- Email: [email protected]
Noise behavior in SiGe devices
Solid-State Electronics 45 (2001) 1891±1897
Review
Noise behavior in SiGe devices M. Regis a,b, M. Borgarino a, L. Bary a, O. Llopis a, J. Graeuil a, L. Escotte a, U. Koenig c, R. Plana a,* a LAAS-CNRS, 7 Av du Colonel Roche, 31077 Toulouse Cedex 04, France SiGe Inc. 2680, Queensview Drive, Suite 150, Ont., Ottawa, Canada K2B8J9 Daimler Chrysler Research Center, Wilhelm Runge Strasse 11, 89081 Ulm, Germany b
c
Accepted 8 May 2001
Abstract This paper presents an overview of SiGe technologies and their corresponding noise properties both in the high frequency and low frequency range. We demonstrate that SiGe bipolar technology exhibits impressive low frequency noise performance with an excess corner noise frequency in the 1 kHz range. These results have been validated by low phase noise microwave oscillators based on SiGe material. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: SiGe Heterojunction; Noise ®gure; 1/f noise; Low phase noise
Contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1891 2. SiGe technological overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.1. The SiGe HBT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.2. The SiGe FET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 3. High frequency noise properties of SiGe devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 4. Low frequency noise and phase noise behavior of SiGe devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1894 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1895 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1896
1. Introduction The next generation of telecommunication systems will feature a dramatic change in term of electrical per*
Corresponding author. Fax: +33-561-336-969. E-mail address: [email protected] (R. Plana).
formance. Among the latter, an important issue is the ``low noise character'' that should be achieved to improve the sensitivity and the bit rate error of future telecommunication systems. This makes silicon a new contender for RF and microwave circuits. It is well understood that silicon is a very low cost and low noise technology. The problem was related to the fact that silicon was considered as a ``slow semi-conductor'' for a
0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 1 ) 0 0 2 2 9 - 5
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long time. This drawback has been overcome by introducing a heterostructure concept through silicon germanium technology (SiGe). This makes it possible to achieve both bipolar and ®eld eect transistors devices through band gap engineering of silicon. Of course as the SiGe/Si heterostructure forms a strained layer, it is important to assess the noise behavior of these kind of devices. It has been previously demonstrated that the noise performance of a RF and microwave receiver comes from two dierent kind of noise sources. The ®rst one deals with the noise of the ampli®er and the mixer that is due to thermal noise and shot noise generated within the active devices. The second kind of noise is a little more complicated and is related to noise of the RF and microwave oscillator represented by phase noise. It has been shown that phase noise of a microwave oscillator comes from low frequency noise sources which are up-converted into phase noise through the nonlinear behavior of the active device. In this paper, we try to give an outlook on the noise properties exhibited by SiGe devices. Section 1 will address a brief overview of SiGe technology. Section 2 will focus on the high frequency noise of SiGe devices. In Section 3, we will present the low frequency noise performance of SiGe devices and its impact on phase noise of a microwave oscillator. Conclusions will be given in Section 4.
2. SiGe technological overview 2.1. The SiGe HBT In SiGe HBTs the thin base is made from a SiGe alloy. For the amount of Ge in the base region and the associated doping pro®le two technological concepts can be used to realize an heterojunction bipolar transistor. The ®rst one leaves the doping pro®le nearly unchanged with respect to a BJT but a small and graded Ge content is added from the emitter to collector junction. The Ge grading induces band gap narrowing along the base region resulting in a drift ®eld which improves the base transit time. The advantage of this concept is related to its BiCMOS compatibility. One drawback is associated
with the low base doping level which degrades the noise performance of the devices and which is overcome by scaling down the emitter. Note that the SiGe base is formed by selective or nonselective epitaxial growth. The second approach results in a true HBT (Ne Nb) since both base and emitter pro®les are changed and the Ge content ranges from 20% to 30%. The corresponding band gap narrowing allows a very high base doping level which is favourable for low noise. Among the others advantages we can list: higher injection eciency, lower emitter transit time and lower base transit time. Nevertheless some drawbacks occur related to the lattice mismatch of the SiGe alloy with respect to silicon, the large surface recombination due to the poor quality of the SiGe oxide. This approach further requires low temperature budget processing which is not compatible with current BiCMOS technology. We will refer in this paper the former as ``gradual'' (G) HBT and the latter as ``abrupt'' (A). In order to complete this section, we propose in Table 1 a comparison of the microwave performances of the dierent SiGe bipolar technologies. We can observe that most of them lead to a good performance in the millimeter-wave range which makes feasible the design of circuits at least up to 10 GHz. We just end the discussion with a new kind of SiGe technology developed by Osten et al. [10] which introduces some carbon in the SiGe base layer resulting to the suppression of the transient enhanced diusion of boron atom which is a limitation for lateral emitter scaling. SiGe:C HBTs feature fmax in the 70 GHz range and cuto frequency in the 50 GHz range. SiGe HBT technology becomes to be ready and some foundries are already available. Concerning the unipolar devices (also called FET devices) the situation is more complex and the technology is still at the ``laboratory state''. Nevertheless, we give some guidelines concerning these devices. 2.2. The SiGe FET Most of the present SiGe FETs come from the modulation doped type (MODFETs). They consist of a thin well with quantized states in which the carriers move collision-free. They will take bene®ts about a very high mobility. N wells are Si and p wells are SiGe typi-
Table 1 A review of the microwave performances of SiGe HBT Company
Process
Ft (GHz)
Fmax (GHz)
Type
Reference
Hitachi IBM DC ST Motorola SiGe Inc Siemens
UHV/CVD CVD MBE RPCVD RTCVD CVD CVD
130 117 116 50 ± 35 70
± 65 160 70 45 50 74
G G A G G G G
[1] [2,3] [4,5] [6] [7] [8] [9]
M. Regis et al. / Solid-State Electronics 45 (2001) 1891±1897
cally up to 30%. More details can be found in Refs. [11± 13]. Finally, other works address p channel MOSFETs consisting in a SiGe channel on Si substrate or Ge channel on Ge substrate [14,15]. Recently, an fmax of 81 GHz has been found for a 0.18 lm T-gate n MODFET [16] and theoretical performances of n and p channel devices have demonstrated transconductance above 1 S/ mm and cut-o frequencies in the 200 GHz range [17]. Nevertheless in this kind of device, the noise performances are not those expected mainly due to gate leakage current and numerous surface states. These points will be further discussed later in the paper.
