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5Self-heating Compensation of SiGe HBT
Y.F. Adamov, V.P. Timochenkov
National Research University of Electronic Technology, Moscow УДК 621.385
Компенсация саморазогрева в SiGe ГБТ
Ю.Ф. Адамов, В.П. Тимошенков
Национальный исследовательский университет МИЭТ
In this paper, self-heating problem of the SiGe HBT is described. Two schematic realizations for compensation of thermo- heated process in bipolar transistors are proposed.
Keywords: Heterojunction Bipolar Transistors (HBTs), Silicon Germanium (SiGe), Thermal impedance.
Рассмотрены проблемы, связанные с саморазогревом SiGe биполярных гете-ропереходных транзисторов. Предложены схемы, обеспечивающие компенсацию эффекта термоэффектов, основанные на использовании КМОП элементной базы.
Ключевые слова: гетеропереходный биполярный транзистор; кремний-германий (SiGe); термосопротивление.
High speed applications like car radar modules (24 and 77GHz), wireless LAN (40/60 GHz) system [1] as well as 100Gb/s data communication [2] require to have high speed Heterojunction Bipolar Transistors (HBT) which should operate at high current. High current mode of operation will effect of self-heated process in transistor structure which reflected on characteristics of the devices. Thermal issue is one of the key factors limiting the performance, reliability and improvement of the devices and integrated circuits. Therefore compensation of thermal effect on schematic level is very important task.
It is well known that the temperature of transistors active region is dependent from his working mode of operation and temperature resistance of transistor structure. Self-heating of hetero-junction bipolar transistor (SiGe HBT) structure with germanium doped base will decrease of base-emitter voltage at constant collector current or increase collector current at constant base-emitter voltage. At self-heating condition positive electro-temperature feedback is exist. Transistor self-heating will increase collector current and stimulate future temperature increasing.
Time constant of self-heating strongly dependent from transistor structure and his size and will decrease if size of transistor is decrease. The value of time constant is about microsecond if modern process is used. Self-heating effect has spectrum from zero to tens of megahertz.
Modern telecommunication and radio techniques are required increasing of frequency response which strongly required to improve speed parameters of HBT. Comparison of regular Si bipolar transistor (BT) and HBT shows that HBT has in 5-7 time better frequency response at high current density. But high current density is the reason of self-heating effect. So design technique should take in account this process.
In integrated circuits (IC) of radio bands high path filters (HPF) are used for eliminating of low frequency heating processes of HBTs. In ultra wide band blocks special temperature compensated schematics are required.
Self-heating process is introduced in modern models of HBT and silicon bipolar transistors such as VBIC, HiCUM, or MEXTREM [3, 4]. But engineering calculations of internal thermo-resistance are needed to be done. The examples of schematics which compensate self-heating effect will be present below.
© Y.F. Adamov, V.P. Timochenkov, 2015
High frequency amplifier based on HBTs with common emitter are presented in Fig.1 [5]. It consists of amplification stage 1, reference current block 2, which stabilized collector current of HBT when temperature of p-n junction will change (without of self-heating), and block 3 which compensates self-heating. Current of block 2 supplies to the base of output transistor by two paths. Current in first path is depended from ambient temperature and defined from junction temperature of HBT transistor in reference block. Current in second path is depended from difference of base-emitter voltage for HBT transistor in reference block and base emitter voltage of transistor in amplification stage. This current supplies by differential amplification stage. Inputs of this differential stage are connected to base of HBT in amplification stage and reference block throw low path filter (LPF). When temperature is increased current gain of transistor is decreased. If self-heating process is going on base-emitter voltage is decreased at constant base current. Difference between base-emitter voltage for HBT in reference block and base-emitter voltage for HBT in amplification stage will change output current in compensation stage and base current of HBT in amplification stage. Gain of compensation stage is calculated by using of gain temperature dependence and Vbe temperature dependence.
