A study on Thermal Modeling and Management of InGaP/GaAs HBT for Power Amplifier Applications

Abstract

In this dissertation, a study on thermal modeling and management of InGaP/GaAs heterojunction bipolar transistor (HBT) for power amplifier (PA) applications is performed. The GaAs HBT device mainly used for a microwave PA has been developed using many investigations for performance optimization. In terms of thermal optimization, however, since the previous thermal models excessively required long time and/or complicated analysis, the circuit designers cannot apply a suitable model for various PA design. In order to solve the issue above, the simplified model for fast/comprehensible/accurate analysis is required to use for circuit designers. Firstly, a novel extraction method to estimate the thermal resistance (Rth) of a multi-finger InGaP/GaAs HBT device is presented based on the simplified Fourier’s law. By calculating the thermal coupling resistance between fingers on top of the self-heating thermal resistance, Rth of multi-finger devices for various dimensions are accurately calculated while maintaining the simplicity of calculation. To verify the idea, the proposed method is compared with the measurement-based method as well as the analytic methods such as the finite-element method (FEM) and solution of 3-D Laplace’s equation. For 4-finger HBT devices, the extracted Rth-results showed good agreements with the analytic methods within an error of 9% and the measurement-based results with a deviation of 7.4%, thus convincing the usefulness of the proposed method. Secondly, a simple/fast Rth-extraction method to reflect the thermally coupled effects of multi-cell devices for InGaP/GaAs HBT is presented. By calculating the thermal coupling resistance between cells using the same method used for a multi-finger device, Rth of multi-cell devices used in PA practically are also accurately calculated while maintaining the simplicity of calculation. To validate, the proposed method is compared with the measurement-based method as well as the analytic method as the finite-element method (FEM). For the two structures, Binary-type and Linear-type, the extracted Rth-results showed good agreements with the analytic method within an error of 8% and the measurement-based results with a deviation of 4.5%. Additionally, the calculated junction temperature results showed good agreement with the thermal profiles measured by IR-scope within a deviation of 5°C. Finally, an optimized device design suggestion for PA applications is presented based on proposed method for Rth-extraction of multi-finger device and multi-cell structure. Since a conventional device design has the thermal problem focused on the center, the performance degradation is occurring in conventional PA. To resolve the problem above, this study presents the two suggestions: device layout optimized thermally and device optimization by base ballast resistor. To verify the usefulness of proposed approaches, the 1.88 GHz PAs using an InGaP/GaAs HBT process for practical PA applications were fabricated and measured under CW operating mode. Compared to a conventional PA under the test condition of P3dB, the proposed PAs showed that output power and efficiency improved 0.1~0.2 dB and 1.0~1.6%, respectively. Furthermore, thermal profile of proposed PAs was measured by IR-scope and showed maximum temperature reducing of ~8°C, compared with conventional PA. In addition, the thermal runaway test was run. The proposed PAs for Binary / Linear devices showed that the current-gain-collapse and maximum output power are improved as ~3% / ~5% and 0.2 dB / 0.3 dB, respectively. Thus, since the usefulness of proposed method for simple/fast/accuracy analysis has been established by verifications, this study can help to develop PA applications for circuit designer.|본 논문에서는 전력증폭기용 InGaAsP/GaAs HBT의 열 모델링 및 관리에 관한 연구를 수행하였다. 마이크로파 전력증폭기에서 대부분 사용되고 있는 GaAs HBT 디바이스는 성능 최적화를 위해 많은 기술들이 개발되었다. 하지만, 열 적 최적화의 관점에서 기존의 열 모델들이 굉장히 많은 시간과 복잡한 분석을 요구하기 때문에, 회로 설계자들은 다양한 전력증폭기 설계를 위해서 적합한 모델을 적용할 수 없었다. 위와 같은 문제를 해결하기 위해서는, 회로 설계자들이 사용하기에 빠르고, 쉬우며, 또한 정확한 분석을 위한 간소화된 모델이 요구된다. 우선, 멀티-핑거 (multi-finger) InGaAs/GaAs HBT 디바이스의 열 저항을 예측하기 위해서 Fourier’s law를 기초로 한 새로운 추출 방법을 제안한다. 자기 가열 열 저항 (self-heating thermal resistance)뿐만 아니라 핑거들 간의 열 적 결합 저항(thermal coupling resistance)까지 계산함으로써, 계산의 간단함을 유지하면서 다양한 크기를 갖는 멀티-핑거 디바이스의 열 저항 값이 정확하게 계산된다. 아이디어 검증을 위해서, 제안된 방법은 유한요소법(FEM)과 3-D 라플라스 공식의 해석법과 같은 분석적 방법들뿐만 아니라, 측정기반 방법과도 비교 되었으며, 4-finger HBT 디바이스에 대해서 추출된 열 저항 결과들은 분석적 방법들과 9%이내, 측정기반 방법과 9%이내의 오차로 좋은 일치함을 보였다. 다음으로는, InGaP/GaAs HBT용 멀티-셀(multi-cell) 디바이스들의 열 적 결합 저항을 반영하기 위한 쉽고 빠른 열 저항 추출 방법을 제시한다. 멀티-핑거 디바이스에서 제안된 방법을 이용하여 셀 간의 열 적 결합 저항을 계산함으로써, 이 또한 계산의 간단함을 유지하면서 실제 전력증폭기에 사용되고 있는 멀티-셀 디바이스들의 열 저항 값이 정확하게 계산된다. 