3. High frequency noise properties of SiGe devices The high frequency noise behavior of an active device is assimilated to a two ports network where the noise is analyzed through the determination of the microwave noise parameters (minimum noise ®gure, equivalent noise resistance and optimum noise impedance). In a bipolar device, the high frequency noise is driven by the thermal noise (i.e. the emitter, base resistance), the shot noise (i.e. the DC bias) and the dierent transit time constant within the device (i.e. cut-o frequency). From previous works developed by Escotte [18] and Voiginescu [19], it is possible to derive the minimum noise ®gure expression as follows: s n f 2Ic fT2 n2 fT2 Fmin 1 Re Rb 1 2 b fT Vt bf b f2 1 where n represents the ideality factor of the junction, b the current gain of the device, Ic the collector current. Re and Rb represent the emitter and base resistance respectively, fT the cut-o frequency, Vt the thermodynamic voltage and f the operating frequency. From this expression, we can see that high current gain (b), high fT and low base and emitter resistance are suitable to minimize the minimum noise ®gure value. Note that SiGe technology allows to ful®ll all these requirements due to the SiGe heterostructure which pro-
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vides a decoupling between the current gain and the geometrical dimensions of the device contrary to BJT technology. Table 2 summarizes the high frequency noise performance of a set of silicon based devices. The results indicate that abrupt HBT is able to provide a lower value of Fmin than the gradual HBT due to the higher base doping. Nevertheless in the case of gradual SiGe HBT, appropriate shape of the base layer (Ge rate and doping) as well as a speci®c design of the extrinsic base resistance should feature similar performances as reported from theoretical simulations [23]. We have to note that for most of the RF and microwave applications, the high frequency noise performance featured by SiGe bipolar technology is sucient to meet the technical requirements. Concerning the noise performance of FET devices, they can be simply assess through the following expression [24]: p f q Fmin 1 2 P 2 gm0 Rs Rg fT where P is an empirical parameter associated with the technology, Rs and Rg are the source and gate resistance respectively. From this expression, it is understood that we have to increase the cut-frequency and to minimize the source and gate resistance to achieve a low noise ®gure. The situation concerning the SiGe FET is not well established as the technology is still under progress and the today results are not representative of the capabilities of this kind of device. Fig. 1 presents the minimum noise ®gure evolution versus frequency for a SiGe n-MODFETs featuring gate geometry of 0:25 50 lm2 . The results indicate a minimum noise ®gure value ranging from 2 to 5 dB which is not a good result compared to those obtained with III±V devices. This relatively high value for the minimum noise ®gure is mainly attributed to an excess gate leakage due to the strained heterostructure. We can nevertheless expect improvements in the future. Concerning the p-MODFETs device, Koester et al. [25] have presented minimum noise
Table 2 Comparative study of noise performances of microwave silicon based devices Device
Fmin (dB) @2 GHz
Fmin (dB) @10 GHz
Reference
SiGe abrupt SiGe gradual Si BJT conventional SiGe graduala
0.5 0.6 1
0.8 1.8 2.2
[20] [21] [22]
<0.4
0.8
[23]
a
Theoretical results.
Fig. 1. Minimum noise ®gure evolution of a nSiGe MODFET.
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®gure ranging from 1.1 to 3 dB from 1 to 18 GHz which is to our knowledge the best result reported on SiGe FET device. We have to note that this result has been obtained on devices realized on high resistivity substrate and that better performances could be obtained by using sapphire substrate. We believe that there is provision to improve the noise behavior of FET device to get performance closer to III±V based devices. The next section of this paper will address the low frequency noise behavior of SiGe device and their in¯uence on the phase noise of microwave oscillator.
