Multi-finger Heterojunction Bipolar Transistors Research Articles

A Study on Self-Heating and Mutual Thermal Coupling in SiGe Multi-Finger HBTs

In this paper, the self-heating and mutual thermal coupling in a state-of-the-art SiGe:C multi-finger heterojunction bipolar transistor (HBT) was investigated in static dc operation conditions. Multi-finger HBT structure was created using Sentaurus structure editor with dimensions similar to the layout of SiGe:C multi-finger HBTs in ST-Microelectronics BiCMOS55 (B55) technology (f T > 300 GHz, f max > 400 GHz) as per ST’s BiCMOS55 process design kit guidelines. Three-dimensional thermal technology computer aided design (TCAD) simulations were carried out to obtain the temperature distribution in static dc operation. The lattice temperature (T Lattice) and heat flux (F Heat) distribution inside the device were studied. The impact of back-end-of-line (BEOL) layers on static thermal behavior of the state-of-the-art SiGe:C multi finger HBTs was also investigated. The temperature dependent thermal resistance of different fingers of the trench isolated SiGe multi-finger HBT was extracted without and with back-end-of-line (BEOL) effect. An electro-thermal dc compact model of self-heating and mutual thermal coupling in multi-finger HBTs was proposed and applied to compare the modeling results with the TCAD simulation results. Very good agreement was achieved between results obtained from TCAD simulation and those obtained from compact model-based simulation.

Design and Thermal Analysis of SiGe HBT with Non-Uniform Segmented Emitter Fingers and Non-Uniform Emitter Finger Spacing

A novel multi-finger power SiGe heterojunction bipolar transistor (HBT) with non-uniform segmented emitter fingers and non-uniform emitter finger spacing was proposed to improve the thermal stability. Thermal simulation for a five-finger power SiGe HBT with novel structure was conducted with ANSYS software. Three-dimensional temperature distribution on emitter fingers was obtained. Compared with non-uniform segmented emitter fingers structure and non-uniform emitter finger spacing structure, the maximum junction temperature of novel structure reduce significantly, the thermal resistance reduce, temperature distribution were significantly improved. Thermal stability was effective enhanced.

Design and Thermal Analysis of SiGe HBT with Segmented Emitter Fingers and Non-Uniform Emitter Finger Spacing

A novel multi-finger power SiGe heterojunction bipolar transistor (HBT) with segmented emitter fingers and non-uniform emitter finger spacing was proposed to improve the thermal stability. Thermal simulation for a five-finger power SiGe HBT with novel structure was conducted with ANSYS software. Three-dimensional temperature distribution on emitter fingers was obtained. Compared with traditional emitter structure, the maximum junction temperature of novel structure reduce significantly from 429.025K to 414.252K, the thermal resistance reduce from 159K/W to 141K/W, temperature distribution were significantly improved. Thermal stability was effective enhanced.

Novel Multi-Finger Power HBTs with Non-Uniform Segmented Emitter for Thermal Stability Enhancement

With the aid of a two-dimensional thermal-electrical model, the thermal stability of multi-finger power heterojunction bipolar transistors (HBTs) with uniform segmented emitter, and non-uniform segmented emitter are studied. It shows that both of them could lower the peak temperature and improve the non-uniformity of the collector current density distribution, and thus improve the thermal stability, compared with an HBT which has traditional non-segmented emitter structure. At the same time, the improvement capability of thermal stability for HBT with non-uniform segmented emitter is superior to that of HBT with uniform segmented emitter, ascribing to selectively divide the emitter fingers into various segments according to the difference of heat dissipation capability in emitter fingers. Furthermore, the improvement capability of thermal stability for HBT with non-uniform segmented emitter is enhanced exponentially as segment spacing increases. However, for HBT with uniform segmented emitter, the ability is less sensitive to the increasing of the segment spacing as it exceeds 6μm. Therefore, the technique of non-uniform segmented emitter is a better method for enhancing the thermal stability of power HBTs.

Improvement on thermal stability of power heterojunction bipolar transistor using non-uniform finger spacing

One method of non-uniform finger spacing is proposed to enhance thermal stability of multiple finger power heterojunction bipolar transistor. Using coupling thermal resistance to characterize the influence of the change of emitter spacing on thermal coupling, the relation between coupling thermal resistance and emitter spacing is obtained. Temperature distribution on the emitter fingers of multi-finger heterojunction bipolar transistor is studied by a numerical electro-thermal model. The results show that the heterojunction bipolar transistor with non-uniform finger spacing has a smaller temperature difference between fingers thant the traditional uniform finger spacing heterojunction bipolar transistor under the same power dissipation. So the method of non-uniform finger spacing is very effective to enhance the thermal stability.

