Improvement of an SiGe HBT Structure by Changing the Germanium Profile (original) (raw)
Microwave and Optical Technology Letters, 2005
In this paper, the effects of Ge content and profile shape on the performance of a SiGe-based heterojunction bipolar transistor (HBT) are investigated. The common-emitter current gain, the early voltage, and the transit time of SiGe HBTs are calculated and computed for different Ge profiles as well as the total Ge content in the base, considering uniform base-doping. Then the effects of the above on the cutoff frequency f and the maximum oscillation frequency fmax of the HBT are shown. It is seen that both the forward current gain and the transit time decrease with the change in profile from box to triangle, but the early voltage increases with a similar change. The increase of Ge content for the same profile results in a decrease of transit time. From the analysis, a large increase in f and fmax can be predicted with a suitable choice of Ge profile and the total Ge content in the base. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 47: 247–254, 2005; Published online in Wiley InterScience (www.interscience.wiley.com).
Semiconductor Science and Technology, 2014
We describe a reliable technique to separate the different contributions to the apparent base resistance (R B = R Bx + X R Bi ) of silicon germanium carbon (SiGe:C) heterojunction bipolar transistors (HBTs). The extrinsic base resistance (R Bx ) is quantified using small-signal measurements. The base-collector junction distribution factor (X) and the intrinsic base resistance (R Bi ) are extracted from high frequency noise (MWN) measurements. This method is applied to five different SiGe:C HBTs varying in base doping level and germanium content. The results show that high doping levels improve high frequency noise performances while germanium gradient helps to maintain outstanding dynamic performances. This method could be used to elucidate the base technological configuration that ensures low noise together with remarkable dynamic performances in state-of-the-art SiGe:C HBTs.
Parasitic energy barriers in SiGe HBTs
IEEE Electron Device Letters, 2000
The improvement in electrical performance of SiGe heterojunction bipolar transistors compared to conventional homojunction transistors depends strongly on the position of the SiGe/Si transition relative to the emitter and collector junctions. Parasitic energy barriers can easily be introduced during processing. Measurements and calculations of experimental n-p-n HBT's will be presented, showing that a parasitic conductionband barrier at the base-collector junction reduces the collector current and the cutoff frequency. A simple analytical model explains the fT degradation, caused by the reduction of the collector current and a pileup of minority carriers in the base. With the model the effective height and width of the barrier can also be derived from the measured collector current enhancement factor Ic(SiGe)/Zc(Si).
SiGe Heterojunction Bipolar Transistors
2003
Physical Constants and Properties of Silicon and Silicon-Germanium xvii List of Symbols xix xii CONTENTS 12.4 Gate Delay Estimation 244 12.5 Optimization Procedure 12.6 Optimization of Silicon Bipolar Technology 246 12.7 Optimization of Silicon-Germanium HBT Technology 251 References 255 Index 257 PREFACE xv in Chapter 9. These include buried layer, epitaxy, isolation, selectiveimplanted-collector, base and emitter. Examples are then given of four types of bipolar process: double polysilicon self-aligned bipolar, single polysilicon bipolar, complementary bipolar and BiCMOS. Silicon-germanium heterojunction bipolar technology is introduced in Chapter 10 and the two approaches of differential epitaxy and selective epitaxy are outlined. Silicon-germanium-carbon HBT processes and germanium implanted HBT processes are also described. The main application of SiGe HBT technologies is in radio frequency circuits and so integrated circuit passives are described, including resistors, capacitors, inductors, and varactor diodes. Chapters 11 and 12 describe the use of bipolar transistors and SiGe HBTs in circuits. Chapter 11 describes compact bipolar transistor models, beginning with the Ebers-Moll model and building towards the Gummel-Poon model in easy-to-understand stages. The well known SPICE2G bipolar transistor model is described in detail and the chapter concludes with consideration of the VBIC95 and Mextram bipolar transistor models. In Chapter 12 optimization of the overall process, transistor and circuit design is discussed using a quasi-analytical expression for the gate delay of an ECL logic gate in terms of all the time constants of the circuit. The application of the gate delay expression is demonstrated by case studies for the double polysilicon self-aligned bipolar technology and the SiGe HBT technology. Many people have contributed directly and indirectly to the writing of this book, and it would be impossible to find the space to thank them all. Nevertheless, I would like to identify a number of colleagues who have made particularly large contributions to this project. First, acknowledgements should go to my colleagues in the Microelectronics Group at Southampton University, with whom I have had numerous stimulating discussions about device physics. These include Henri Kemhadjian, Greg Parker, Arthur Brunnschweiler, Alan Evans, Kees de Groot and Darren Bagnall. A debt of gratitude is also owed to my past and present research students, who have contributed greatly to my understanding of device physics in general and bipolar transistors in particular. These include Bus Soerowirdjo,
Effects of Boron and Germanium Base Profiles on SiGe and SiGe:C BJT Characteristics
32nd European Solid-State Device Research Conference, 2002
We present results of an experiment with different boron and germanium profiles aimed at the optimization of the vertical profile in SiGe BJT. Simulation and experimental results show the importance of correct positioning of the germanium profile relative to boron profile to achieve maximum fTpeak. Introduction of the low-doped base region at the emitter side if well optimized improves the base current ideality and low-current fT without fTpeak reduction.