4. Low frequency noise and phase noise behavior of SiGe devices Contrary to the FET device, where only one noise generator is needed related to the drain current ¯uctuations, in a bipolar device the noise behavior is fully described by the determination of both the base current and collector current ¯uctuations including their correlation [26]. Nevertheless this measurement technique features some drawbacks like narrow band range frequency, high sensitivity to the external environment and this why we prefer another alternative consisting to measure the input noise voltage and current generators including their correlation using a multiple impedance technique. This technique is time consuming but it provides more reliable measurements over a very large frequency range [27]. We have further demonstrated that, the input noise generators lead to a better accuracy in term of phase noise prediction [28,29]. Concerning the performances of abrupt device, a too high Ge ratio results to a tremendous degradation of the input noise current generator as reported in Fig. 2. This behavior is related to the strain in the base layer which creates some dislocation as well as surface states
Fig. 2. Input noise current generator versus Ge ratio in the base for abrupt HBT.
resulting to an increase of the recombination rate. We have further demonstrated that the passivation of the device plays a very important role in the low frequency noise performance. Nevertheless, on optimized devices, the recombination rate is minimized and we have measured impressive noise performance where the 1=f noise corner frequency is lower than 1 kHz which is a similar result than those obtained on BJT devices. Concerning the performances of the graded HBT, they are similar, 1=f noise component and an excess corner noise frequency lower than 1 kHz [30,31]. Vempati et al. have observed that most of noise is related to trapping± detrapping eect in the intrinsic emitter±base region [32]. Others works have shown that some additional noise can be found in the emitter resistance [33]. Others authors have found that some noise could be generated in the extrinsic base region related to some front edge implantation damage [34] as well as some surface problems associated with the cleaning method used to realize the CMOS devices [30]. Concerning the gradual SiGe HBT, the evolution of the input noise voltage versus frequency indicate that higher the emitter length lower is the noise. The same behavior is achieved for the input noise current generator. The evolution of the correlation coecient indicates a value dierent of 1 con®rming that not only one noise source is present within the device. From measurements versus bias and emitter length, we have de®ned a scaleable low frequency noise model involving 1=f noise source located at the emitter±base junction, the output of the device, the emitter and the base resistance and at the extrinsic base region related to some surface recombination. In order to compare the noise performance of SiGe HBT with others HBT devices, we present in Fig. 3 the evolution of the noise voltage versus frequency for three typical HBT, one SiGe BiCMOS
Fig. 3. Input noise voltage generator versus frequency for a set of microwave HBT.
M. Regis et al. / Solid-State Electronics 45 (2001) 1891±1897
Fig. 4. Frequency evolution of the input noise voltage for a nSiGe MODFET.
HBT, a GaInP/GaAs HBTs and an InP/InGaAs HBTs. The results con®rm the attractive capabilities of SiGe devices with respect to the III±V one mainly related with lower traps density and lower surface recombination velocity in silicon based device. Concerning the low frequency noise properties of SiGe FET device, there is very few data in the literature. Okhonin [35] presented some data concerning the low frequency properties of pSiGe MOSFET where they demonstrated that nothing wrong was given by the Ge introduction. Nevertheless the data they published concern only a very speci®c bias level and we believe that they are not representative of the ``real'' low frequency noise behavior. Fig. 4 presents the frequency evolution of the input noise voltage generator for a nSiGe MODFET where we can observe a high 1=f noise component as well as a ``lorentzian'' noise component in the 10 kHz range which is a typical behavior already observed in III±V HEMT or PHEMT. The next section will address the phase noise behavior of this kind of technology. In order to evaluate the suitability of an active devices with respect to the phase noise applications, we consider two quantities: (1) RF or microwave residual phase noise near the carrier for an open loop con®gured device; (2) near carrier oscillator RF or microwave phase noise. In Fig. 5, we have plotted the residual phase noise measurements performed at 10 GHz on a set of abrupt SiGe HBTs. The results demonstrate that like on the low frequency noise range, the devices must feature a good passivation and to be free of recombination to exhibit good residual phase noise performances. This behavior indicates that, the device exhibiting ultimate microwave performances with no passivation will not be suited for low phase noise applications. The next step concerns the real phase noise capabilities. In order to investigate this point, we built a 4 GHz parallel feedback oscillator with
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Fig. 5. Residual phase noise at 10 GHz for three kind of SiGe abrupt HBT.
Fig. 6. Comparative study of 4 GHz oscillator realized with dierent kind of active devices.
devices featuring dierent emitter length and the phase noise measurements demonstrated that higher is the emitter length lower is the phase noise. Finally, Fig. 6 shows a comparison of 4 GHz oscillators realized with dierent active devices and the results indicate that there is no dierence between Si and SiGe technology which demonstrate superior phase noise performances of at least 10 dB.
