The Hot-Spot Phenomenon and its Countermeasures in Bipolar Power Transistors by Analytical Electro-Thermal Simulation

This communication deals with a theoretical study of the hot spot onset (HSO) in cellular bipolar power transistors. This well-known phenomenon consists of a current crowding within few cells occurring for high power conditions, which significantly decreases the forward safe operating area (FSOA) of the device. The study was performed on a virtual sample by means of a fast, fully analytical electro-thermal simulator operating in the steady state regime and under the condition of imposed input base current. The purpose was to study the dependence of the phenomenon on several thermal and geometrical factors and to test suitable countermeasures able to impinge this phenomenon at higher biases or to completely eliminate it. The power threshold of HSO and its localization within the silicon die were observed as a function of the electrical bias conditions as for instance the collector voltage, the equivalent thermal resistance of the assembling structure underlying the silicon die, the value of the ballasting resistances purposely added in the emitter metal interconnections and the thickness of the copper heat spreader placed on the die top just to the aim of making more uniform the temperature of the silicon surface.Comment: Submitted on behalf of TIMA Editions (http://irevues.inist.fr/tima-editions

SPICE modelling of photoluminescence and electroluminescence based current-voltage curves of solar cells for concentration applications

Quantitative photoluminescence (PL) or electroluminescence (EL) experiments can be used to probe fast and in a non-destructive way the current-voltage (IV) characteristics of individual subcells in a multi-junction device, information that is, otherwise, not available. PL-based IV has the advantage that it is contactless and can be performed even in partly finished devices, allowing for an early diagnosis of the expected performance of the solar cells in the production environment. In this work we simulate the PL- and EL-based IV curves of single junction solar cells to assess their validity compared with the true IV curve and identify injection regimes where artefacts might appear due to the limited in-plane carrier transport in the solar cell layers. We model the whole photovoltaic device as a network of sub-circuits, each of them describing the solar cell behaviour using the two diode model. The sub-circuits are connected to the neighbouring ones with a resistor, representing the in-plane transport in the cell. The resulting circuit, involving several thousand subcircuits, is solved using SPICE

Heavy and light hole minority carrier transport properties in low-doped n-InGaAs lattice matched to InP

Minority carrier diffusion lengths in low-doped n-InGaAs using InP/InGaAs double-heterostructures are reported using a simple electrical technique. The contributions from heavy and light holes are also extracted using this methodology, including minority carrier mobilities and lifetimes. Heavy holes are shown to initially dominate the transport due to their higher valence band density of states, but at large diffusion distances, the light holes begin to dominate due to their larger diffusion length. It is found that heavy holes have a diffusion length of 54.5 +/- 0.6 microns for an n-InGaAs doping of 8.4 x 10^15 cm-3 at room temperature, whereas light holes have a diffusion length in excess of 140 microns. Heavy holes demonstrate a mobility of 692 +/- 63 cm2/Vs and a lifetime of 1.7 +/- 0.2 microsec, whereas light holes demonstrate a mobility of 6200 +/- 960 cm-2/Vs and a slightly longer lifetime of 2.6 +/- 1.0 microsec. The presented method, which is limited to low injection conditions, is capable of accurately resolving minority carrier transport properties.Comment: 16 pages, 13 figure

Modeling charge transport in C60-based self-assembled monolayers for applications in field-effect transistors

We have investigated the conductance properties of C60-containing self-assembled monolayers (SAMs), which are used in organic field-effect transistors, employing a combination of molecular-dynamics simulations, semiempirical electronic structure calculations and Landauer transport theory. The results reveal the close relation between the transport characteristics and the structural and electronic properties of the SAM. Furthermore, both local pathways of charge transport in the SAMs and the influence of structural fluctuations are analyzed.Comment: 10 figure