cal for both types of the devices. This clearly shows ... [4] M. Asif Khan, X. Hu, G. Simin, A. Lunev, J. Yang, R. Gaska, and M. S. Shur, IEEE Electoron. Device Lett.
Fluctuations and Noise Letters Vol. 0, No. 0 (2001) 000–000 © World Scientific Publishing Company
LOW-FREQUENCY NOISE IN AlGaN/GaN HETEROSTRUCTURE FIELD EFFECT TRANSISTORS AND METAL OXIDE SEMICONDUCTOR HETEROSTRUCTURE FIELD EFFECT TRANSISTORS
S. L. RUMYANTSEV†, N. PALA, M. S. SHUR, Department of Electrical, Computer, and Systems Engineering and Center for Integrated Electronics and Electronics Manufacturing, CII 9017, Rensselaer Polytechnic Institute, Troy NY 12180-3590 M. E. LEVINSHTEIN AND P. A. IVANOV Solid State Electronics Division, The Ioffe Physical-Technical Institute of Russian Academy of Sciences, 194021, St. Petersburg, Russia M. ASIF KHAN, G. SIMIN, AND J. YANG Department of Electrical and Computer Engineering, University of South Carolina, Columbia, SC, 29208 X. HU, A. TARAKJI, R. GASKA Sensor Electronic Technology, Inc., 21 Cavalier Way, Latham, NY 12110
Received (received date) Revised (revised date) Accepted (accepted date) The dependence of the 1/f noise on 2D electron concentration in the channel nCh of AlGaN/GaN Heterostructure Field Effect Transistors and Metal Oxide Semiconductor Heterostructure Field Effect Transistors has been studied and compared. The dependencies of Hooge parameter αCh for the noise sources located in the channel of the transistors on sheet electron concentration are found identical for both types of devices. The increase of the Hooge parameter αCh with the decrease of the channel concentration observed in both types of devices confirms that the noise sources are located in the region under the gate in the AlGaN/GaN heterostructure and that electron tunneling from the 2D electron gas into the traps in GaN or AlGaN layers is a probable noise mechanism. Keywords: 1/f noise, AlGaN/GaN, HFETs, MOSHFETs, 2DEG concentration.
1.
Introduction
GaN-based Heterojunction Field Effect Transistors (HFETs) are expected to find applications in high power, high frequency, and low noise electronics [1-3]. Recently, a novel SiO2/AlGaN/GaN Metal-Oxide-Semiconductor Heterostructure Field Ef†
On leave from the Ioffe Institute of Russian Academy of Sciences, 194021 St-Petersburg, Russia
Low frequency noise in AlGaN/GaN HFETs and MOS-HFETs
fect Transistor (MOSHFET) device has been demonstrated [4,5]. The gate leakage current of this devices is 4 - 6 orders of magnitude lower than that of a conventional AlGaN/GaN HFET. The general noise properties of AlGaN/GaN MOSHFETs have been studied in Refs. [6,7]. The investigations of the gate voltage dependence of the drain current noise is an effective tool to analyze the physical nature and localization of the noise sources (see, for example, [8-11]). Our recent studies of HFETs using this method showed that the tunneling of the electrons from the two dimensional electron gas in the device channel into traps in the bulk GaN or AlGaN might cause the 1/f noise [11]. In this paper we report on the comparative study of the AlGaN/GaN HFETs and MOSHFETs that confirms this conclusion. 2.
Experimental details
The AlGaN/GaN heterostructures used in this study were grown by metalorganic chemical vapor deposition (MOCVD) on a semi-insulating 4H-SiC substrate. They consisted of a 50-nm-thick AlN buffer layer, 0.4-µm-thick undoped GaN layer, followed by Al0.2Ga0.8N barrier layer, which was doped with silicon to approximately 2×1018 cm-3. As always in our growth, we added trace amounts of indium to all nitride layers, which considerably improves the materials quality. HFETs and MOSHFETs were fabricated on the same wafer. In order to fabricate MOSHFET and HFET devices on the same wafer, prior to the gate metallization a 7 nm SiO2 layer was deposited on a part of the heterostructures using Plasma Enhanced Chemical Vapor Deposition. The transistors had the source-drain spacing of 4-5 µm, the gate length, L, of 11.5 µm, and a gate width, W, in the range of 50 -150 µm. The Transmission Line Model (TLM) structures and transistors were fabricated on the same wafers. All dc and noise measurements were performed in linear (Ohmic) regime at a small drain-source dc bias. 3.