5. Conclusions In this paper, we have presented an outlook about the SiGe technology and their noise properties both in the high frequency and low frequency range. We demonstrated that SiGe HBT exhibited very attractive high
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frequency noise performances due to the improvement given by the SiGe/Si heterojunction. Concerning the FET devices, there is still provision for progress in term of noise to compete with III±V devices speci®cally at the gate level to avoid the gate leakage. The most important advantage of SiGe bipolar technology deals with their low frequency noise performances as well as appropriate design is applied in order to minimize the strain and the surface recombination. The impact on phase noise performances has been also demonstrated con®rming the superiority of SiGe technology over the III±V ones. References [1] Oda K, Ohue E, Tanabe M, Shimamoto H, Onai T, Washio K. 130 GHz fT SiGe HBT technology. Tech Dig IEDM 97, Washington, 7±10 December 1997. p. 791±4. [2] Crabbe EF, Meyerson BS, Stork JMC, Harame DL. Vertical optimization of very high frequency epitaxial Siand SiGe-base bipolar transistors. Tech Dig IEDM 93. p. 83±6. [3] Harame D, Larson L, Case M, Kovacic S, Voiginescu S, Tewksbury T, Nguyen-Ngoc D, Stein K, Cressler J, Jeng SJ, Malinowski J, Groves R, Eld E, Sunderland D, Rensch D, Gilbert M, Schonenberg K, Ahlgren D, Rosenbaum S, Glenn J, Meyerson B. SiGe HBT technology: device and applications issues. Tech Dig IEDM 95, 1995. p. 731±4. [4] Schupen A, Gruhle A, Erben U, Kibbel H, Konig U. SiGe-HBTs with high ft at moderate current densities. Electron Lett 1994;14(30):1187±9. [5] Gruhle A, Schupen A. Recent advances with SiGe heterojunction bipolar transistors. Thin Solid Films 1997;294(1± 2):246±9. [6] Chantre A, Marty M, Regolini JL, Mouis M, De Pontcharra J, Dutartre D, Morin C, Gloria D, Jouan S, Chaudier F, Aussous M, Roche M. A highly manufacturable 0.35 lm SiGe HBT technology with 70 GHz fmax . Proc of ESSDERC'98, Bordeaux, 8±10 September 1998. p. 448±51. [7] Hong M, De Fresart E, Steel J, Zlotnicka A, Stein C, Tann G. High performance SiGe epitaxial base bipolar transistor produced by a reduced pressure CVD reactor. IEEE Electron Dev Lett 1993;14:450±2. [8] Borgarino M, Kovacic S, Lafontaine H, Feng Zhou Z, Plana R. Low noise considerations in SiGe BiCMOS technology for RF applications. Wireless'99 Conference, Munich, 3±8 October 1999. [9] Meister TF, Schafer H, Franosch M, Molzer W, Au®nger K, Scheler U, Walz C, Stolz M, Boguth S, Bock J. SiGe base bipolar technology with 74 GHz fmax and 11 ps gate delay. Tech Dig IEDM'95, 1995. p. 739±42. [10] Osten HJ, Knoll D, Hienmann B, Tillack B. Carbon doping of SiGe heterobipolar transistors. Proc of topical meeting on Silicon Monolithic Integrated Circuits in RF Systems, Ann Arbor, MI, USA, 17±18 September, 1998. p. 19±23. [11] Ismail K, Meyerson BS, Wang PJ. High electron mobility in modulation doped Si/SiGe. Appl Phys Lett 1991;58(19): 2117±9.
[12] Koenig U, Schaeer F. Si/SiGe modulation doped ®eld eect transistor with two electron channels. Electron Lett 1991;27(16):1405±7. [13] Koenig U, Schatter F. P-type Ge channel MODFETs with high transconductance grown on Si substrates. IEEE Electron Dev Lett 1993;14:205±7. [14] Murakami E, Nakagawa K, Nishida A, Miyao M. Fabrication of a strain-controlled SiGe/Ge MODFET with ultrahigh hole mobility. IEEE Trans Elect Dev 1994; 41(5):857±61. [15] Voiginescu SP, Salama CAT, Noel JP, Kamins TI. Optimized Ge channnel pro®les for VLSI Compatibe Si/ SiGe p-MOSFETs. Proc IEDM'94, 1994. p. 369±72. [16] Gluck M, Hackbarth T, Koenig U, Haas A, Hock G, Kohn E. High fmax n-type Si/SiGe MODFETs. Electron Lett 1997;33:335±6. [17] Hagelauer R, Ostermann T, Koenig U, Gluck M, Hock G. Performance estimation of Si/SiGe hetero-CMOS circuits. Electron Lett 1997;33:208±10. [18] Escotte L, Roux JP, Plana R, Graeuil J, Gruhle A. Noise modelling of microwave heterojunction bipolar transistors. IEEE Trans Electron Dev 1995;42(5):883±9. [19] Voinigescu SP, Maliepaard MC, Showell JL, Babcok GE, Marchesan D, Schroeter M, Schwan P, Harame DL. A scalable high frequency noise model for bipolar transistors with application to optimal transistor sizing for lownoise ampli®er design. IEEE J Solid State 1997;32(9): 1430±8. [20] Schumacher H, Erben U, Durr W. SiGe heterojunction bipolar transistors ± the noise perspective. Solid-State Electron 1997;41(10):1485±92. [21] Cressler JD. SiGe HBT technology: a new contender for Si based RF and microwave circuit applications. IEEE Trans Microwave Theory Tech 1998;46(5):572±89. [22] Bock J, Felder A, Meister T, Franosh M, Au®nger K, Wurzer M, Schreiter R, Boguth S, Treitinger L. 1996 Symposium on VLSI Techno, Digest of technical papers, 1996. p. 108±9. [23] Ansley WE, Cressler JD, Richey DM. Base-pro®le optimization for minimum noise ®gure in advanced UHV/CVD SiGe HBTs. IEEE Trans Microwave Theory Tech 1998; 46(5):653±60. [24] Cappy A et al. Noise modelling in submicrometer gate twodimensional electron gas ®eld eect transistor. IEEE Trans Electron Dev 1985;32(12):2787±95. [25] Koester SJ, et al. High-performance SiGe p-MODFETs grown by UHV-CVD. Proc EDMO'99, London, 1999. p. 27±32. [26] Bruce S, Vandamme LKJ, Rydberg A. Measurement of low frequency base and collector current noise and coherence in SiGe heterojunction bipolar transistors using transimpedance ampli®er. IEEE Trans Elect Dev 1999;46(5):993±1000. [27] Plana R, Escotte L. Noise properties of microwave heterojunction bipolar transistors. Invited paper 21st International Conference on Microelectronics (MIEL'97), Nis (Yougoslavie), 14±17 September 1997, vol. 1, p. 215± 22. [28] Graeuil J, Plana R. Low-frequency noise properties of microwave transistors and its application to circuit design.
