R.G. Johnson

Electrothermal modeling and measurement for spatial power combining at millimeter wavelengths

In this paper, the first completely physical coupled electrothermal model, suitable for large-signal simulation of MESFET- and HEMT-based MMICs and MMIC arrays, on a timescale suitable for computer-aided design, is presented. The model is validated experimentally by high-resolution thermal imaging of a MMIC 38-GHz three-stage balanced amplifier, mounted on a Cu/FR-4 substrate and cooled entirely by natural convection and radiation into free space.

A physics-based electro-thermal model for microwave and millimetre wave HEMTs

An electro-thermal physical model is described for HEMT microwave and millimetre wave simulations which includes temperature effects due to self-heating within the device. Comparisons of the results from the model with measured data are made and it is found that good agreement is obtained without fitting of any of the parameters. The model is compatible with CAD requirements and is particularly suited to large-signal applications.

Thermal transients in microwave active devices and their influence on intermodulation distortion

A fully physical transient thermal model is used to investigate the effects of temperature on the intermodulation distortion performance of microwave devices. A 24 mm, 60 finger PHEMT is used to compare measurements with predictions from the model. Results are in very good agreement and are a strong indication of thermally induced intermodulation distortion.

Electro-Thermal Modelling of Monolithic and Hybrid Microwave and Millimeter Wave IC's

The first completely physical electro-thermal model is presented that is capable of describing the large signal performance of MESFET- and HEMT-based, high power microwave and millimeter wave monolithic and hybrid ICs, on timescales suitable for CAD. The model includes the effects of self-heating and mutual thermal interaction on active device performance with full treatment of all thermal non linearities. The electrical description is provided by the rapid quasi-2D Leeds Physical Model and the steady-state global thermal description is provided by a highly accurate and computationally inexpensive analytical thermal resistance matrix approach. The order of the global thermal resistance matrix describing 3-dimensional heat flow in complex systems, is shown to be determined purely by the number of active device elements, not the level of internal device structure. Thermal updates in the necessarily iterative, fully coupled electro-thermal solution, therefore reduce to small matrix multiplications implying orders of magnitude speed-up compared to the use of full numerical thermal solutions capable of comparable levels of detail and accuracy.

Fully physical coupled electro-thermal simulations and measurements of power FETs

A fully physical coupled electro-thermal model is presented. It is fast, efficient, suitable for CAD applications and capable of describing power FETs, MMICs and MMIC arrays. Results are presented which show the model gives good agreement with measurements for large power devices.

Fully physical time-dependent compact thermal modelling of complex non linear 3-dimensional systems for device and circuit level electro-thermal CAD

An fully analytical spectral domain decomposition approach to solution of the nonlinear time-dependent heat diffusion equation in complex volumes is introduced. Its application to device/circuit level electro-thermal simulation on CAD timescales is illustrated. The full treatment in coupled electro-thermal CAD of thermal nonlinearity due to temperature dependent diffusivity is described. Thermal solutions are presented in the form of thermal impedance matrix expressions for thermal subsystems. These include double Fourier series solutions for rectangular multilayers, which are an order of magnitude faster to evaluate than existing semi-analytical Fourier solutions based on DFT-FFT. They also include double Fourier series solutions for arbitrarily distributed volume heat sources and sinks, constructed without use of Greens function techniques, and for rectangular volumes with prescribed fluxes on all faces. These analytical solutions allow treatment of arbitrary device structures without invoking conventional numerical methods. They provide minimal boundary condition independent compact thermal models, allowing CAD timescale coupled electro-thermal solution for complex systems, without requiring lumped element RC network extraction or node reduction. The time-independent thermal resistance matrix description of device structure is illustrated by a fully physical, coupled electro-thermal study of the interaction of substrate thickness and surface convection in power HEMTs. The thermal time-dependent implementation is illustrated by circuit level harmonic balance simulation of a 3/spl times/3 MMIC amplifier array.

Electro-thermal modelling of microwave transistors and MMICs for optimised transient and steady-state performance

The influence of the physical layout of MESFET-based MMICs, on active device temperature and I-V characteristics, is investigated. These transient and steady state simulations represent the first reported, fully physical, coupled electro-thermal studies on CAD timescales. Calculated temperature rises are compared against experimental results obtained by thermal imaging.

An investigation of breakdown in power HEMTs and MESFETs utilising an advanced temperature-dependent physical model

This paper presents a physical model and experimental validation for the breakdown process in HEMTs and MESFETs. The model is integrated into a fast quasi-two-dimensional physical simulation. The model takes account of the tunnelling effects in the region of the gate metallization. A new thermal model monitors the channel temperature and controls the tunnelling mechanism. The effects of the substrate conduction on breakdown in HEMTs is highlighted. Experimental results are presented which confirm the physical interpretations of the numerical model.

Electro-thermal modelling and measurement of thermal time constants and natural convection in spatial power combining grid arrays

The first theoretical study of thermal time constants in MMIC grid arrays is presented, indicating the significance of time dependent thermal effects for spatial power combining. Model validation against thermal images of passive grid arrays provides the first experimental demonstration of the impact of natural convection on grid array beam forming.

Multi-Tone Microwave Simulation of HEMTs using a Physics-Based Electro-Thermal CAD Model

An electro-thermal physical model is described for HEMT multi-tone microwave and millimetre wave simulations. Comparisons of the results from the model with measured data are made and good agreement is obtained without fitting any of the parameters. The model is compatible with CAD requirements and is applied to multi-tone microwave and millimetre wave simulations for MMICs.