in self-aligned SUSiGe heterojunction bipolar transistor (HBT) is reported. The observed low-frequency noise exhibits a pure llf shape, probably related to carrier ...
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IEEE ELECTRON DEVICE LETI‘ERS, VOL. 16, NO. 2, FEBRUARY 1995
llf Noise in Self-Aligned Si/SiGe Heterojunction Bipolar Transistor R. Plana, L. Escotte, J. P. Roux, J. Graffeuil, A. Gruhle, and H. Kibbel
Abstract-The first characterizationof the low-frequencynoise in self-aligned SUSiGe heterojunction bipolar transistor (HBT) is reported. The observed low-frequency noise exhibits a pure llf shape, probably related to carrier number fluctuationsat the pseudomorphic Emitter-Base heterointerface.
I. INTRODUCTION
I
T is well established today that 111-V based heterojunction bipolar transistors could be the more convenient devices for low phase noise microwave oscillator applications [ 11-43] since they exhibit both a reduced llf noise and high frequency capabilities [4], [5]. But in most HBT’s, both current gain and llf noise are affected by surface recombination current in the extrinsic base region due to the higher surface recombination velocity of Gallium Arsenide with respect to Silicon materials. Recently, good maximum operating frequency in Si/SiGe heterojunction bipolar transistor have been demonstrated ( f = ~ 91 GHz and fmax = 65 GHz) [6]-[8]. However, for specific microwave applications where the low-frequency (LF) noise must be minimized, the potentialities of these devices have still to be investigated. The structure and technology procedure of these devices have been previously described [9]. The current gain of our device ranges between 100 to 200 for an emitter size of 1pm x 20 pm. This device features a current gain cut off frequency of 80 GHz and a maximum oscillation frequency of 30 GHz only as a consequence of a non minimized base resistance. 11. LOW-FREQUENCY NOISE MEASUREMENTS The procedure of on-wafer HBT’s L.F noise characterization in common emitter configuration have been previously described [lo] and is based on the measurement of the input referred noise versus different input terminations. Between 250 Hz and 100 kHz, this technique provides the input noise current (i,) and noise voltage (e,) including their correlation. The optimum noise input termination resistance with = respect to the minimum noise figure is obtained from .-/, The correlation between these noise generators is depicted by the correlation resistance R,,, defined as R,,, = Seni; /Si, where Seni; represents the interspectral intensity between e, and 2,. Fig. 1 shows the input noise current
( S z n ( f ) )and voltage (Se,(f)) spectra measured at Ib = 45 PA, IC = 9 mA and V,, = 1 V (an higher collector-emitter voltage must be avoided due to low breakdown voltage). It can be seen that these spectra exhibit a l / f L Y ( a x 1) frequency dependence which is a favorable feature over most 111-V microwave HBT’s devices where usually one or several additional bumps (associated with one or several Lorentzians) cause some degradation of the overall noise level of the device. Note that the 1 Hz amplitude of the current llf noise source Sz, is found to be about 3.10-17 A’ which compares well with data previously obtain in more classical HBT’s [12], [14]. In order to investigate more precisely the noise properties of these devices, we have conducted an analysis of the noise generators according to van der Ziel [ 111 and Kleinpenning [121 theories. and voltage The spectral densities of the input current (2), (e,) noise generators and their interspectral density can be expressed as follows:
Sen =4kTri
and (3) where ri = r;b + re (rib represents the spreading base resistance and re the access emitter resistance), r , represents the dynamic resistance of the Emitter-Base heterojunction defined by r , = n U t / I b and P the DC current gain. Finally i e b and i,, are the short circuited noise currents at the EmitterBase heterojunction and at the Emitter-Collector terminal of the intrinsic device (i.e excluding re and &). Since these two noise currents result from two uncorrelated components, their spectral intensities can be written as:
a,,,
Manuscript received August 26, 1994; revised October 24, 1994. R. Plana, L. Escotte, J. P. Roux, and J. Graffeuil are with LAAS-CNRS and Universite Paul Sabatier Toulouse 7 Avenue du Colonel Roche 31077 Toulouse Cedex, France. A. Gruhle and H. Kibbel are with Daimler-Benz AG, Research Center Ulm, Wilhern-Runge-Str.11 D-7900 Ulm, Germany. IEEE Log Number 9408119.
