W. Gerlach

Blocking capability of planar devices with field limiting rings

Abstract The results of breakdown voltage investigations of planar devices with field limiting rings are described. The two-dimensional Poisson equation is solved using the finite difference method. The question of the optimal ring spacings for devices with more than one field ring and the influence of surface charges on the blocking capability are extensively studied. The influence of the following device parameters is discussed: ring spacing, ring width, doping gradient and surface charge density. An analytical model for simplified calculations showing the principal characteristics of field ring devices is presented. Furthermore some experimental results of EBIC measurements are shown.

On the turn-off behaviour of the NPT-IGBT under clamped inductive loads

Abstract One-dimensional non stationary analytical models are presented describing the turn-off behaviour of the non-punch-through IGBT (NPT-IGBT) driven in a chopper circuit with a clamped inductive load. For this purpose the ambipolar device equations are solved by the integral method. Analytical expressions are derived for the carrier profiles in the n − -base during the extraction phase and the early tail phase. Analytical relations are also given for the time dependent anode voltage and the power losses in the extraction phase and the current decay during the early tail phase. Additionally, the ratio between the electron and the hole current at the p + -emitter during the early tail phase is investigated.

Two-dimensional analytical models of the carrier distribution in the on-state of the IGBT

Abstract Analytical models are presented describing the dependence of the carrier distribution in the conductivity modulated base of NPT-IGBTs on the device parameters. This dependence is shown to be a direct result of the two-dimensional current flow. Special attention is paid to the dependence of the conductivity modulation between the cathode wells on the cell design and the current level. A physical interpretation of this dependence is given taking advantage of conformal mapping. Moreover analytical expressions for the emitter efficiency in NPT-IGBTs are derived.

The influence of surface charge and bevel angle on the blocking behavior of a high-voltage p + -n-n + device

The breakdown voltage of p+-n-n+devices is investigated. The two-dimensional Poisson equation is solved using the finite difference method. The questions of optimal bevelling and the influence of surface charges on the blocking capability are extensively studied. Furthermore, it is investigated to replace a small bevel angle at the n+- region by a mesa-like structure.

The effect of nonlinear coupling of reciprocal mobilities on the charge carrier distribution in semiconductor power devices

Abstract A new differential equation, describing the charge carrier distribution in the weakly doped base of a pin-diode under high injection conditions has been derived. The effect of electron-hole scattering on the mobilities, the ambipolar diffusion coefficient and the resulting carrier profile has been considered in a consistent way. Also discussed is the validity of Einsteins relation in the electron-hole plasma. Moreover, the simulation of the resulting transport equations are compared with measured carrier distributions in different psn-diodes for high current densities (about 1000 A/cm2). The measurements were performed by the analysis of the infrared recombination radiation.

Determination of the carrier lifetime from the open-circuit voltage decay of p-i-n rectifiers at high-injection levels

The method of finite elements has been used to determine the open-circuit voltage of p-i-n rectifiers at current turn-off from very high forward currents taking into account carrier-carder scattering, Auger recombination, and recombination in the end regions. It has been shown that the evaluation of the linear decay of the open-circuit voltage after Davies gives the true value of the carrier lifetime in the i-region only if the recombination in the end regions is negligible. Under the influence of the recombination in the end regions, the open-circuit voltage shows two linear decay ranges. The Davies formula gives a considerably smaller value for the lifetime, when the first linear decay of the open-circuit voltage is evaluated. However, the true value of the carrier lifetime can be determined, after Davies in the second linear range of the open-circuit voltage decay, with a high accuracy.

Determination of the carrier lifetime in power transistors from the measurement of the transient collector current

