Efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with nonuniform current spreading

We demonstrate that the efficiency droop phenomenon in multiple quantum well InGaN/GaN light-emitting diodes (LEDs) may be connected to the current crowding effect. A numerical model of internal quantum efficiency calculation is presented that takes into account nonuniform lateral carrier injection in the active region. Based on this model, we examine the effect of current crowding on the efficiency droop using comparison of simulated internal quantum efficiency of InGaN LEDs with low and high uniformity of current spreading. The results of simulations and measurements show that the devices with low uniformity of current spreading exhibit higher efficiency droop and lower roll-off current value

Technique and setup for diagnostics of p-n junction–package thermal resistance in high-power gallium nitride LEDs

We present a setup and procedure of studying p-n junction–package thermal resistance in high-power light-emitting diodes (LEDs) from their thermal relaxation. A set of LEDs mounted on a metal-core printed circuit board (MCPCB) were studied. The contributions to the total thermal resistance from a heavy heat sink, MCPCB, heat slug and LED chip are separated

The Effect of Current Crowding on the Internal Quantum Efficiency of InAsSb/InAs Light-Emitting Diodes

The effect of current crowding on the internal quantum efficiency (IQE) of InAsSb/InAs light- emitting diodes (LEDs) operating in the middle�infrared (mid�IR) range (λ = 3–5 μm) has been studied. Calculations based on a modified model of recombination coefficients show that current crowding leads to a significant decrease in the IQE of LEDs, which is especially pronounced in longer�wavelength devices (23% at λ = 3.4 μm versus 39% at λ = 4.2 μm). The obtained results indicate that the effect of current crowding should be taken into consideration as an additional nonthermal mechanism of IQE decrease in mid-IR LEDs

A non-standard analysis of a cultural icon: The case of Paul Halmos

We examine Paul Halmos' comments on category theory, Dedekind cuts, devil worship, logic, and Robinson's infinitesimals. Halmos' scepticism about category theory derives from his philosophical position of naive set-theoretic realism. In the words of an MAA biography, Halmos thought that mathematics is "certainty" and "architecture" yet 20th century logic teaches us is that mathematics is full of uncertainty or more precisely incompleteness. If the term architecture meant to imply that mathematics is one great solid castle, then modern logic tends to teach us the opposite lession, namely that the castle is floating in midair. Halmos' realism tends to color his judgment of purely scientific aspects of logic and the way it is practiced and applied. He often expressed distaste for nonstandard models, and made a sustained effort to eliminate first-order logic, the logicians' concept of interpretation, and the syntactic vs semantic distinction. He felt that these were vague, and sought to replace them all by his polyadic algebra. Halmos claimed that Robinson's framework is "unnecessary" but Henson and Keisler argue that Robinson's framework allows one to dig deeper into set-theoretic resources than is common in Archimedean mathematics. This can potentially prove theorems not accessible by standard methods, undermining Halmos' criticisms. Keywords: Archimedean axiom; bridge between discrete and continuous mathematics; hyperreals; incomparable quantities; indispensability; infinity; mathematical realism; Robinson.Comment: 15 pages, to appear in Logica Universali

Temperature dependence of contact resistance of Au-Ti-Pd2Si-n+-Si ohmic contacts

We investigated temperature dependence of contact resistance of an Au-Ti-Pd2Si ohmic contact to heavily doped n+-Si. The contact resistance increases with temperature owing to conduction through the metal shunts. In this case, the limiting process is diffusion input of electrons to the metal shunts. The proposed mechanism of contact resistance formation seems to realize also in the case of wide-gap semiconductors with high concentration of surface states and dislocation density in the contact

Temperature-dependent efficiency droop in InGaN-based light-emitting diodes induced by current crowding

Temperature-dependent internal quantum efficiency (IQE) of multiple quantum well InGaN/GaN light-emitting diodes (LEDs) has been investigated. IQE versus current relation is analysed using the modified rate equation model that takes into account the current crowding effect at different temperatures. The results of calculations are consistent with the fact that droop in IQE at higher currents originates from Auger recombination increased by current crowding. It is shown that unusual experimentally observed temperature dependence of the efficiency droop can be explained by stronger lateral nonuniformity of carrier injection at low temperatures without any assumptions about carrier delocalization from In-rich regions in quantum wells

Degradation processes in LED modules

Electrical-heat-light degradation model of a light-emitting module has been developed in this work. The Monte-Carlo method was used to calculate the reliability time of LED modules with different halfwidth of LED chip series resistance. Separation of LED chips with different series resistance before assembling may increase the time of emission in a stable mode up to 10%

Determination of the Schottky barrier height in diodes based on Au–TiB₂–n-SiC 6H from the current-voltage and capacitance-voltage characteristics

We present the results of investigation of the barrier height and ideality factor in Schottky barrier diodes based on Au–TiB₂–n-SiC 6H relying on measuring the current-voltage and capacitance-voltage characteristics. Improving the accuracy of the methods that take into account the effect of the series resistance in calculating the ideality factor and barrier height has been shown with the Cheung method and direct approximation one. It has been ascertained that an inconsistency between real currentvoltage characteristics and its model – the temperature dependence of the barrier height, the ideality factor dependence on the voltage – introduces the basic error into the calculated parameters in the diode under study