Entries by Ingrid Schwarz

Atlas Simulation of GaN-Based Super Heterojunction Field Effect Transistors Using the Polarization Junction Concept

January 1, 2014in /by

Wide-bandgap semiconductors such as SiC and GaN have attracted much attention because they are expected to break through the material limits of silicon. In particular, AlGaN/GaN HEMTs are generally promising candidates for switching power transistors due to their high electric field strength and the high current density in the transistor channel giving a low on-state resistance.

Modeling the Optical Response of Phonon-dressed Excitons in OLED Simulations

January 1, 2014in /by

We demonstrate the modeling of optical response of exciton-polarons based on the well established Holstein Hamiltonian to model coupled exciton-phonon systems in organic molecular chains. Our approach uses Green’s functions to compute the density of states and the linear optical susceptibility, and thus eliminates the conventional and computationally expensive step of diagonalizing a large Hamiltonian matrix. We exploit this technique further to focus exclusively on the optically active states when computing the linear optical response, and significantly reduce the computational effort to construct the optical susceptibility. In this article, we demonstrate the computation of absorption and emission spectra of Alq3 at 4.2 K and at room temperature using our model. Using the two parameters of the Holstein model, the inhomogeneous broadening energies, and a phenomenological reorganization energy of the solute, we obtain excellent fits to established experimental results. We then use this model inside the larger simulation of a 3-layer organic light emitting (OLED) structure composed of Alq3, Alq3:DCJTB, and α-NPD, which are the electron transport, emissive, and hole transport layers respectively. In our methodology, we also couple the optical response into the rate equations for exciton dynamics in addition to computing the spectrum of light output by the device.

Hints, Tips, and Solutions – Simulate Rapid Thermal Annealing Temperature Transients in Vacuum

October 1, 2013in /by

When a wafer is clamped only at its edges and is heated with an infra-red source in a vacuum, the majority of the heat loss occurs through black body radiation. The vacuum prevents thermal conduction mechanisms for the majority of the wafer area. It is also important to realize that if both sides of the wafer are exposed to the vacuum without contact to a conductive heat sink, both the front and back sides of the wafer will radiate heat and so black body heat radiation must be simulated from both surfaces for accurate results.

Simulation of a DBR Edge Emitting Laser with External Air Gap Tuning Mirror

October 1, 2013in /by

A methodology for simulating an edge emitting laser is demonstrated that incorporates a distributed Bragg Reflector (DBR) as one of the mirrors, with the other being a variable air gap external planar mirror that is used as a method for fine tuning the lasing wavelength. This type of calculation requires non uniformity of the dielectric’s static and dynamic (pumping) properties in both the transverse and longitudinal directions, together with self consistent coupling to the external tuning cavity. A key challenge addressed is the proper treatment of the open nature of the cavity, such that the amplitudes of the outgoing waves leaving the facets are consistent with the cavity interior and facet reflectivity. Mode competition and hopping are natural consequences of the methodology developed. In the model developed here, the spatial hole burning effects, cross and self-saturation effects can be included while accounting for the degree of spatial overlap of modes. We also account for diffraction losses at the output facet which can become important in the case of strong index guiding.

Anode Shorts Layout Dependence of Bi-mode IGBT (BiGT) On-state Characteristics

July 1, 2013in /by

A new class of high-voltage semiconductor devices, known as a reverse conducting IGBT (RC-IGBT), has emerged in recent years from research efforts to diminish the usage of external anti-parallel free-wheeling diode chips for IGBT switching applications. The RC-IGBT device concept is based on the monolithic integration of a freewheeling diode into an IGBT chip. Hereby, an anti-parallel diode is formed through the embedment of an n+ region in an anode/collector region of the IGBT. Both the n+ region and the p+ anode are then shorted together by an anode/collector contact. Although the RC-IGBT has many advantages over the conventional IGBT, especially in terms of manufacturing cost, total chip size and reliability of power modules, it is detrimental to the snapback effect at forward conduction of IGBT mode.

Hints, Tips and Solutions – Compare the Results of Breakdown Simulations with Hatakeyama’s Impact Ionization Model for 4H-SiC Based Power Devices

July 1, 2013in /by

In a 4H-SiC crystal, the stacking sequence of a Si-C bilayer plane extends along the crystallographic c-axis direction, also referred to as the <0001> direction (see Figure 1). With regard to crystallographic orientation, avalanche breakdown process in 4H-SiC uniquely features anisotropy of avalanche breakdown field, i.e., impact ionization coefficients.