Results and Discussion
For both HFETs and MOSHFETs, the capacitance - voltage measurements were performed at frequency of 1 kHz on the test transistors with the large gate area. For the entire range of the gate biases, Vg, from zero to the threshold voltage, VT, the gate capacitance was practically constant manifesting the linear dependence of the electron channel sheet concentration nCh on the gate bias. The measured gate threshold voltages were equal to –6 V and –2 V for MOSHFETs and HFETs, respectively. A more negative threshold voltage of the MOSHFET is partially linked to an additional voltage drop across the silicon dioxide layer. The thickness of GaAlN and SiO2 layers extracted from the capacitance measurements was in good agreement with the data obtained from technological estimations. The noise spectra of the drain current fluctuations had the form of 1/f Γ noise with Γ close to unity (Γ = 1.0-1.17). At low drain biases, the spectral noise density was proportional to the square of the drain voltage SId ~ Vd2 . Using the techniques described in details in Ref. [12], we checked that neither contact noise nor noise from gate leakage current contributed much to the total output noise of HFETs. Fig. 1 shows the dependence of the relative spectral density of short circuit current fluctuations SId/Id2 multiplied to the gate area ACh on normalized gate voltage Vgn = (Vg - VT)/VT for HFETs and MOSHFETs.
10
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10
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2
2
SId/Id *ACh, cm /Hz
Rumyantsev et al.
MOSHFET
10
-1
10
0
Vgn Fig. 1 The dependencies of the of the relative spectral noise density of the drain current fluctuations on normalized gate voltage Vgn = (Vg - Vt)/Vt for HFETs and MOSHFETs. Frequency of analysis f=1Hz.
As seen in Fig.1, the gate voltage dependencies of noise for both types of the devices are identical in spite of different threshold voltages for the HFETs and MOSHFETs. At low values of Vgn , the noise in MOSHFETs is somewhat smaller than that in HFETs. One of the reasons for this difference might be the different electron concentration even at the same values of Vgn. Another possible reason for the different noise level can be the different ratio of the channel resistance to the contact resistance for HFETS and MOSHFETs. In order to take into account these two factors, we estimated the Hooge parameter αCh in the channel under the gate [13]:
α Ch =
S RCh fN 2 RCh
(1)
where N is the total number of the conduction electrons in the channel under the gate, f is the frequency of the analysis, RCh is the channel resistance, SRCh /RCh2 is the relative spectral density of the channel resistance fluctuations. Assuming that the noise sources are not correlated and located in the channel, and in the source-gate, and gate-drain regions, the spectral noise density of the drain current fluctuations can be presented as follows:
S Id S RCh + S ac = I d2 Rds2
(2)
Low frequency noise in AlGaN/GaN HFETs and MOS-HFETs
where Rds = Rch+Rs+Rd+Rc is the drain-source resistance at low drain bias, Rs and Rd are the series resistance of source - gate and gate - drain regions, respectively Rc is the contact resistance, SRch and Sac are spectral densities of the Rch and (Rs+Rd) fluctuations, respectively. The spectral noise density Sac does not depend on gate voltage, as the contribution from the channel resistance SRch varies with the gate voltage. Assuming that both SRch and Sac comply with the Hooge formula (1) the dependence of αCh on the channel concentration nCh can be found as:
S α ( R + Rd ) 2 ACh nCh Rds2 α Ch = 2I − 0 s 2 Aac n ac Rds2 RCh Id
(3)
where Aac is the area of the drain-gate, gate-source openings, nac is the electron concentration in these regions,
nCh =
1 Vg CdV g q ∫Vg1
,
(4)
C is the gate capacitance per unit area, Vg1 is the gate voltage which was taken to be smaller than the threshold voltage VT. Since at Vg < VT the gate capacitance is very small, the chosen value of Vg1 does not significantly affect the dependence nCh(Vg) given by eq. (4). The channel resistance RCh was found as RCh = Rds - Rc - Rd -Rs. The values Rc, Rs, and Rd, were found from the TLM measurements [14] and from the known transistor dimensions. In Eq.(3), α0 is Hooge parameter for ungated regions of the transistor. We estimated the value of α0 assuming that at zero gate voltage the value of the Hooge parameter is the same for the channel and ungated regions.