Design consideration of the thermal and electro stability of multi-finger HBTs based on different device structures

The thermal and electro stability of multi-finger heterojunction bipolar transistors (HBTs) with different structures were analyzed and discussed simultaneously. The thermal stability of the devices with different layout structures was assessed by the DC-IV test and thermal resistance calculation. Their electro stability was assessed by the calculation of the stability factor K based on the S parameter of the HBT. It is found that HBTs with higher thermal stability are prone to lower electro stability. The trade-off relationship between the two types of stability was explained and discussed by using a compact K-factor analytic formula which is derived from the small signal equivalent circuit model of HBT. The electro stability of the device with a thermal ballasting resistor was also discussed, based on the analytic formula.

Assessment of Thermal Instabilities and Oscillations in Multifinger Heterojunction Bipolar Transistors Through a Harmonic-Balance-Based CAD-Oriented Dynamic Stability Analysis Technique

We present a novel analysis of thermal instabilities and oscillations in multifinger heterojunction bipolar transistors (HBTs), based on a harmonic-balance computer-aided-design (CAD)-oriented approach to the dynamic stability assessment. The stability analysis is carried out in time-periodic dynamic conditions by calculating the Floquet multipliers of the limit cycle representing the HBT working point. Such a computation is performed directly in the frequency domain, on the basis of the Jacobian of the harmonic-balance problem yielding the limit cycle. The corresponding stability assessment is rigorous, and the efficient calculation method makes it readily implementable in CAD tools, thus allowing for circuit and device optimization. Results on three- and four-finger layouts are presented, including closed-form oscillation criteria for two-finger devices.

Thermalelectro-feedback model for multi-emitter finger power heterojunction bipolar transistor. Part I: Temperature profile

Poor thermal conductivity of GaAs, a self-heating phenomenon which results in the rapid rise of device temperature, is the major factor that limits and even degrades the electrical performance of GaAs-based heterojunction bipolar transistor (HBT) operated at high power densities. On the basis of this consideration, a numerical model is presented to study the interaction mechanism between the thermal and electrical behavior of AlGaAs/GaAs HBT with multiple-emitter fingers. The model mainly comprises a numerical model applicable for multi-finger HBT that solves the three-dimensional heat transfer equation. The device design parameters that influence the temperature profile and current distribution of the device are identified, and optimization concerning the device performance is made.

Computer simulation of multifinger heterojunction bipolar transistor with self-heating and thermal coupling models

In this paper, we simulate the nonlinearity of a multifinger heterojunction bipolar transistor (HBT) operated at radio frequency (RF). We directly solve the nonlinear differential equations of the HBT large-signal model with the electrical–thermal feedback equations in time domain using the waveform relaxation (WR) and monotone iterative (MI) methods. The temperature dependence of energy band gap ( E g), current gain, saturation current and thermal conductivity are also taken into consideration. With the developed simulator, the power-added efficiency (PAE), 1-dB compression point ( P 1-dB) and output third-order intercept point (OIP3) of a three-finger HBT are calculated. Our results illustrate the effects of self-heating and thermal coupling among different fingers play important roles in the nonlinearity of the multifinger power transistors. Furthermore, the proposed method allows us to evaluate the thermal effects on linearity of the multifinger power transistors and perform optimum design for these devices.

Simulation of the thermal response of self-heating power transistors using a time step adaption method

An implementation of the Rosenbrock-Wanner scheme with embedded time-step adaption in the thermal simulation of AlGaAs-GaAs heterojunction bipolar transistors (HBT) is presented. The simulations are based on discrete 3-D models of the device using the finite integration technique for the solution of the thermal equation. The thermally coupled current response of the device in DC operation is obtained by a nonlinear EM/sub 3/ model. Several simulations featuring the thermal crosstalk and the current collapse instability demonstrates the utility of time-step adaption in simulating the alternating slow-fast transients in multifinger HBTs.

A self-consistent model for temperature and current distribution in abrupt heterojunction bipolar transistors

The thermal behavior of abrupt heterojunction bipolar transistors (HBTs) has been studied by coupling the thermionic-field-emission injection mechanism at the emitter-base heterojunction with the thermal-electric feedback phenomenon. The exact quantum mechanical injection mechanism rather than the semiclassical WKB approximation is used in the present calculation to self-consistently calculate the thermionic and tunneling components of current. Moreover, the total current and temperature are self-consistently evaluated by testing the convergence on both current and temperature. The calculation shows correctly that the degree of the partitioning between the thermionic and tunneling components are bias- as well as temperature-dependent. It is shown that even a single emitter finger can have a highly nonuniform temperature and current distribution across it, leading to the current collapse phenomenon. At high power levels, this may give rise to a current collapse phenomenon similar to that observed for the multifinger HBTs.

Design of multi-finger HBTs with a thermal–electrical model

Multi-finger heterojunction bipolar transistors (HBTs) with uniform spacing exhibit a higher temperature at the center of devices. The temperature distribution on the emitter fingers of the HBT is studied with a three-dimensional thermal–electrical model. Using this model, multi-finger HBTs are designed with non-uniform spacing to improve temperature distribution. Depending on the number of emitter fingers, different design approaches are demonstrated. For a six-finger or 12-finger HBT, the design is more straightforward. For a complex structure such as a 26-finger HBT, an efficient design procedure is necessary. In all of these cases, the calculated results show significant temperature reduction on non-uniform spacing devices.