M. Regis et al. / Solid-State Electronics 45 (2001) 1891±1897 Invited paper at 24th European Microwave Conference, Cannes, September 1994. p. 1±13. [29] Llopis O, Dienot JM, Verdier J, Plana R, Gayral M, Graeuil J. Analytic investigation of frequency sensitivity in microwave oscillators: application to the computation of phase noise in a dielectric resonator oscillator. Ann Telecom 1996;51(3±4):121±9. [30] Jouan S, Planche R, Baudry H, Ribot P, Chroboczek JA, Dutartre D, Gloria D, Laurens M, Linares P, Marty M, Monroy A, Morin C, Pantel R, Perrotin A, de Pontcharra J, Regolini JL,Vincent G, Chantre A. A high-speed low 1=f noise SiGe HBT technology using epitaxially-aligned polysilicon emitters. IEEE Trans Electron Dev 1999;46(7): 1525±1531. [31] Vempati L, Cressler JD, Babcock JA, Jaeger RC, Harame D. Low-frequency noise in UHV/CVD epitaxial Si and
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[35]
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SiGe bipolar transistors. IEEE J Solid State Circ 1996; 31(10):1458±67. Cressler JD, Vempati L, Babcock JA, Jaeger RC, Harame D. Low-frequency noise characteristics of UHV/CVD epitaxial Si and SiGe base bipolar transistors. IEEE Electron Dev Lett 1996;17(1):13±5. Gabl R, Au®nger K, Bock J, Meister TF. Low-frequency noise characteristics of advanced Si and SiGe bipolar transistors. Proc of ESSDERC'97, September 1997. p. 90±4. Tang R, Ford J, Pryor B, Anandakugan S, Welch P, Burt C. Extrinsic base optimisation for high performance RF SiGe heterojunction bipolar transistor. IEEE Electron Dev Lett 1997;18(9):426±8. Okhonin S, et al. Low frequency noise in SiGe P-channel MOSFET designed for 0.13 lm technology. Proc ESSDERC'98. p. 504±7.
Review
Noise behavior in SiGe devices M. Regis a,b, M. Borgarino a, L. Bary a, O. Llopis a, J. Graeuil a, L. Escotte a, U. Koenig c, R. Plana a,* a LAAS-CNRS, 7 Av du Colonel Roche, 31077 Toulouse Cedex 04, France SiGe Inc. 2680, Queensview Drive, Suite 150, Ont., Ottawa, Canada K2B8J9 Daimler Chrysler Research Center, Wilhelm Runge Strasse 11, 89081 Ulm, Germany b
c
Accepted 8 May 2001
Abstract This paper presents an overview of SiGe technologies and their corresponding noise properties both in the high frequency and low frequency range. We demonstrate that SiGe bipolar technology exhibits impressive low frequency noise performance with an excess corner noise frequency in the 1 kHz range. These results have been validated by low phase noise microwave oscillators based on SiGe material. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: SiGe Heterojunction; Noise ®gure; 1/f noise; Low phase noise
Contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1891 2. SiGe technological overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.1. The SiGe HBT. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 2.2. The SiGe FET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1892 3. High frequency noise properties of SiGe devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1893 4. Low frequency noise and phase noise behavior of SiGe devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1894 5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1895 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1896
1. Introduction The next generation of telecommunication systems will feature a dramatic change in term of electrical per*
Corresponding author. Fax: +33-561-336-969. E-mail address: [email protected] (R. Plana).
formance. Among the latter, an important issue is the ``low noise character'' that should be achieved to improve the sensitivity and the bit rate error of future telecommunication systems. This makes silicon a new contender for RF and microwave circuits. It is well understood that silicon is a very low cost and low noise technology. The problem was related to the fact that silicon was considered as a ``slow semi-conductor'' for a
0038-1101/01/$ - see front matter Ó 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 0 1 ) 0 0 2 2 9 - 5
1892
M. Regis et al. / Solid-State Electronics 45 (2001) 1891±1897
long time. This drawback has been overcome by introducing a heterostructure concept through silicon germanium technology (SiGe). This makes it possible to achieve both bipolar and ®eld eect transistors devices through band gap engineering of silicon. Of course as the SiGe/Si heterostructure forms a strained layer, it is important to assess the noise behavior of these kind of devices. It has been previously demonstrated that the noise performance of a RF and microwave receiver comes from two dierent kind of noise sources. The ®rst one deals with the noise of the ampli®er and the mixer that is due to thermal noise and shot noise generated within the active devices. The second kind of noise is a little more complicated and is related to noise of the RF and microwave oscillator represented by phase noise. It has been shown that phase noise of a microwave oscillator comes from low frequency noise sources which are up-converted into phase noise through the nonlinear behavior of the active device. In this paper, we try to give an outlook on the noise properties exhibited by SiGe devices. Section 1 will address a brief overview of SiGe technology. Section 2 will focus on the high frequency noise of SiGe devices. In Section 3, we will present the low frequency noise performance of SiGe devices and its impact on phase noise of a microwave oscillator. Conclusions will be given in Section 4.