+ r;’SZe6+ ySaec ( r f ,+ r,)’ P
+
(4)
siec= 2qIc +
(5)
Sieb = 2qIb
and
The first terms 2qIb, 2qIc account for the shot noise sources and the second terms and stand for the flicker noise spectral intensities at intrinsic device terminals. Finally S;bIf and S;!f stand for the l l f noise spectral intensity in the resistive parts of the device (access base and emitter
0741-3106/95$04.00 0 1995 IEEE
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PLANA et al.: NOISE IN SELF-ALIGNED Si/SiGe HETEROJLJNCTION BIPOLAR TRANSISTOR
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Input Noise Voltage (v2/Hz)
Input Noise Current ( h H z )
10 -I5
10 -I9
10 10
-16
10
-’’
10 -” 10 -” 10 33
10 -18
io3
10’
10 104
lo6
105
10’
Frequency (Hz) Fig. 1. Input noise voltage spectral intensity (m) and input noise current spectral intensity (0)versus frequency at I* = 4 5 p A I c = 9mA and V,, = 1 V-Full lines represent Calculated Noise using (6) and (8).
resistances). Presently, since (3 >> 1 and assuming that StlLf have the same order of magnitude than S:Lf (a classical situation observed in Silicon BJT’s [13]), ( 2 ) and (3) can be simplified as:
Sin = Sieb
(6)
Optimum Noise Resistance Ropl and Correlation ResistanceIL, (Ohms) 120
O
-
110
-
100
-
0 0 0 0
and (7)
and hence R,,, = T ; . Therefore R,,, should not vary with frequency. The observed variations of Ropt and R,,, versus frequency are reported in Fig. 2. Below 50 kHz these two quantities keep a constant equal value which is about 80 ohms. This behavior substantiates previous statements. Subsequently it can be inferred that rkb T , is about 80 ohms as it was expected from dynamic measurements and base sheet resistance value of 800 a / O . Additionally the similarity between hPt and R,,, denotes that the second term in (1) is the more relevant flicker noise term (the l/f noise across rLb and re is unimportant with respect to the Emitter-Base flicker noise). Equation (1) therefore simplifies as:
+
Sen = 4lcTr; + rL2Sie6.
(8)
According to (4), (6) and (8), one can define two noise comer frequencies fci and fcW where the excess noise and the white background (Nyquist or shot) noise take equal magnitudes. Fig. 1 shows a value of fCw lower than 100 kHz (for the noise voltage) and a value of f C ; in the range of 1 MHz
10’
.
lof
10’
I 0’
Frrqueney (Hz)
Fig. 2. Optimum noise resistance (0)and noise correlation resistance (m) versus frequency between 250 Hz and 100 kHz at Ib = 45 pA, I , = 9 mA and V,, = 1 V.
(for the noise current). These values outperform those usually observed in classical GaAdGaAlAs HBT’s [lo] and are not very different from those recently reported in GaInP/GaAs HBT’s [4] and both in AlInAsAnGaAs and GaAlAslGaAs HBT’s with guardring [15]. The increase of Ropt observed beyond 50 kHz is attributed to the fact that, when approaching fcw, the first term in (8) can no longer be neglected and therefore contributes to an increase of Rapt. The difference in f,; and fCw is related to the different magnitudes of the thermal and shot noise sources at the E-B heterojunction. We can speculate that the excess noise is primarily generated at the Emitter-Base heterojunction and
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IEEE ELECTRON DEVICE LETERS, VOL. 16, NO. 2, FEBRUARY 1995
l/f Noise Coefficient (A’)
Shot Noise (A*&)
-I6
10
-17
10
10’
10’
Base Bias Current (pA)
Fig. 3.
l/f noise at 1 Hz and shot noise spectral intensity variations versus Ib
the relatively high l l f noise source is probably attributed to the fact that the E-B heterdjunction is pseudomorphic and then prone to defects which, in turn, produce number fluctuations noise. This behavior is different with respect to classical HBT’s where the excess noise is generated not only at the E-B heterojunction but also at the extrinsic base surface (emitter periphery) and in the resistive parts of the device (rLband T , ) [12] which turns into Ropt > R,,, [lo]. To investigate more deeply other possible physical origins of excess noise in this device, we have performed additional noise measurements versus base current ranging from 4 to 45 pA and we have fitted the measured noise spectra using expression (6) to extract the llf noise coefficient. The evolution of this coefficient and of the shot noise source (2qIb) versus base current are displayed in Fig. 3: they vary linearly with the base current Ib. Any carrier recombination at emitter periphery would have resulted (as in 111-V devices) in noise varying as Ib” (with n > 1).Therefore the noise produced in that way is not prominent in our unpassivated devices (thanks to the low surface recombination velocity). This could have been expected from the examination of their near ideal base current. Therefore, in SilSiGe HBT’s, mobility fluctuation noise cannot be totally ruled out but carrier number fluctuations at the Emitter-Base heterojunction seems the more likely dominant process producing a higher flicker noise magnitude in comparison with homojunction Si BJT’s [ 161. Nevertheless a further decrease of this flicker noise can be expected from an improvement of the heterointerface quality. 111. CONCLUSION
In this letter, we have reported preliminary results about L.F noise in self-aligned Si/SiGe HBT’s. These devices are not prone to Lorentzian noise as most 111-V devices. The observed excess noise is only of l l f type. A carrier number fluctuations process at the Emitter-Base heterojunction is the more likely origin of the dominant flicker noise component. Finally, we believe that there is still a provision for a significant further L.F noise performance enhancement in these new devices since their processing techniques will certainly be largely improved in a near future.