Direct extraction technique to derive the junction temperature of HBT's under high self-heating bias conditions

A new technique is presented that can directly extract the mean device junction temperature of heterojunction bipolar transistors (HBTs) under high self-heating operating conditions. The method uses three trivial DC measurements of the device where the junction temperature is known to be the same. This paper details the technique and applies it to both closely and widely spaced multi-finger HBT's, and compares the results to methods already known.

Thermal behavior depending on emitter finger and substrate configurations in power heterojunction bipolar transistors

Analytical and experimental results are used to investigate thermal behavior depending on the emitter-finger area and substrate thickness in multifinger heterojunction bipolar transistors (HBT's). The temperature distributions along the depth direction of the substrate are obtained from a three-dimensional (3-D) analysis. The calculated results show a 3-D effect which depends upon the emitter finger area. This effect weakens with increasing the emitter-finger area. As the effect weakens, thinning the substrate effectively reduces the junction temperature (thermal resistance). This is verified by the measured results obtained from a simple thermal-resistance measurement.

Transient analysis of collector current collapse in multifinger HBT's

The authors report for the first time a time-domain analysis of thermal instability in multifinger heterojunction bipolar transistors (HBT's). This is based on a transient quasi-three-dimensional (3-D) electrothermal model that selfconsistently solves the thermal and electrical equations. This model is designed to evaluate the thermal time constant of GaAs-based power HBT's employing emitter thermal shunt and emitter ballast resistance.

Simulation of Collector Current Distribution in Heterojunction Bipolar Transistors Using Newly Proposed Empirical Expression

This paper proposes a new empirical expression of the collector current in multifinger HBTs (Heterojunction Bipolar Transistors) as a function of the junction temperature, which is essential for optimizing the device layout and the ballasting resistor. The novel calculation yields more precise agreement with measured data than the conventional one. This scheme should be useful for elucidating the actual collector current distribution over each finger in multifinger HBTs.

A numerical study of the effect of base and collector structures on the performance of [formula omitted] multi-finger HBTs

A numerical analysis is presented to investigate different base and collector structures on the d.c. and a.c. performance of AlGaAs GaAs multi-finger heterojunction bipolar transistors (HBTs). The simulation is carried out using a two-dimensional device simulator called MEDICI. Five possible structures are studied and compared. The results show that different structures give rise to different electric fields in the base-collector junction and lattice temperatures in the HBT, which consequently affect the HBT's cutoff frequency and current gain, respectively. The physical mechanisms governing these changes are also discussed in detail.

1 W Ku-band AlGaAs/GaAs power HBT's with 72% peak power-added efficiency

High power and high-efficiency multi-finger heterojunction bipolar transistors (HBT's) have been successfully realized at Ku-band by using an optimum emitter ballasting resistor and a plated heat sink (PHS) structure. Output power of 1 W with power-added efficiency (PAE) of 72% at 12 GHz has been achieved from a 10-finger HBT with the total emitter size of 300 /spl mu/m/sup 2/. 72% PAE with the output power density of 5.0 W/mm is the best performance ever reported for solid-state power devices with output powers more than 1 W at Ku-band.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Geometry Considerations for Thermal Design of Microwave Heterojunction Bipolar Transistors

An efficient general algorithm for three-dimensional thermal simulation of semiconductor chips and its exemplary application to AlGaAs/GaAs heterojunction bipolar transistors (HBT) is presented. Firstly, the assumption of stationary heat sources as a boundary condition is validated by pulsed scattering parameter measurements. Secondly, the simulation method based on the fast Fourier transform is introduced. The model is verified by liquid crystal thermography, infrared thermography and comparison to another simulation method. In the third part of the paper, the model is applied to the self-heating problem in multifinger HBTs. Geometrical design rules for minimization of the maximum temperature and temperature gradients between the individual emitters are deduced. A tradeoff between the emitter size, the number of emitters, the power level and cooling measures such as substrate thinning must be carried out in order to control the thermal properties with respect to safe operation and reliability.

The collapse of current gain in multi-finger heterojunction bipolar transistors: its substrate temperature dependence, instability criteria, and modeling

One undesirable phenomenon observed when AlGaAs/GaAs heterojunction bipolar transistors (HBT's) are operated under high power density is the collapse (of current gain). The collapse manifests itself by a distinct abrupt decrease of collector current in the transistor common-emitter current-voltage (I-V) characteristics. In this investigation, we study the substrate temperature dependence of the collapse. A unified equation is introduced to relate the collapse instability criterion with other thermal instability criteria proposed for silicon bipolar transistors. The effects of the thermal instability on the collapse behavior of 2-finger and 1-finger HBT's are examined. We also present a numerical model to adequately describe the collapse in multi-finger HBT's having arbitrary geometry. The I-V characteristics and regression plots of both ballasted and unballasted HBT's are compared.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>