Analysis of temperature dependence of Si-Ge HBT
Proceedings of the 8th International Conference on VLSI Design, 1995
In this paper the dependence of characteristics of SiGe heterojunction bipolar transistors on Ge mle-fraction and also variation of gain with temperature are presented. The simulation is carried out using a two dimensional device simulator, BISOF, based on finite element method. It is observed that the current gain of graded HBT improves when the temperature falls from 300 K to 200 K which matches well with the available experimental results.
Germanium profile design options for SiGe LEC HBTs
Solid-State Electronics, 2004
Silicon-Germanium (SiGe) heterojunction bipolar transistors (HBTs) with a low emitter doping concentration (LEC) are investigated with respect to their electrical characteristics in dependence of the Germanium profile shape. The study is based on one-dimensional (1D) device simulation using a realistic doping and Ge profile as baseline. While keeping the doping profile unchanged the Ge profile is modified to evaluate its impact on major electrical figure of merits such as transit frequency, ideality factor, early voltage, noise figure, as well as on process control monitors (PCMs) such as internal base sheet resistance and area specific depletion capacitances. The variations in electrical characteristics and PCMs are briefly explained on a theoretical basis with regard to the impact on compact and statistical modeling.
Electric field effects associated with the backside Ge profile in SiGe HBTs
Solid-State Electronics, 2002
A comprehensive investigation of electric field effects associated with the backside Ge profile in SiGe heterojunction bipolar transistors (HBTs) is conducted using calibrated simulations. We show for the first time that the backside Ge retrograde can alter the local electric field distribution in the base-collector space-charge region near the SiGe to Si heterojunction, thereby affecting the impact ionization and the apparent neutral base recombination (NBR) ðI B ðV CB Þ=I B ðV CB ¼ 0ÞÞ of SiGe HBTs. The changes in the electric field induced by the Ge-induced band offsets contributes to a decrease of the observed impact ionization between comparably doped SiGe HBTs and Si bipolar junction transistors (BJTs), as well as an improved V CB dependence of I B (apparent decrease in NBR). Experimental data on SiGe HBTs with various Ge profiles and a Si BJT control are used to support our claims. Ó
Ge-profile design for improved linearity of SiGe double HBTs
IEEE Electron Device Letters, 2000
The influence of Ge-profile design on SiGe HBT linearity-harmonic distortion has been quantified using finite element physical device simulation. It was demonstrated that proper Ge-profile tailoring allows the linearity to be improved for both low-and high-current operation. High injection heterojunction barrier effects are shown to have a significant influence on the higher order harmonics. The influence of the Ge-profile design on linearity was found to be comparable to the influence from the epitaxial collector doping profile.
On the optimization of SiGe-base bipolar transistors
Electron Devices, …, 1996
Advanced epitaxial growth of strained SiGe into a Si substrate enhances the freedom for designing high speed bipolar transistors. Devices can be designed by altering the Ge percentage, a procedure known as bandgap engineering. An optimization study on NPN SiGe-base bipolar transistors has been performed using computer simulations focussing on the effect of the Ge profile on the electrical characteristics. In this study it is shown that the base Gummel number is of major importance on the maximum cutoff frequency and the Ge-grading itself, which induces a quasielectric field, is of minor importance. Because of the outdiffusion of the boron dope in the base and the relatively thin critical layer thickness of approximately 600 A it appears that a box-like Ge profile with the leading edge approximately in the middle of the base is optimal. The predicted maximum cutoff frequency is 45 GHz, a sheet resistance of 8.5 kR/O and current gain of 80. The optimized device was fabricated and measurements were performed showing good agreement with the simulations. from calculations especially of the maximum cutoff frequency ft,max and the current gain H f , for different Ge profiles.