Power HBTs Research Articles

Characterization and modeling the effect of temperature on power HBTs InGaP/GaAs

The variation and stability of HBT’s parameters at different temperatures are important for utilizing these devices in high-power integrated circuits. The temperature dependence of the DC current gain of bipolar transistors, as a key device parameter, has been extensively investigated. A major issue of the power HBT’s is that the current gain is decreased with junction temperature due to self-heating effect. Hence, how to stabilize the DC current gain and RF performances is important issue to develop the power HBTs. This work describes the DC and high-frequency temperature dependence of InGaP/GaAs HBT’s. The substrate temperature (T) was varied from 25 to 150°C. The static and dynamic performances of the HBT are degraded at high temperature, due to the reduced of carrier mobility with increasing temperature. The current gain (β) decreases at high temperatures; from 140 to 127 at 25 to 150°C, while the decreases in the peak Ft and Fmax are observed from about 110 GHz to 68 GHz and from 165 GHz to 53 GHz respectively in the temperature range of 25 to 150°C.

Simultaneous Optimization of Ge Doping Profile and Layout in Multi-Finger Power SiGe HBTs for High Thermal Stability

Considering the effect of electro-thermal feedback on the thermal stability of multi-finger power SiGe HBT, optimization of SiGe HBT on both Ge doping profile (electrical aspect) and layout (thermal aspect) is demonstrated in this paper. A novel stepped Ge doping profile of SiGe HBT with a grading Ge concentration in base region is proposed to improve the temperature coefficient of current gain, meanwhile, the cut-off frequency of HBT is also increased considerably due to the accelerated electric field caused by the Ge concentration grading. However, there is still an uneven temperature distribution. Therefore, the layout of multi-finger HBT with nun-uniform emitter finger length is optimized to compensate the thermal coupling effects and hence the uneven temperature distribution is improved obviously. It is shown that the device with simultaneous optimization of Ge doping profile and layout could be in thermal stability over a wide temperature range, which presents useful guideline to design microwave power HBTs.

Novel Emitter-Segmented Power HBT with 2-D Non-Uniform Segment Spacing for High Thermal Stability

With the aid of the thermal model, thermal resistance matrix of emitter-segmented power HBT is proposed to represent thermal effects. The effect of 2-dimensional (2-D) inter-segment spacing on thermal stability of device is studied. It is shown that the increase of inter-segment spacing could effectively decrease thermal coupling resistance, lower temperature of center segments, and hence improves the thermal stability. Furthermore, a novel emitter-segmented power HBT with 2-D non-uniform segment spacing is proposed, in which the non-uniformity of segment temperature is improved by 75.26% and the maximum power level difference of emitter segment is improved by 55.84%, when compared with uniform segment spacing HBT. Therefore the technique of 2-D non-uniform segment spacing is a better method for enhancing the thermal stability of power HBTs.

Effects of Proton Radiation on Device Modeling of SiGe Power HBTs

The effects of proton irradiation on device modeling of SiGe power HBTs are investigated in this work. On-wafer DC, small- signal AC, linearity and large-signal load-pull measurements were performed before and after proton irradiation. Minor degradation in DC, small-signal AC and large-signal load-pull characteristics and minor improvement in linearity are well modeled with the extraction of model parameter changes after radiation. It is found that the major changes that influence the device performance of SiGe power HBTs after proton radiation are the increase of base and emitter resistances.

Impact of Proton Radiation on the Large-Signal Power Performance of SiGe Power HBTs

The effects of proton irradiation on the RF power performance of SiGe power HBTs are, for the first time, reported in this work. Large emitter area high-power SiGe HBTs fabricated in a commercial BiCMOS process were irradiated with proton, at fluences up to 2times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13 </sup> p/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Besides DC and small-signal AC characterizations, on-wafer large-signal high-power performance was characterized by load-pull measurements for pre- and post-radiation devices. It is shown that, in addition to DC and small-signal AC performance, the power performance of SiGe power HBTs also exhibits excellent tolerance to high-fluence proton radiations. Only a minor degradation (from the worst measurement case, 0.7 dB degradation in power gain, 8% degradation in PAE) was measured for post-radiation devices under class-AB bias at 1.9 GHz. Moreover, the source and load impedance matching points tuned for the optimum power performance of the devices, which are critical in the design of power amplifiers, are also shown to be robust to proton radiation. This work demonstrates the potential of SiGe power HBTs in the applications of power amplifiers for wireless application under severe radiation environment even without any intentional radiation hardening

Large-Signal Modeling of Proton Radiated SiGe Power HBTs

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3-W SiGe power HBTs for wireless applications

High-performance SiGe power HBTs are developed using 4-metal industry SiGe BiCMOS platform for 1.9 GHz portable wireless communications. The values of ballasting resistors are carefully selected to ensure thermally stable operation of these devices at high power levels. Three-Watt RF power with concurrent 67% PAE and power gain of 9.8 dB is achieved from 0.9 μm emitter finger power SiGe HBTs, resulting in a RF power density of 1.13 mW/μm 2. The power performance of these devices under different bias conditions are also studied. These results show the great potential of SiGe HBTs for on-chip high-power amplifications.

AlGaInP/GaAs power HBTs design and fabrication

High-power-density and high-efficiency AlGaInP/GaAs power HBTs have been successfully designed and fabricated by using 14 double-finger emitters in parallel and related input and output matched circuits. The device material structure and layout design are discussed and optimized in detail. An output power of 1.07 W (power density of 2 mW/μm2) with a power-added efficiency of 64% at 2 GHz has been achieved. ©2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 332–334, 2000.