ContentsA method is presented that determines the carrier lifertime in bipolar transistors from the collector current decay after optical carrier generation. The transient collector current is calculated by solving the one-dimensional diffusion equation. Parameters are the carrier lifetime τ, the ratiod/L of base width to diffusion length and the penetration depth 1/α of the incident light. It is shown that after a transition phase, the collector current decays exponentially with the carrier lifetime of the bulk material as time constant. The smallerd/L, the faster the collector current decays exponentially. Experimental results prove the applicability of the method to determine the low-level lifetime as well as the high-level lifetime in power transistors. Furthermore, the influence of the lateral diffusion is investigated by changing the ratior/L of the radius of the illuminated area and the diffusion length. Within the variation 1.5≦r/L≦6.5, no such influence can be observed.ÜbersichtDie vorliegende Arbeit stellt eine Methode vor, die Ladungsträgerlebensdauer in Transistoren aus dem transienten Kollektorstrom nach optischer Trägergeneration zu bestimmen. Der Kollektorstromverlauf wird durch Lösung der eindimensionalen Diffusionsgleichung in Abhängigkeit von der Trägerlebensdauer τ, dem Verhältnisd/L aus Basisweite und Diffusionslänge und der Eindringtiefe 1/α des anregenden Lichtes berechnet. Es wird gezeigt, daß der Kollektorstrom nach einer Übergangsphase exponentiell mit der Trägerlebensdauer abklingt. Je kleinerd/L ist, desto schneller ist ein rein exponentieller Stromverlauf erreicht. Meßergebnisse zeigen die gute Anwendbarkeit auf Leistungstransistoren zur Bestimmung der Hoch- und Niedriginjektionslebensdauer. Weiterhin wird das Verhältnisr/L aus dem Radius der beleuchteten Fläche und der Diffusionslänge verändert, um einen Einfluß der lateralen Diffusion auf das Meßergebnis festzustellen. Innerhalb der vorgenommenen Variation 1.5≦r/L≦6.5 kann kein Einfluß beobachtet werden.

FGTO — A fine structured GTO-thyristor with improved switching characteristics

ContentsA fine structured GTO thyristor, abbreviated FGTO, has been developed to improve the switching characteristics. The new device has a cathode finger width of only 20 μm and can be turned off without a negative gate bias at an anode current level of more than 200 A/cm2. The turn-off time can be shortened down totoff≦500 ns by increasing the negative gate current. A snubber circuit is not necessary to turn off the FGTOs. The maximum rate of rise of anode voltage du/dl reaches more than 10 kV/μs. Analytical models are developed to describe the turn-off transients of the FGTOs. Good agreement between the theoretical and experimental results could be achieved.ÜbersichtZur Verbesserung des Schaltverhaltens ist ein feinstrukturierter GTO-Thyristor, abgekürzt FGTO, entwickelt worden. Dieses neue Bauelement hat eine Kathoden-streifenbreite von nur 20 μm und kann ohne negative Gatevorspannung bei einer Anodenstromdichte von mehr als 200 A/cm2 abgeschaltet werden. Die Abschaltzeit läßt sich durch Erhöhung des negativen Gatestroms bis zutoff ≦500 ns verkürzen. Beim Abschalten von FGTOs ist keine RC-Beschaltung erforderlich. Der maximale du/dl-Wert erreicht mehr als 10 kV/μs. Zur Beschreibung des Abschalt-verhaltens von FGTOs werden analytische Modelle entwickelt, die eine gute Übereinstimmung mit den Meßergebnissen liefern.

Theorie der PCD-Methode für Licht geringer Eindringtiefe

ÜbersichtDie Theorie der PCD-Methode wird erweitert auf den Fall, daß das Licht in der Halbleiterprobe vollständig absorbiert wird. Das transiente Verhalten der Photoleitfähigkeit wird sowohl für rechteckförmige Lichtpulse beliebiger Pulsdauer als auch für δ-förmige Lichtpulse analysiert. Ausführlich diskutiert wird das Abklingen der Photoleitfähigkeit. Besondere Beachtung wird der Auswirkung der Oberflächenerekombination auf die effektive Lebensdauer der Überschußträger und die Abklingrate der Überschußträger unmittelbar nach dem Ende des Lichtpulses geschenkt.ContentsThe theory of the PCD-method is extended to conditions, where the light is fully absorbed in the semiconductor sample. The transient behaviour of photoconductivity is analysed for rectangular light pulses of arbitrary pulse length as well as for δ-pulses. The decay of photoconductivity is extensively discussed. Most emphasis is given to the influence of surface recombination on the effective lifetime of excess carriers and on the decay rate of the excess carriers immediately after the end of the light puls.

Injection-level dependence of charge carrier mobility in high-injection plasma

Abstract The contradictions between results of commonly used charge carrier transport equations resulting from electron-hole scattering on the one hand and experimental data concerning transport coefficients in an electron-hole plasma on the other are pointed out. To resolve them, the influence of the injection level on the transfer of charge carrier momentum to the phonon system is investigated. The effective momentum transfer mobility for electrons and holes is derived, taking into account the indirect influence of electron-hole scattering on the momentum transfer to acoustical and high-energetic phonons in silicon. The result of the calculation is compared with measurements, and an analytical fitting formula for the implementation of the injection dependent phonon scattering into device simulators is presented.