-2
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a
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1/nCh
HFET -4
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10
13
nCh, cm
-2
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Fig. 2 The dependencies of the Hooge parameter αCh on 2D sheet channel concentration nCh for HFETs and MOSHFETs.
Rumyantsev et al.
Figure 2 shows the dependencies of the Hooge parameter αCh on 2D sheet channel concentration nCh calculated for the HFETs and MOSHFETs. As seen in Fig. 2, both the absolute values of αCh and their dependencies on the channel concentration are identical for both types of the devices. This clearly shows that SiO2 layer does not affect the noise properties. 4.
Conclusions
At low channel concentrations, αCh decreases with the increase of nCh approximately as αCh ~ 1/nCh, reaches a minimum and then increases with a further increase of nCh. The dependence αCh ~ 1/nCh indicates that the spectral density of the channel resistance fluctuations SRch does not depend on channel concentration. This is a very typical situation for Si MOSFETs where the noise arises from tunneling of electrons from semiconductor to traps in the oxide. The amplitude of that surface noise depends only on the trap density on the Fermi level [15,16]. Similar noise mechanism consisting of tunneling of electrons from 2D gas to GaN or AlGaN might be dominant in GaAlN/GaN-based transistors as well. The increase of αCh at high concentrations, nCh, can be attributed to the electron spillover from the 2D-channel to a parallel low electron-mobility “parasitic conduction" channel (see also [11]). The noise level in such “parasitic channel” should be very large and corresponds to α ~ (10-2 ÷1) for the bulk GaN [17-19]. Acknowledgment The work at RPI was supported by the Office of Naval Research. Project monitor Dr. J. Zolper. The work at USC was supported by the Ballistic Missile Defense Organization (BMDO) under Army SSDC contract DASG60-98-1-0004, monitored by Dr. Brian Strickland and Dr. Kepi Wu. The work at SET, Inc. was supported by BMDO under SBIR Phase II contract F33615-01-C-1911 and monitored by Dr. J. Gillespie of WPAFB. The authors would like to thanks Dr. E. Borovitskaya for useful discussions. References [1] A. Tarakji, G. Simin, N. Ilinskaya, X. Hu, A. Kumar, A. Koudymov, J. Zhang, M. Asif Khan, M.S. Shur and R. Gaska, Appl. Phys. Lett., 78, N 15, pp. 2169-2171 (2001) [2] S. T. Sheppard, K. Doverspike, W. L. Pribble, S. T. Allen, J. W. Palmour, L. T. Kehias, and T. J. Jenkins, IEEE Electoron Device Lett. 20, 161, (1999) [3] M. E. Levinshtein,S. L. Rumyantsev, M. S. Shur R. Gaska, and M. Asif Khan, “Low frequency and 1/f noise in wide gap semiconductors: Silicon Carbide and Gallium Nitride”, Review, Special Issue of 'IEE Proc. - Circuits, Devices, and Systems, to be published, October 2001. [4] M. Asif Khan, X. Hu, G. Simin, A. Lunev, J. Yang, R. Gaska, and M. S. Shur, IEEE Electoron Device Lett. 21, 63, (2000) [5] M. Asif Khan, X. Hu, A. Tarakji, G. Simin, J. Yang, R. Gaska, and M. S. Shur, Appl. Phys. Lett., v. 77, N 9, pp. 1339-1341 (2000) [6] N.Pala, R. Gaska, S.Rumyantsev, M.S.Shur, AsifKhan, X.Hu, G. Simin, J, Yang, “Low frequency noise in AlGaN/GaN MOS-HFETs”, Electron. Lett, v.36, no.3, pp. 268-270, (2000) [7] S. L.Rumyantsev, N. Pala, M. S. Shur, M. E. Levinshtein, R.Gaska, X. Hu, J. Yang, G. Simin, and M. Asif Khan , “Low frequency noise in GaN-based transistors” Seventeenth Biennial IEEE/Cornell University Conference on Advanced Concepts in High Performance Devices, August 7 -9, 2000, Proceedings, pp. 257-264.