2. SiGe technological overview 2.1. The SiGe HBT In SiGe HBTs the thin base is made from a SiGe alloy. For the amount of Ge in the base region and the associated doping pro®le two technological concepts can be used to realize an heterojunction bipolar transistor. The ®rst one leaves the doping pro®le nearly unchanged with respect to a BJT but a small and graded Ge content is added from the emitter to collector junction. The Ge grading induces band gap narrowing along the base region resulting in a drift ®eld which improves the base transit time. The advantage of this concept is related to its BiCMOS compatibility. One drawback is associated
with the low base doping level which degrades the noise performance of the devices and which is overcome by scaling down the emitter. Note that the SiGe base is formed by selective or nonselective epitaxial growth. The second approach results in a true HBT (Ne Nb) since both base and emitter pro®les are changed and the Ge content ranges from 20% to 30%. The corresponding band gap narrowing allows a very high base doping level which is favourable for low noise. Among the others advantages we can list: higher injection eciency, lower emitter transit time and lower base transit time. Nevertheless some drawbacks occur related to the lattice mismatch of the SiGe alloy with respect to silicon, the large surface recombination due to the poor quality of the SiGe oxide. This approach further requires low temperature budget processing which is not compatible with current BiCMOS technology. We will refer in this paper the former as ``gradual'' (G) HBT and the latter as ``abrupt'' (A). In order to complete this section, we propose in Table 1 a comparison of the microwave performances of the dierent SiGe bipolar technologies. We can observe that most of them lead to a good performance in the millimeter-wave range which makes feasible the design of circuits at least up to 10 GHz. We just end the discussion with a new kind of SiGe technology developed by Osten et al. [10] which introduces some carbon in the SiGe base layer resulting to the suppression of the transient enhanced diusion of boron atom which is a limitation for lateral emitter scaling. SiGe:C HBTs feature fmax in the 70 GHz range and cuto frequency in the 50 GHz range. SiGe HBT technology becomes to be ready and some foundries are already available. Concerning the unipolar devices (also called FET devices) the situation is more complex and the technology is still at the ``laboratory state''. Nevertheless, we give some guidelines concerning these devices. 2.2. The SiGe FET Most of the present SiGe FETs come from the modulation doped type (MODFETs). They consist of a thin well with quantized states in which the carriers move collision-free. They will take bene®ts about a very high mobility. N wells are Si and p wells are SiGe typi-
Table 1 A review of the microwave performances of SiGe HBT Company
Process
Ft (GHz)
Fmax (GHz)
Type
Reference
Hitachi IBM DC ST Motorola SiGe Inc Siemens
UHV/CVD CVD MBE RPCVD RTCVD CVD CVD
130 117 116 50 ± 35 70
± 65 160 70 45 50 74
G G A G G G G
[1] [2,3] [4,5] [6] [7] [8] [9]
M. Regis et al. / Solid-State Electronics 45 (2001) 1891±1897
cally up to 30%. More details can be found in Refs. [11± 13]. Finally, other works address p channel MOSFETs consisting in a SiGe channel on Si substrate or Ge channel on Ge substrate [14,15]. Recently, an fmax of 81 GHz has been found for a 0.18 lm T-gate n MODFET [16] and theoretical performances of n and p channel devices have demonstrated transconductance above 1 S/ mm and cut-o frequencies in the 200 GHz range [17]. Nevertheless in this kind of device, the noise performances are not those expected mainly due to gate leakage current and numerous surface states. These points will be further discussed later in the paper.
