V. Krozer

Characterisation of reliability of compound semiconductor devices using electrical pulses

Transmission line pulses (TLP) with 0-60V amplitude and 100ns pulse width have been applied for accelerated lifetime tests of GaAs devices. 1/f noise, RF noise and I/V measurements are applicable for the characterisation of the reliability of these devices. Different failure mechanisms can be identified by applying square pulses of varying amplitude and different polarity on a variety of samples. Correlation between anomalies in 1/f noise, RF noise and I/V characteristics has been determined. Using this novel method, the determination of failure threshold levels for current density, electric field and charge carrier temperature is possible and critical spots in device design can be ascertained.

Modeling and design aspects of millimeter-wave Schottky varactor frequency multipliers

Design and optimization of frequency multipliers for millimeter and submillimeter wavelengths have been previously performed using harmonic-balance techniques together with equivalent circuit models. Using this approach it is difficult to simultaneously design and optimize the diode device and the multiplier circuit. This work demonstrates results from numerical semiconductor simulation coupled with the harmonic-balance technique. The good agreement between the calculated and published experimental data for the output power and efficiency is essentially due to the incorporation of impact ionization in the numerical model. Details on the device behavior and circuit operation at different power levels are provided.

Limitations in THz power generation with Schottky diode varactor frequency multipliers

We discuss the limitations in power generation with Schottky diode and HBV (heterostructure barrier varactor) diode frequency multipliers. It is shown that at lower frequencies the experimental results achieved so far approach the theoretical limit of operation for the employed devices. However, at increasing frequencies the power drops with f/sup -3/ instead of the f/sup -2/ predicted by theory. In this contribution we provide an overview of state-of-the-art results. A comparison with theoretically achievable multiplier performance reveals that the devices employed at higher frequencies are operating inefficiently and the design and fabrication capabilities have not reached the maturity encountered at lower THz frequencies.

Model for the decrease in HBT collector current under DC stress based on recombination enhanced defect reactions

Abstract Based on simplifying assumptions a model is developed to describe decreasing HBT collector current under DC bias stress. The underlying processes are recombination enhanced defect reactions (REDR), namely defect generation and defect annealing. The simplification of the derived equation under particular boundary conditions leads to a form of the Eyring relationship generally employed to describe the decrease in collector current under DC bias stress. Additionally, pulsed tests with an emitter current density of JE=200 kA/cm2 have been performed. The comparison between measured data from these tests and their fit proofs the applicability of the derived equation. Of course this can not be an evidence for the real running physical processes.

Thermal Coupling in Multi-finger Heterojunction Bipolar Devices

IThe thermal characteristics of power HBT devices are considered. The analysis includes emitter finger coupling and limitation in power operation due to selfheating. An essential part of the generated heat is dissipated through the top device layers and metal contacts. A strongly uneven temperature distribution in the emitter finger of multi-finger devices is the result of not sufficient temperature drain through the emitter air-bridge. Sufficient thermal dissipation can be additionally obtained by temperature drain through the base and collector metallised areas on top of the device. An optimum distance between individual emitter fingers exists for maximum thermal coupling. This yields better thermal homogeneity.

Characterisation of Schottky Diode Performance by Numerical Simulation Coupled with Harmonic Balance

The electrical and RF performance characteristics of submillimetre-wave Schottky diodes are investigated using an accurate physical model which combines drift-diffusion current transport with thermionic and thermionicfield emission currents imposed at the Schottky contact. The model includes self-consistently image force effect, tunneling transport, and current dependent recombination velocity at the Schottky contact. This physical simulator has been coupled to a harmonic balance simulator as a non-linear element. The integrated device-circuit simulator has allowed to study in detail the limiting factors of varactor operation. The contribution of large-signal capacitance excitation is discussed. The influence of external loads at different harmonics, bias and power level is also analysed.

Schottky diode-based mixers design and optimization at millimetre and submillimetre-wave bands

In this paper we present some design and optimization aspects for Schottky-based mixers at the millimetre and submillimetre-wave bands by using an in-house mixer CAD tool. This simulator takes into account both the external circuit and a physical model for the semiconductor device. For mixer analysis, proper time-frequency conversion techniques have been implemented, which combine good accuracy and low computational cost. This CAD tool allows the joint optimization of the Schottky diode and the external circuit. Another important advantage is the possibility to study and optimize the internal structure of the Schottky diode for optimum performance.

Investigation of interface charges at the heterojunction discontinuity in HBT devices

Abstract In this paper we investigate the impact of interface charges at heterojunctions on the performance of heterostructure bipolar transistors (HBT). Interface charges can modify the limiting process for the carrier transport in a device. Therefore, intentional interface charges introduced by δ-doped layers are basic tools for interface engineering. An accurate modelling of heterointerfaces which includes thermionic-field emission, surface charges, and surface dipoles allows to analyse the electrical performance of some modern devices based on band gap and interface engineering. It is demonstrated that the limiting transport process at an abrupt heterojunction can be shifted from thermionic emission towards drift-diffusion due to the presence of interface charges. We will also show that controlling the number and polarity of interface charges enables to improve HBT device performances.

A Galerkin Method Solution for Arbitrary Electromagnetic Problems using a Surface-Volume Formulation

An efficient numerical method to analyze the electromagnetic field in presence of bodies of arbitrary shape and material is presented. The method has been applied to the problem of the field distribution in the far field of a mobile phone antenna near a head and a hand of a person. Resonant microwave circuits and scattering applications demonstrate the versatility and effectiveness of the present method. Other problems have also been studied, which can be conductors, dielectrics or and arbitrary combinations of both. The procedure is based on the surface and volume equivalence principles and a Galerkin method is used to solve the integral equations.