Multi-finger Bipolar Transistors Research Articles

Optimizing Finger Spacing in Multifinger Bipolar Transistors for Minimal Electrothermal Coupling

We present a compact modeling framework to optimize finger spacing for improving the thermal stability in multifinger bipolar transistors with shallow-trench isolation. First, we present an accurate physics-based model for total junction temperature in all the fingers of a transistor. Other than validating the model with 3-D TCAD simulations and measured data, we demonstrate its efficacy to achieve finger spacing optimization with the aid of an iterative algorithm. Since the proposed technique is scalable from the viewpoint of the number of fingers within a transistor and their geometries, the proposed framework is found to work seamlessly for various emitter finger numbers.

Extraction of True Finger Temperature From Measured Data in Multifinger Bipolar Transistors

In this brief, we propose a step-by-step strategy to accurately estimate the finger temperature in a silicon-based multifinger bipolar transistor structure from conventional measurements. First we extract the nearly zero-power self-heating resistances (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th,ii</sub> (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> )) and thermal coupling factors ( <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cij</sub> (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> )) at a given ambient temperature. Now, by applying the superposition principle on these variables at nearly zero-power, where the linearity of the heat diffusion equation is preserved, we estimate an effective thermal resistance (R <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th,i</sub> (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> )) and the corresponding revised finger temperature T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ). Finally, the Kirchhoff's transformation on T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) yields the true temperature at each finger (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</sub> (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a,</sub> P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> )). The proposed extraction technique automatically includes the effects of back-end-of-line metal layers and different types of trenches present within the transistor structure. The technique is first validated against 3-D TCAD simulation results of bipolar transistors with different emitter dimensions and then applied on actual measured data obtained from the state-of-the-art multifinger SiGe HBT from STMicroelectronics B5T technology. It is observed that the superposition of raw measured data at around 40 mW power underestimates the true finger temperature by around 10%.

An Efficient Thermal Model for Multifinger SiGe HBTs Under Real Operating Condition

In this work, we present a simple analytical model for electrothermal heating in multifinger bipolar transistors under realistic operating condition where all fingers are heating simultaneously. The proposed model intuitively incorporates the effect of thermal coupling among the neighboring fingers in the framework of self-heating bringing down the overall model complexity. Compared to the traditional thermal modeling approach for an ${n}$ -finger transistor where the number of circuit nodes increases as ${n}^{{2}}$ , our model requires only ${n}$ -number of nodes. The proposed model is scalable for any number of fingers and with different emitter geometries. The model is validated with 3-D thermal simulations and measured data from STMicroelectronics B4T technology. The Verilog-A implemented model simulates 40% faster than the conventional model in a transient simulation of a five-finger transistor.

Static Thermal Coupling Factors in Multi-Finger Bipolar Transistors: Part I—Model Development

In this part, we propose a step-by-step strategy to model the static thermal coupling factors between the fingers in a silicon based multifinger bipolar transistor structure. First we provide a physics-based formulation to find out the coupling factors in a multifinger structure having no-trench isolation (cij,nt). As a second step, using the value of cij,nt, we propose a formulation to estimate the coupling factor in a multifinger structure having only shallow trench isolations (cij,st). Finally, the coupling factor model for a deep and shallow trench isolated multifinger device (cij,dt) is presented. The proposed modeling technique takes as inputs the dimensions of emitter fingers, shallow and deep trench isolations, their relative locations and the temperature dependent material thermal conductivity. Coupling coefficients obtained from the model are validated against 3D TCAD simulations of multifinger bipolar transistors with and without trench isolations. Geometry scalability of the model is also demonstrated.

Influence of Concurrent Electrothermal and Avalanche Effects on the Safe Operating Area of Multifinger Bipolar Transistors

The impact of the concurrent action of electrothermal and avalanche effects on the reduction of the safe operating area is experimentally investigated for a wide number of single-, two-, and three-finger bipolar transistors fabricated in SiGe, GaAs, and silicon-on-glass technologies. The results of the analysis are substantiated by a SPICE-like simulation tool that allows the monitoring of the temperatures of the individual fingers and provides an in-depth understanding of the individual influence of the positive feedback mechanisms. Design strategies for minimizing the effects on device operation, like the implementation of ballasting resistors and emitter segmentation, are also analyzed.

Thermally induced current bifurcation in bipolar transistors

Thermally induced current bifurcation in multifinger bipolar transistors is studied by means of three novel and efficient methods that are based on the same electrothermal model for the low to medium current operation of the device: the limit model, the perturbation model and the FEAT simulation program. The results are substantiated by experimental measurements and numerical simulations. The new analysis techniques have been used to supply detailed information on which parameters, both individual and coupled, influence the current bifurcation and the behavior around the bifurcation point, particularly for the case of two-finger devices. In addition, different bipolar technologies and device designs are compared with respect to electrothermal feedback. It is shown that transistors with a negative temperature coefficient of the current gain (such as SiGe-base devices) made on thermally conductive substrates (such as silicon-on-copper technologies) can be designed to have an electrothermal stability superior to that of other Si bipolar technologies.

A simplified model for the effect of interfinger metal on maximum temperature rise in a multifinger bipolar transistor

The prediction of a simple lumped representation of heat sharing through emitter interconnect in high-power multiemitter bipolar devices is compared to numerical thermal simulation and found to exhibit nonphysical results. Using numerical simulation, interfinger metal heat flow is characterized qualitatively in three dimensions and the requirements for a more accurate model are determined. A new modeling approach based on these insights, using segmented emitters and a coarse representation of the metal structure, yields results within 2% of those obtained from numerical thermal simulation for a wide variety of device geometries and substrate materials, with a simulation time reduction of more than an order of magnitude. Using the new model, an extensive series of simulations is performed for devices fabricated in Si, GaAs, and InP substrates using Al and Au for metallization. Reduction in maximum temperature due to the presence of emitter interconnect in these structures is found to be in the range of 5%-15%.

Optimum design for a thermally stable multifinger power transistor

The thermal stability of multifinger bipolar transistors has been analyzed theoretically. Coupled equations are solved to study the onset of instability and its dependence on the distributions of ballasting resistors. Analytical expressions were derived for the emitter ballasting distribution for optimum stable operation. Compared to conventional methods with uniform ballasting, the optimized design can significantly increase the stable operating current of the transistor. An absolutely stable operating condition is also derived. At this condition, the device never becomes unstable.

Optimum design for a thermally stable multifinger power transistor with temperature-dependent thermal conductivity

The thermal stability of multifinger bipolar transistors has been analyzed theoretically. Coupled equations are solved to study the onset of instability and its dependence on the distributions of ballasting resistors. We extended our previous work on the multiple-finger transistor thermal stability from the simple coupled thermal-electrical feedback equation to the more accurate I-V equation and taking the temperature dependence of the thermal conductivity into consideration. Transistors with three-fingers and N-fingers have been analyzed. Two design procedures, uniform current design and uniform temperature design, of the best ballasting resistor distribution for optimum thermal stability operation were developed. Using these design flows, we can design the best ballasting resistor needed for thermal stable operation under the specified current level or specified junction temperature.