Hans Peter Felsl

Ruggedness analysis of 3.3kV 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.

Different types of avalanche-induced moving current filaments under the influence of doping inhomogeneities

The influence of small doping inhomogeneities on the behavior of current filaments appearing in p^+-n^--n^+ diodes during the reverse-recovery period under extreme turn-off conditions is investigated. It is shown that depending on the strength and the distance between the inhomogeneities, different types of filament dynamics may appear, resulting in irregular or regular hopping or a continuously traveling filament. The hopping filament dynamics is essentially controlled by the long-range inhibitory effect of local plasma extraction, while the traveling filament is dominated by the mutual interaction of charge carrier generation and plasma extraction in the region between two neighboring inhomogeneities. Electrothermal simulations have shown that local heating induced by a traveling filament may result in the generation of a non-moving thermal-induced filament which can lead to destructive thermal runaway. The onset of thermal runaway is extremely sensitive to the thermal resistance of the contacts.

Comparison of critical current filaments in IGBT short circuit and during diode turn-off

Device simulations of two-dimensional structures with a huge lateral extension indicate that the combination of lateral current flow and a vertical reduction of space-charge region are necessary for the filament formation during the short-circuit operation of IGBTs and the turn-off of diodes. The shape of the electric-field distribution is introduced as new criterion to estimate the rising branch of the short-circuit destruction curve in the Ic-Vce SOA-diagram. The impact of the stray inductance Lstray and the gate resistor RG on the short-circuit destruction limit are experimentally investigated. The presented results allow a deeper understanding of the different electrical short-circuit failure mechanisms (turn-on, quasi-stationary state, turn-off) and their relation to one another.

A diode structure with anode side buried p doped layers for damping of dynamic avalanche

In this paper we present a novel 3.3 kV diode concept with anode side buried p doped layers. These p doped layers could retard the arising of the electric field and consequently postpone the onset of the dynamic avalanche at the pn-junction. Isothermal numerical simulation results suggested that this novel structure could permit balanced improvement of the static as well as the dynamic behaviors.

Optimization of the selenium field-stop profile with respect to softness and robustness

Tailoring the field-stop layer is an effective measure to increase the switching softness and to improve the avalanche robustness of high-voltage power diodes. Diodes and IGBTs with deeper field-stop layers having a superior switching behavior can be created with a relatively low temperature budget by using selenium as field-stop dopant. In this work, we show that a further improvement of the diode performance can be achieved by using a selenium double-field-stop layer. Providing a CIBH-diode with such a selenium double field-stop layer results in a very soft and robust diode. The characteristic of the selenium traps (donor levels and capture cross sections) are determined by DLTS and by frequency resolved admittance spectroscopy measurements. Simulations of the reverse recovery behavior using these trap properties show good agreement with experimental results and are essential for finding further optimized field stop profiles.

Switching ruggedness and surge-current capability of diodes using the self-adjusting p emitter efficiency diode concept

The surge-current ruggedness of free-wheeling diodes can be improved by implementing the self-adjusting p emitter efficiency diode concept (SPEED). Simulations indicate that the switching ruggedness is reduced because of the occurrence of cathode-side filaments during reverse-recovery. Experiments confirm the weak switching performance of such a diode in comparison to a conventional diode. By implementing the controlled injection of backside holes concept cathode-side filaments can be suppressed. However, this measure is not sufficient to regain the switching ruggedness of a conventional diode. It is also necessary to fully embed the p + -areas of the SPEED anode in the low-doped p-type area to avoid high electrical field strengths at the p + p-junction and pinning of anode-side filaments. However, anode-side adjustments for improving the switching ruggedness can reduce the benefit of the SPEED concept regarding the surge-current capability.

Measurements and simulations of the turn-off behaviour of diodes with deep energy levels of Se implanted in Si

The switching softness of high-voltage power diodes is significantly influenced by the design of the field-stop layer and its dopant atoms. The influence of the energy levels of selenium - used as donor in the field-stop layer - on the turn-off behaviour of CIBH diodes has been investigated by DLTS-measurements and device simulations.

Thyristors and IGBTs with integrated self-protection functions

Recent activities in insulated gate bipolar transistor (IGBT) and thyristor development focus on the integration of protection functions in order to improve the reliability of the entire electronic system. It is shown how various protection functions can be integrated into symmetric and asymmetric light-triggered thyristors with a blocking capability up to 13 kV. Furthermore, different measures to provide IGBTs with an overvoltage protection are discussed and experimental results revealing the successful implementation of such a protection function are presented.

Investigation of deep levels in SiC-Schottky diodes with frequency resolved admittance spectroscopy

We show that the technique of Frequency Resolved Admittance Spectroscopy (FRAS) allows the identification of deep traps with reduced measurement effort compared to Deep Level Transient Spectroscopy (DLTS). This is shown by the analysis of radiation defects introduced in n-type 4H-SiC 1700V Schottky diodes.