Modeling the bias of the dependence of the base-collector capacitance of power heterojunction bipolar transistors

This paper introduces a modified formulation for the minority charge of a recently published heterojunction bipolar transistor large-signal model that results in an accurate description of the base-collector capacitance bias dependence of power HBTs. The accompanying parameter extraction procedure determines the stored minority charge through simultaneous optimization of the cutoff frequency and the base-collector capacitance bias dependencies. The modified model results in accurate predictions of the HBT small- and large-signal characteristics over a wide range of bias, frequencies, and RF excitations.

Design considerations for millimeter-wave power HBTs based on gain performance analysis

Critical design issues involved in optimizing millimeter-wave power HBTs are described. Gain analysis of common-emitter (CE) and common-base (CB) HBTs is performed using analytical formulas derived based on a practical HBT model. While CB HBT's have superior maximum-gain at very high frequencies, their frequency limit is found to be determined by the carrier transit time delay. Thus, to fully exploit the potential gain in a CB HBT, it is essential to maintain a high f/sub T/ even at high collector voltages. The advantage of using CB HBT's in a multifingered device geometry is also discussed. Unlike CE HBTs, CB HBTs are capable of maintaining a high gain even if the device size is scaled up by increasing the number of emitter-fingers. Moreover, it is found that reducing the wire parasitic capacitance allows emitter ballasting resistance to be used without affecting the gain. Fabrication of HBTs based on these design considerations led to excellent power performance in a CB unit-cell HBT at 25-26 GHz, featuring output power of 740 mW and power-added efficiency of 42%.

Nonlinear modeling of power FETs and HBTs

Large-signal modeling and simulation techniques for microwave and millimeter-wave transistors are described and reviewed. The relative merits of physical, physics-based and nonlinear equivalent circuit models are summarized. Several examples of MESFET, HBT, and HEMT simulation results are given. © 1996 John Wiley & Sons, Inc.

Nonlinear modeling of power FETs and HBTs

Large-signal modeling and simulation techniques for microwave and millimeter-wave transistors are described and reviewed. The relative merits of physical, physics-based and nonlinear equivalent circuit models are summarized. Several examples of MESFET, HBT, and HEMT simulation results are given. © 1996 John Wiley & Sons, Inc.

Optimization of the HBT and bump configuration for bump heat sink structure and application to HBT power MMICs

Suitable HBT and bump configuration for the BHS (Bump Heat Sink) structure are experimentally investigated. The best results in terms of thermal and electrical characteristics is obtained from the combination of a single emitter finger and an “H” shaped bump. Power HBTs are demonstrated with typical power added efficiencies η add of 70% at 8.0 W and a linear power vs transistor size relation up to 10 W at 0.9 GHz CW with V cc = 6 V, using the optimized structure. A three-stage HBT MMIC power amplifier and a power amplifier module for GSM class 4 are demonstrated with η add of 53% at 5.0 W, for V cc = 6 V CW operation, also using the optimized structure.

A self-consistent DC-AC two dimensional electrothermal model for GaAlAs/GaAs microwave power HBTs

A 2D self-consistent model has been developed to investigate the coupled electrothermal problem in GaAlAs/GaAs HBTs under DC and AC conditions. Electrical and thermal aspects of device behavior are simultaneously investigated by means of a nodal analysis of the distributed and physics-based model using a circuit simulator. The effects of base widening and the temperature dependence of the thermal conductivity are taken into account. Rigorous simulations are compared to the results given under different approximations. In particular, the sensitivity to the temperature dependence of the electron mobility is used to highlight the beneficial role of the negative temperature coefficient of the current gain in determining the thermal stability of the device. An HF performance criterion (maximum stable gain) is calculated and then compared to the results derived from the isothermal approximation. >

Accurate analysis of emitter ballasting in HBT power transistors

An accurate analysis of emitter ballasting, taking into account the current gain dependence on the temperature and the collector current is presented in this paper. Numerical computations of current-voltage characteristics have been performed using the emitter ballasting resistance as a parameter. Power HBTs for mobile communications have been fabricated. Load-pull measurements at the L band have shown 30 dBm maximum output power and 45% power added efficiency with 8.5dB associated gain.

Double-layer collector for heterojunction bipolar transistors

A double-layer collector with appropriate profiles has been shown to increase the maximum collector current without compromising the breakdown voltage. Power HBTs with emitter ballasting resistors were grown on Si and GaAs substrates and fabricated to test the various collector designs. A maximum collector current density of 3.5 × 10 4 A cm −2 and a collector-emitter breakdown voltage of 28 V were realized for a double-layer collector HBT with ten 5 × 25 μm 2 emitter fingers. This high current density obtained by the novel collector design represents a 60% improvement over the conventional HBT with slightly higher breakdown voltage which is in agreement with predictions.

Ultrahigh power efficiency operation of common-emitter and common-base HBT's at 10 GHz

The DC and RF characteristics of microwave power HBTs are described. Ultrahigh power-added efficiency is reported for AlGaAs-GaAs HBTs operating at 10 GHz in common-emitter (CE) and common-base (CB) modes. A record high 67.8% power-added efficiency with 11.6 dB associated gain was achieved with a CE HBT at a CW output power of 0.226 W, corresponding to a power density of 5.6 W/mm. With a CB HBT, 62.3% power-added efficiency with 11.85 dB gain and 0.385 W total CW power was demonstrated. Power saturation characteristics of CE and CB HBTs are compared. The importance of bias schemes is discussed. High-efficiency operation in near class B mode is described and compared with FET operation. An advantage of HBT over FET is the low leakage current during the off half cycle in class B operation. Stability conditions for CE and CB HBTs are discussed. >