protons and 1.9 MeV alphas. Radiation defects were then characterized by capacitance deep-level transient spectroscopy and C-V measurements. Results show that both projectiles produce identical, strongly localized damage peaking at ion’s projected range. Radiation defects have a negligible effect on dynamic characteristic of irradiated 4H-SiC Schottky diodes, however acceptor character of introduced deep levels and their high introduction rates deteriorate diode’s ON-state resistance already at very low irradiation fluences.

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.

High-Power Silicon p-i-n Diode With the Radiation Enhanced Diffusion of Gold

Fast recovery p-i-n diode with anode p-n junction modified by the radiation-enhanced diffusion (RED) of gold is presented. The RED of gold is shown to provide the local lifetime control of excess carriers, the compensation of n-base doping profile from n-type to p-type, and the enhancement of concentration of two gold-related deep levels. The deep level Au-/0(EC-0.549 eV) controls the low-level lifetime, whereas the gold-hydrogen pair (EC-0.215 eV) the high-level lifetime. This feature eliminates the drawback of negative temperature coefficient of forward voltage drop of the RED with palladium and platinum, where only a single deep level, which controls the high-level lifetime, is enhanced. The RED of gold provides the maximal reverse bias safe operation area at the annealing temperature of 600°C, whereas the RED of palladium at 650°C.

Point Defects in 4H-SiC Epilayers Introduced by 4.5 MeV Electron Irradiation and their Effect on Power JBS SiC Diode Characteristics

Electronic properties of radiation damage produced in 4H-SiC by electron irradiation and its effect on electrical parameters of Junction Barrier Schottky (JBS) diodes were investigated. 4H‑SiC N‑epilayers, which formed the low‑doped N-base of JBS power diodes, were irradiated with 4.5 MeV electrons with fluences ranging from 1.5x1014 to 5x1015 cm-2. Radiation defects were then characterized by capacitance deep-level transient spectroscopy and C-V measurement. Results show that electron irradiation introduces two defect centers giving rise to acceptor levels at EC‑0.39 and EC‑0.60 eV. Introduction rate of these centers is 0.24 and 0.65 cm‑1, respectively. These radiation defects have a negligible effect on blocking and dynamic characteristics of irradiated diodes, however, the acceptor character of introduced deep levels and their high introduction rates deteriorate diode’s ON-state resistance already at fluences higher than 1x1015 cm‑2.

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).

Operation of 4H-SiC high voltage normally-OFF V-JFET in radiation hard conditions: Simulations and experiment

Abstract Operation of high-voltage 4H-SiC vertical-JFET in radiation hard environment was investigated by simulation and experiment. Commercial 1700 V normally-OFF SiC JFETs in TO-247 package were irradiated with fast neutrons to fluences of 4.0 × 1014 cm− 2 (1 MeV Si equivalent) and the effect of radiation on their characteristics was then thoroughly analyzed. Four degradation mechanisms were identified, of which the most important is the increase of JFETs ON-state resistance due to the mobility degradation and removal of carriers from transistors light doped channel and drift regions. As a result, the JFET ON-state losses grow and, at fluences higher than 4 × 1014 cm− 2, the low doped n-regions are fully compensated and transistor loses its functionality. On the contrary, irradiation slightly improves JFETs switching characteristics. The effect of neutron irradiation on operation of SiC V-JFET in a real application was then investigated on the step-UP 15 V/60 V DC-DC converter where the SiC JFET was used as an active switch. Converter characteristics were analyzed by means of the mixed-mode simulation using the developed 2D model of the neutron irradiated transistor. Results showed that the duty cycle of the PWM regulator is growing due to the increase in the voltage drop on the switching JFET. This effect, which is caused by the abovementioned increase the JFETs ON-state resistance, increases power dissipation and deteriorates converter efficiency. Finally, the effect of neutron irradiation on operation SiC V-JFET in the 850 V/24 V auxiliary flyback switching mode power supply was analyzed. We showed that the growth of the ON-state resistance increases transistors conduction losses and decreases converter efficiency. Exceeding the fluence of 3.3 × 1014 cm− 2 neutrons then causes JFET overheating and subsequent destruction.