Fig.1. Self-heating compensation in SiGe HBT for common emitter stage: 1 - amplification stage; 2 - reference current source with current mirror; 3 - block of compensation self-heating process
Second example is presented in Fig.2 [6]. This is wide band differential amplifier with block correction of thermo-electric coupling in HBTs. It includes differential amplification stage 1, loading element 2 and block correction of thermo-electric coupling 3. Inputs of block correction are connected to inputs of amplifier throw LPF. Additionally compensation block can provide extension of region of common mode signal. The value of current for input differential stage I1-I2 is calculated as difference between main current source I1 and current source of correction circuit I2. When level of common mode signal is decreased current for main source compensated by decreasing of compensation current.
For self-heating compensation of input HBTs in correction block there is additional input stage (4) based on n channel metal oxide transistors (NMOS). Signal coming to the input of the additional stage throw LPF which selects low frequency spectrum of the temperature dependent signal. Additional and main input stages are switch ON/OFF in opposite polarity and his output current is summing on load. At high frequency operation mode there is no temperature difference for HBTs so there is no self-heating. If input signal contains spectrum with frequency 1 MHz or less, self-heating of input HBTs are existed. Current of colder transistor is decreased but current of hotter transistor is increased. It will lead of differential signal on inputs of additional stage. Increasing of output current for more hotter HBT will compensate the value of the current in additional stage based on NMOS transistors. Self-heating process in MOS transistors are negligible, because they operate at low frequencies. In that case it is possible to provide low current density for this NMOS transistors.
Fig.2. Self-heating compensation in SiGe HBT for differential stage: 1 - differential stage; 2 - load elemets; 3 - block of compensation self-heating process; 4 - block of compensation common mode signal
Self-heating simulation at different temperature was done for schematic presented in Fig.3. In this configuration reference block consist of transistors 74.1-74.8 and amplification stages based on 78, 719, 710-712. Feedback path organized by NMOS transistors 713-714, 76-77, 71-72. Temperature dependent current throw 720 generated by reference block and current mirror based on transistors 716-717 and 720. Output signal was checked on collectors of the transistors 718-719. In the temperature range from minus 25 to 125 °C and power supply voltage variation from 3.0 to 3.5 volt output voltage of the differential amplifier changes by no more than 10%.
Fig.3. Schematic for compensation of self-heating process
It was shown that correct functionality of the analog blocks based on hetero-junction bipolar transistors require to have additional block which will correct static characteristics when self heating of the HBTs are exist. Schematic of this blocks are proposed.
References
1. Schwerd M., Seck M., Huttner T. et al. A manufacturable 0.35 /spl mu/m 150 GHz f/sub T/ SiGe:C bipolar RF technology, in 2003 Topical Meeting on Silicon Monolithic Integrated Circuits in RF Systems, 2003. Digest of Papers, 2003, pp. 10-13.
2. Moller M. High-speed electronic circuits for 100 Gb/s transport networks, in Optical Fiber Communication (OFC), collocated National Fiber Optic Engineers Conference, 2010 Conference on (OFC/NFOEC), 2010, pp. 1-3.
3. Schroter M. Staying current with HICUM // IEEE Circuits and Devices Magazine, 2002, № 18(3), pp. 16-25.
4. Rei H.M., Schroter M. A compact physical large-signal model for high-speed bipolar transistors at high current densities. Part II: Two-dimensional model and experimental results, IEEE Trans. Electron Dev., 1987, № 34, pp. 1752-1761.
5. Patent Russian Federation N 2509407 at June 28, 2012.
6. Patent Russian Federation N 2462813 at October 06, 2011.
Received December 3, 2014
Yuri Adamov - doctor of science, Microelectronics departments MIET. Scientific interests are: High frequency IC design, Telecommunication Radio communication.
Valeri Timoshenkov - doctor of science, Integrated Electronics and Microsystems departments MIET. Scientific interests are: High frequency IC design, Telecommunication Radio communication. E-mail: valeri04@hotmail.com