검증하기 위해서, 제안된 방법은 분석적 방법 및 측정기반 방법과 비교 되었다. 바이너리-타입(Binary-type) 및 리니어-타입(Linear-type)의 두 구조에 대해서, 추출된 열 저항 결과들은 분석적 방법과 5% 이내, 측정기반 방법과 4.5%이내의 오차를 갖는 좋은 일치함을 보였다. 추가적으로, 계산된 접합 온도(junction temperature) 결과들은 IR-scope로부터 측정된 열 사진과 비교하여 5°C 이내의 편차를 보였다. 최종적으로, 제안된 멀티-핑거, 멀티-셀의 열 저항 추출 모델을 기반으로 전력증폭기를 위한 디바이스 설계 최적화 방법을 제시한다. 기존의 디바이스 설계는 가운데로 열이 몰리는 열 집중화 문제를 가지고 있기 때문에, 기존 전력증폭기에서 성능 저하 문제가 발생되고 있다. 이러한 문제를 해결하기 위해서, 본 논문은 두 가지 방법을 제시한다 (열 적으로 최적화된 디바이스 레이아웃, 베이스 발라스트 저항(base ballast resistor)으로의 디바이스 최적화). 제안된 방법의 유용성을 검증하기 위해서, InGaP/GaAs HBT 공정을 이용하여 1.88 GHz 전력증폭기가 제작되었고 CW 동작 모드에서 측정되었다. P3dB 측정 조건에서 기존 전력증폭기와 비교하면, 제안된 전력증폭기들은 출력 전력과 효율에서 각각 0.1~0.2 dB와 1.0~1.6% 개선됨을 보였다. 더불어, 제안된 전력증폭기들의 열 사진은 IR-scope로 측정되었고, 기존 전력증폭기와 비교하여 최대 약 8°C의 온도 감소를 보였다. 추가적으로, thermal runaway 실험을 진행하였다. 바이너리 / 리니어 디바이스에 대해 제안된 전력증폭기는 전류 게인 붕괴 및 최대 출력 전력에서 각각 ~3% / ~5% 및 0.2 dB / 0.3 dB 향상 됨을 보였다. 따라서, 간단하고 빠르면 정확한 분석을 위해서 제안된 방법의 효용성은 검증을 통해서 확인 되었기 때문에, 본 연구는 회로 설계자가 전력증폭기 개발을 위해서 도움이 될 것이다.; In this dissertation, a study on thermal modeling and management of InGaP/GaAs heterojunction bipolar transistor (HBT) for power amplifier (PA) applications is performed. The GaAs HBT device mainly used for a microwave PA has been developed using many investigations for performance optimization. In terms of thermal optimization, however, since the previous thermal models excessively required long time and/or complicated analysis, the circuit designers cannot apply a suitable model for various PA design. In order to solve the issue above, the simplified model for fast/comprehensible/accurate analysis is required to use for circuit designers. Firstly, a novel extraction method to estimate the thermal resistance (Rth) of a multi-finger InGaP/GaAs HBT device is presented based on the simplified Fourier’s law. By calculating the thermal coupling resistance between fingers on top of the self-heating thermal resistance, Rth of multi-finger devices for various dimensions are accurately calculated while maintaining the simplicity of calculation. To verify the idea, the proposed method is compared with the measurement-based method as well as the analytic methods such as the finite-element method (FEM) and solution of 3-D Laplace’s equation. For 4-finger HBT devices, the extracted Rth-results showed good agreements with the analytic methods within an error of 9% and the measurement-based results with a deviation of 7.4%, thus convincing the usefulness of the proposed method. Secondly, a simple/fast Rth-extraction method to reflect the thermally coupled effects of multi-cell devices for InGaP/GaAs HBT is presented. By calculating the thermal coupling resistance between cells using the same method used for a multi-finger device, Rth of multi-cell devices used in PA practically are also accurately calculated while maintaining the simplicity of calculation. To validate, the proposed method is compared with the measurement-based method as well as the analytic method as the finite-element method (FEM). For the two structures, Binary-type and Linear-type, the extracted Rth-results showed good agreements with the analytic method within an error of 8% and the measurement-based results with a deviation of 4.5%. Additionally, the calculated junction temperature results showed good agreement with the thermal profiles measured by IR-scope within a deviation of 5°C. Finally, an optimized device design suggestion for PA applications is presented based on proposed method for Rth-extraction of multi-finger device and multi-cell structure. Since a conventional device design has the thermal problem focused on the center, the performance degradation is occurring in conventional PA. To resolve the problem above, this study presents the two suggestions: device layout optimized thermally and device optimization by base ballast resistor. To verify the usefulness of proposed approaches, the 1.88 GHz PAs using an InGaP/GaAs HBT process for practical PA applications were fabricated and measured under CW operating mode. Compared to a conventional PA under the test condition of P3dB, the proposed PAs showed that output power and efficiency improved 0.1~0.2 dB and 1.0~1.6%, respectively. Furthermore, thermal profile of proposed PAs was measured by IR-scope and showed maximum temperature reducing of ~8°C, compared with conventional PA. In addition, the thermal runaway test was run. The proposed PAs for Binary / Linear devices showed that the current-gain-collapse and maximum output power are improved as ~3% / ~5% and 0.2 dB / 0.3 dB, respectively. Thus, since the usefulness of proposed method for simple/fast/accuracy analysis has been established by verifications, this study can help to develop PA applications for circuit designer.