3. High frequency noise properties of SiGe devices The high frequency noise behavior of an active device is assimilated to a two ports network where the noise is analyzed through the determination of the microwave noise parameters (minimum noise ®gure, equivalent noise resistance and optimum noise impedance). In a bipolar device, the high frequency noise is driven by the thermal noise (i.e. the emitter, base resistance), the shot noise (i.e. the DC bias) and the dierent transit time constant within the device (i.e. cut-o frequency). From previous works developed by Escotte [18] and Voiginescu [19], it is possible to derive the minimum noise ®gure expression as follows: s n f 2Ic fT2 n2 fT2 Fmin 1 Re Rb 1 2 b fT Vt bf b f2 1 where n represents the ideality factor of the junction, b the current gain of the device, Ic the collector current. Re and Rb represent the emitter and base resistance respectively, fT the cut-o frequency, Vt the thermodynamic voltage and f the operating frequency. From this expression, we can see that high current gain (b), high fT and low base and emitter resistance are suitable to minimize the minimum noise ®gure value. Note that SiGe technology allows to ful®ll all these requirements due to the SiGe heterostructure which pro-
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vides a decoupling between the current gain and the geometrical dimensions of the device contrary to BJT technology. Table 2 summarizes the high frequency noise performance of a set of silicon based devices. The results indicate that abrupt HBT is able to provide a lower value of Fmin than the gradual HBT due to the higher base doping. Nevertheless in the case of gradual SiGe HBT, appropriate shape of the base layer (Ge rate and doping) as well as a speci®c design of the extrinsic base resistance should feature similar performances as reported from theoretical simulations [23]. We have to note that for most of the RF and microwave applications, the high frequency noise performance featured by SiGe bipolar technology is sucient to meet the technical requirements. Concerning the noise performance of FET devices, they can be simply assess through the following expression [24]: p f q Fmin 1 2 P 2 gm0 Rs Rg fT where P is an empirical parameter associated with the technology, Rs and Rg are the source and gate resistance respectively. From this expression, it is understood that we have to increase the cut-frequency and to minimize the source and gate resistance to achieve a low noise ®gure. The situation concerning the SiGe FET is not well established as the technology is still under progress and the today results are not representative of the capabilities of this kind of device. Fig. 1 presents the minimum noise ®gure evolution versus frequency for a SiGe n-MODFETs featuring gate geometry of 0:25 50 lm2 . The results indicate a minimum noise ®gure value ranging from 2 to 5 dB which is not a good result compared to those obtained with III±V devices. This relatively high value for the minimum noise ®gure is mainly attributed to an excess gate leakage due to the strained heterostructure. We can nevertheless expect improvements in the future. Concerning the p-MODFETs device, Koester et al. [25] have presented minimum noise
Table 2 Comparative study of noise performances of microwave silicon based devices Device
Fmin (dB) @2 GHz
Fmin (dB) @10 GHz
Reference
SiGe abrupt SiGe gradual Si BJT conventional SiGe graduala
0.5 0.6 1
0.8 1.8 2.2
[20] [21] [22]
<0.4
0.8
[23]
a
Theoretical results.
Fig. 1. Minimum noise ®gure evolution of a nSiGe MODFET.
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®gure ranging from 1.1 to 3 dB from 1 to 18 GHz which is to our knowledge the best result reported on SiGe FET device. We have to note that this result has been obtained on devices realized on high resistivity substrate and that better performances could be obtained by using sapphire substrate. We believe that there is provision to improve the noise behavior of FET device to get performance closer to III±V based devices. The next section of this paper will address the low frequency noise behavior of SiGe device and their in¯uence on the phase noise of microwave oscillator.
4. Low frequency noise and phase noise behavior of SiGe devices Contrary to the FET device, where only one noise generator is needed related to the drain current ¯uctuations, in a bipolar device the noise behavior is fully described by the determination of both the base current and collector current ¯uctuations including their correlation [26]. Nevertheless this measurement technique features some drawbacks like narrow band range frequency, high sensitivity to the external environment and this why we prefer another alternative consisting to measure the input noise voltage and current generators including their correlation using a multiple impedance technique. This technique is time consuming but it provides more reliable measurements over a very large frequency range [27]. We have further demonstrated that, the input noise generators lead to a better accuracy in term of phase noise prediction [28,29]. Concerning the performances of abrupt device, a too high Ge ratio results to a tremendous degradation of the input noise current generator as reported in Fig. 2. This behavior is related to the strain in the base layer which creates some dislocation as well as surface states
Fig. 2. Input noise current generator versus Ge ratio in the base for abrupt HBT.
resulting to an increase of the recombination rate. We have further demonstrated that the passivation of the device plays a very important role in the low frequency noise performance. Nevertheless, on optimized devices, the recombination rate is minimized and we have measured impressive noise performance where the 1=f noise corner frequency is lower than 1 kHz which is a similar result than those obtained on BJT devices. Concerning the performances of the graded HBT, they are similar, 1=f noise component and an excess corner noise frequency lower than 1 kHz [30,31]. Vempati et al. have observed that most of noise is related to trapping± detrapping eect in the intrinsic emitter±base region [32]. Others works have shown that some additional noise can be found in the emitter resistance [33]. Others authors have found that some noise could be generated in the extrinsic base region related to some front edge implantation damage [34] as well as some surface problems associated with the cleaning method used to realize the CMOS devices [30]. Concerning the gradual SiGe HBT, the evolution of the input noise voltage versus frequency indicate that higher the emitter length lower is the noise. The same behavior is achieved for the input noise current generator. The evolution of the correlation coecient indicates a value dierent of 1 con®rming that not only one noise source is present within the device. From measurements versus bias and emitter length, we have de®ned a scaleable low frequency noise model involving 1=f noise source located at the emitter±base junction, the output of the device, the emitter and the base resistance and at the extrinsic base region related to some surface recombination. In order to compare the noise performance of SiGe HBT with others HBT devices, we present in Fig. 3 the evolution of the noise voltage versus frequency for three typical HBT, one SiGe BiCMOS
Fig. 3. Input noise voltage generator versus frequency for a set of microwave HBT.
M. Regis et al. / Solid-State Electronics 45 (2001) 1891±1897
Fig. 4. Frequency evolution of the input noise voltage for a nSiGe MODFET.