Low frequency noise in AlGaN/GaN HFETs and MOS-HFETs
[8] J. A. Garrido, F. Calle, E. Munoz, I. Izpura, J. L. Sanches-Rojas, R. Li, and K. L. Wang, Electron. Lett. 34, 2357 (1998). [9] J. A. Garrido B. E. Foutz, J. A. Smart, J. R. Shealy, M. J. Murphy, W. J. Schaff, and L. F: Eastman, “Low-frequency noise and mobility fluctuations in AlGaN/GaN heterostructure fieldeffect transistors,” Appl. Phys. Lett., 76, (23), pp.3442-3444 (2000) [10] Balandin A.: 'Gate-voltage dependence of low-frequency noise in GaN/AlGaN heterostructure field-effect transistors,' Electron. Letters, 36, (10), pp.912-913 (2000) [11] S. L. Rumyantsev, N. Pala, M. S. Shur, R. Gaska, M. E. Levinshtein, P. A. Ivanov, M. Asif Khan, G. Simin, X. Hu, and J. Yang, “ Concentration dependence of the 1/f noise in AlGaN/GaN Heterostructure Field Effect Transistors” J. of Applied Phys.,, submitted for publication. [12]Rumyantsev S. L., Pala N., Shur M. S., Gaska R., Levinshtein M. E., Khan M. A., Simin G., Hu X., and Yang J.: 'Effect of gate leakage current on noise properties of AlGaN/GaN field effect transistors,' Journ. Appl. Phys., 88, (11), pp. 6726-6730 (2000) [13] F. N. Hooge, T.G.M. Kleinpenning, and L.K.J. Vandamme, "Experimental studies on 1/f noise," Rep. Progr. Phys., 44, no.5, pp. 479-532, (1981) [14] M.S. Shur, "GaAs devices and circuits", Published New York : Plenum Press, c1987. [15] L. K. J. Vandamme, X. Li, and D. Rigaud, “1/f noise in MOS devices, mobility or number fluctuations?”, IEEE Transaction Electron Devices 41, No 11, 1936 – 1945 (1994) [16]M. J. Kirton and M. J. Uhren, “Noise in solid-state microstructures: A new perspective on individual defects, interface states and low-frequency (1/f) noise.” Adv. in Phys., 38, 367 - 468 (1989) [17]M. E. Levinshtein, F. Pascal, S. Contreras, W. Knap, S. L. Rumyantsev, R. Gaska, J. W. Jang, and M. S. Shur. "Low-frequency noise in GaN/GaAlN heterojunctions" Appl. Phys. Lett, 72, pp.3053-3055,(1998) [18] M. E. Levinshtein, S. L. Rumyantsev, R. Gaska, J. W. Yang, M. S. Shur "AlGaN/GaN high electron mobility field effect transistors with low 1/f noise" Appl. Phys. Lett, 73 N8, pp. 10891091 (1998) [19] N. V. Dyakonova, M. E. Levinshtein, S. L. Rumyantsev, " Nature of the Bulk 1/f Noise in GaAs and Si" Sov. Phys. Semicond. 25, no 12, pp.1241-1265, (1991)