REFERENCES [l] N. Hayama, S. R. Lesage, M. Madihian, and K. Honjo, “A lownoise ku-band AlGaAslGaAs HBT oscillator,” IEEE Microwave Theory Techno[.-S Digest, pp. 679-682, 1988. [2] M. A. Khatibzadeh and B. Bayraktaroglu, “Low phase noise heterojunction bipolar transistor oscillator,” Electronics Letters, vol. 26, no. 16, pp. 1246-1247, Aug. 1990. [3] M. Madihian and H.Takahashi, “A low-noise K-Ka Band oscillator using AIGaAs/GaAs heterojunction bipolar transistors,” IEEE Trans. on Microwave Theory Technol., vol. 39, no. 1, pp. 133-136, 1991. [4] R. Plana, J. Graffeuil, S. Delage, H. Blanck, M. A. Di Forte Poisson, C. Brylinski, and E. Chartier,”Low-frequency noise in self-aligned GaInPlGaAs heterojunction bipolar transistors,” Electronics Letters, vol. 28, no. 25, pp. 2354-2356, Dec. 1992. [5] S. Tanaka, H. Hayama, A. Furukawa, T. Baba, M. Mizuta, and K. Honjo, “Low-frequency noise performance of self-aligned InAlAsnnGaAs heterojunction bipolar transistors,” Electronics Letters, vol. 26, no. 18, pp. 1439-1440, 1990. [6] G. L. Patton, S. S. Iyer, S. Delage, S. Tiwari, and J. M. C. Stork, “Silicon-germanium-baseheterojunction bipolar transistors by molecular beam epitaxy,” IEEE Electron Device Lett., vol. 9, pp. 165-167, 1988. [7] G. L. Patton, J. H. Comfort, B. S. Meyerson, and E. F. Crabbe, “75 GHz f~ SiGe-Base HBT‘s,” IEEE Electron Device Lett., vol. 11, p. 171, 1990. [8] A. Gruhle, H. Kibbel, U. Konig, U. Erben, and E. Kasper, “MBE-grown Si/SiGe HBT’s with high 3, fT and fmax.” IEEE Electron Device Lett., vol. 13, p. 206, 1992. [9] A. Gruhle, H. Kibbel, U. Erben, and E. Kasper, “91 GHz SiGe HBT’s grown by MBE,” Electronics Letters, vol. 29, no. 4, pp. 415417, 1993. [lo] M. Tutt, D. Pavlidis, D. Pehlke, R. Plana, and J. Graffeuil, “l/f noise in AIGaAs/GaAs HBT’s using ultrasensitive characterization techniques for identifying noise mechanisms,” 18th G d s and Related Compounds Symp., Seattle, 1991. [ I l l A van der Ziel, X. Zhang, and A. H. Pawlikiewicz, “Location of llf noise sources in BJT’s and HBJT’s-I. Theory,” IEEE Trans. Electron Devices, vol. ED-33, no. 9, pp. 1371-1376, Sept. 1986. [12] T. G. M. Kleinpenning and A. J. Holden, ‘Wf noise in n-p-n GaAdAIGaAs heterojunction bipolar transistors: Impact of intrinsic transistor and parasitic series resistances,”IEEE Trans.Electron Devices, vol. 40, no. 6, pp. 1148-1153, June 1993. [13] T. G. M. Kleinpenning, “Location of the low-frequency noise sources in submicrometer bipolar transistors,” IEEE Trans. Electron Devices, vol. 39, no. 6, pp. 1501-1506, 1992. [14] D. Costa and J. S. Harris, “Low-frequency noise properties of n-p-n AlGaAs/GaAs heterojunction bipolar transistors,” IEEE Trans. Electron Devices, vol. 39, no. 10, pp. 2383-2394, Oct. 1992. [15] N. Hayama and K. Honjo, “llf noise reduction in self-aligned AlGaAdGaAs HBT with AlGaAs surface passivation layer,” IEEE Trans. Electron Devices, vol. 39, no. 9, pp. 2180-2182, Sept. 1992. [16] J. C. Costa, D. Ngo, R. Jackson, D. Lovelance, and N. Camilleri, “Modeling and measurement of llf noise characteristics of silicon BJT’s,” Proc. of IEEE Microwave Theory TechnoL-S Digest, San Diego, pp. 1073-1076, June 1994.
