G. Gradinaru

Prebreakdown and breakdown phenomena in high‐field semiconductor‐dielectric systems

The complexity of the semiconductor‐dielectric system behavior under high electric fields is discussed. Time dependence of the voltage drop, electrical current, and light emissions, and the strong influence of the nature and quality of the surrounding dielectric, as well as the quality of the semiconductor processing and contacts, are analyzed for high‐purity bulk silicon‐dielectric vacuum or gas systems under impulse voltage stress. On the basis of the analysis of the main characteristics of the time response of the system up to breakdown, as well as of the variation of the current components with the applied voltage, a new comprehensive physical model of the prebreakdown and breakdown phenomena in the high‐field semiconductor‐dielectric systems is proposed. The model points out the active behavior of two parts, the semiconductor and the ambient dielectric, and the main role of the semiconductor in the initiation of the breakdown phenomena in the system. Based on a large number of experimental results, t...

The influence of the semiconductor and dielectric properties on surface flashover in silicon-dielectric systems

New experimental results on surface flashover are reported for high field silicon-dielectric systems. Different conditions of the lateral surface, contacts and ambient dielectrics have been studied. The strong influence of the semiconductor quality, and that of the dielectric properties, on the prebreakdown and breakdown response of the system, is demonstrated. All experimental results strongly support the conclusion that surface flashover in silicon systems is a physical process totally different from semiconductor surface breakdown. This conclusion has important practical application in the improvement of the performance of photoconductive power switches, severely limited by premature breakdown effects. >

Electrical properties of high resistivity 6H–SiC under high temperature/high field stress

The influence of ambient temperature and applied electric field on the electrical properties of high resistivity (1–30 kΩ cm), semi-insulating (>100 kΩ cm), and insulating (1011–1012 Ω cm) single-crystal 6H–SiC is reported. Current–voltage (I–V) characteristics of lateral metal-semiconductor-metal test structures were measured in vacuum in a temperature range of 295–730 K and under moderate pulsed electric fields (0.5–80 kV/cm). It is shown that the resistivity of the undoped 6H–SiC varies strongly with the ambient temperature after a temperature/field function dominated by a factor containing the activation (ionization) energy of residual boron of 0.35 eV. The dominant activation energy of semi-insulating Vanadium-compensated material (6H–SiC:V) varies with the ambient temperature, increasing from ∼0 eV at 295–320 K to ∼0.8 eV at T⩾600 K. This result can explain the relatively low decrease of the resistivity of insulating 6H–SiC at very high ambient temperatures and its viability as a substrate for next-...

Prebreakdown and breakdown effects in AlGaN/GaN heterostructure field effect transistors

Prebreakdown and breakdown effects under high electric fields in AlGaN/GaN heterojunction field effect transistors were studied. In the subthreshold regime, at large drain-to-source voltages (>80 V), along with the gate leakage current, a significant source current was measured. For a wide range of gate-to-source bias voltages in OFF and ON regimes, the breakdown voltage was limited by a rapid buildup of the source current, rather than a process dominated by gate leakage.

Surface flashover in silicon-vacuum systems

The properties of the prebreakdown response of silicon-vacuum systems under HV excitation are presented. The most frequent case of system breakdown by surface flashover is treated. The particular properties of the system response in the high-field pulsed regime demonstrate the essential differences between the silicon-vacuum and solid-insulator-vacuum systems. The main ideas of a new physical model of surface flashover in silicon-vacuum systems are presented. The properties of the surface flashover response are discussed in terms of the proposed model. A concept called system surface flashover sensitivity is introduced to provide a better understanding of the surface flashover physical process in silicon-vacuum systems. >

On the negative differential resistance effect in high‐field semiconductor‐dielectric systems

The active role of the dielectric in high‐field semiconductor‐dielectric systems (HFSDS) and the experimental results concerning partial and total surface flashover in HFSDS are presented. The system negative differential resistance effect by partial surface flashover is explained on the basis of a two‐channel model with an appropriate equivalent circuit correlated with the potential distribution along the semiconductor.

Surface filamentation in semi‐insulating silicon

A new field limit of silicon devices stressed at pulsed high fields is proposed, namely, the threshold field for the onset of the phase‐shifted current response of the device. While the classical limit by the breakdown of the semiconductor‐dielectric system, generally by surface flashover, causes a total damage of the semiconductor, the new limit prevents any damage of the device, even at high applied fields. Depending on the overall quality of the device under test, catastrophic failure of the device can be avoided even at large fields of ∼70 kV/cm by not exceeding the threshold for the onset of the phase‐shifted current response, the proposed new field limit. Beyond this limit the device response is unstable and a surface filament, distinctly different from the surface flashover tracks, may appear on the device surface, permanently degrading its quality. Experimental results are presented supporting the new predamage high‐field limitation. Scanning electron microscopy and optical micrographs are present...

High field effects in high resistivity silicon carbide in lateral configurations

Results of the high field performance of single‐crystal bulk 6H–SiC of relatively high resistivity (∼500 Ω cm) are reported. Prebreakdown and breakdown phenomena of SiC at high fields are studied using lateral device geometries, particularly suitable for photoconductive power switches and other high voltage power devices. The influence of the electrode configuration, gap length, sample geometry, and contact technology on the high field responses of SiC is discussed. Ohmic response and relatively large hold‐off fields (≳80 kV/cm) are reported for this material in vacuum and SF6 gas.

IMPROVED VOLTAGE HOLD-OFF CAPABILITY OF PHOTOCONDUCTIVE SILICON IN SF/sub 6/ ENVIRONMENT

A dramatic improvement in the surface breakdown voltage of high purity silicon used for photoconductive switch applications is reported. Experiments on high purity silicon (resistivity > 30 kQ-cm, L = 10 mm, and D = 25 mm ) under SF6 gas at a pressure of one atmosphere, after a special pre-treatment in vacuum, have shown very high stress levels without any total surface flashover or partial voltage collapse effects. Hold-off voltages in excess of about 85 kV/cm with Cr-Cr-Mo-Au contact samples and 40 kV/cm with Au metallized contact samples were obtained consistently. The influence of the quality of the semiconductor (i.e., silicon), in terms of surface finish and contact condition, on the surface breakdown limit under vacuum and SF6 environments, is discussed. Specifically, the role of a special vacuum pre-treatment process as an in-situ cleaning process towards improving voltage hold-off capability is presented.

High field activation of micropipes in high resistivity silicon carbide

Micropipes in high resistivity (p≥5 kΩcm) SiC are highly activated in parallel electric fields (vertical devices) at room temperature starting at very low fields of 5-10 kV/cm, especially in the doped material. No activation of micropipes is observed in high fields (>100 kV/cm) perpendicular to their orientation (lateral devices). In the last case, the high field limitation is due to surface flashover phenomena taking place at 100-175 kV/cm in vacuum ambient and depending strongly on the material growth technology and the gap length. Non-ohmic behavior was not observed in lateral devices up to high applied fields. The high field characterization method is proposed as a powerful tool for the evaluation of the quality of SiC material for next-generation high voltage/high power devices.