HBT, a GaInP/GaAs HBTs and an InP/InGaAs HBTs. The results con®rm the attractive capabilities of SiGe devices with respect to the III±V one mainly related with lower traps density and lower surface recombination velocity in silicon based device. Concerning the low frequency noise properties of SiGe FET device, there is very few data in the literature. Okhonin [35] presented some data concerning the low frequency properties of pSiGe MOSFET where they demonstrated that nothing wrong was given by the Ge introduction. Nevertheless the data they published concern only a very speci®c bias level and we believe that they are not representative of the ``real'' low frequency noise behavior. Fig. 4 presents the frequency evolution of the input noise voltage generator for a nSiGe MODFET where we can observe a high 1=f noise component as well as a ``lorentzian'' noise component in the 10 kHz range which is a typical behavior already observed in III±V HEMT or PHEMT. The next section will address the phase noise behavior of this kind of technology. In order to evaluate the suitability of an active devices with respect to the phase noise applications, we consider two quantities: (1) RF or microwave residual phase noise near the carrier for an open loop con®gured device; (2) near carrier oscillator RF or microwave phase noise. In Fig. 5, we have plotted the residual phase noise measurements performed at 10 GHz on a set of abrupt SiGe HBTs. The results demonstrate that like on the low frequency noise range, the devices must feature a good passivation and to be free of recombination to exhibit good residual phase noise performances. This behavior indicates that, the device exhibiting ultimate microwave performances with no passivation will not be suited for low phase noise applications. The next step concerns the real phase noise capabilities. In order to investigate this point, we built a 4 GHz parallel feedback oscillator with
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Fig. 5. Residual phase noise at 10 GHz for three kind of SiGe abrupt HBT.
Fig. 6. Comparative study of 4 GHz oscillator realized with dierent kind of active devices.
devices featuring dierent emitter length and the phase noise measurements demonstrated that higher is the emitter length lower is the phase noise. Finally, Fig. 6 shows a comparison of 4 GHz oscillators realized with dierent active devices and the results indicate that there is no dierence between Si and SiGe technology which demonstrate superior phase noise performances of at least 10 dB.
5. Conclusions In this paper, we have presented an outlook about the SiGe technology and their noise properties both in the high frequency and low frequency range. We demonstrated that SiGe HBT exhibited very attractive high
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frequency noise performances due to the improvement given by the SiGe/Si heterojunction. Concerning the FET devices, there is still provision for progress in term of noise to compete with III±V devices speci®cally at the gate level to avoid the gate leakage. The most important advantage of SiGe bipolar technology deals with their low frequency noise performances as well as appropriate design is applied in order to minimize the strain and the surface recombination. The impact on phase noise performances has been also demonstrated con®rming the superiority of SiGe technology over the III±V ones. References [1] Oda K, Ohue E, Tanabe M, Shimamoto H, Onai T, Washio K. 130 GHz fT SiGe HBT technology. Tech Dig IEDM 97, Washington, 7±10 December 1997. p. 791±4. [2] Crabbe EF, Meyerson BS, Stork JMC, Harame DL. Vertical optimization of very high frequency epitaxial Siand SiGe-base bipolar transistors. Tech Dig IEDM 93. p. 83±6. [3] Harame D, Larson L, Case M, Kovacic S, Voiginescu S, Tewksbury T, Nguyen-Ngoc D, Stein K, Cressler J, Jeng SJ, Malinowski J, Groves R, Eld E, Sunderland D, Rensch D, Gilbert M, Schonenberg K, Ahlgren D, Rosenbaum S, Glenn J, Meyerson B. SiGe HBT technology: device and applications issues. Tech Dig IEDM 95, 1995. p. 731±4. [4] Schupen A, Gruhle A, Erben U, Kibbel H, Konig U. SiGe-HBTs with high ft at moderate current densities. Electron Lett 1994;14(30):1187±9. [5] Gruhle A, Schupen A. Recent advances with SiGe heterojunction bipolar transistors. Thin Solid Films 1997;294(1± 2):246±9. [6] Chantre A, Marty M, Regolini JL, Mouis M, De Pontcharra J, Dutartre D, Morin C, Gloria D, Jouan S, Chaudier F, Aussous M, Roche M. A highly manufacturable 0.35 lm SiGe HBT technology with 70 GHz fmax . Proc of ESSDERC'98, Bordeaux, 8±10 September 1998. p. 448±51. [7] Hong M, De Fresart E, Steel J, Zlotnicka A, Stein C, Tann G. High performance SiGe epitaxial base bipolar transistor produced by a reduced pressure CVD reactor. IEEE Electron Dev Lett 1993;14:450±2. [8] Borgarino M, Kovacic S, Lafontaine H, Feng Zhou Z, Plana R. Low noise considerations in SiGe BiCMOS technology for RF applications. Wireless'99 Conference, Munich, 3±8 October 1999. [9] Meister TF, Schafer H, Franosch M, Molzer W, Au®nger K, Scheler U, Walz C, Stolz M, Boguth S, Bock J. SiGe base bipolar technology with 74 GHz fmax and 11 ps gate delay. Tech Dig IEDM'95, 1995. p. 739±42. [10] Osten HJ, Knoll D, Hienmann B, Tillack B. Carbon doping of SiGe heterobipolar transistors. Proc of topical meeting on Silicon Monolithic Integrated Circuits in RF Systems, Ann Arbor, MI, USA, 17±18 September, 1998. p. 19±23. [11] Ismail K, Meyerson BS, Wang PJ. High electron mobility in modulation doped Si/SiGe. Appl Phys Lett 1991;58(19): 2117±9.
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