In this work, the extrinsic dynamic behaviour and noise performance of a 225 nm isolated-gate InAs/AlSb HEMT have been studied by means of Monte Carlo simulations. A very good agreement with experimental results has been achieved for fT. Discrepancies between experimental and simulated fmax have been observed and attributed to the experimental frequency dispersion of gd and Cds. The simulations of the intrinsic and extrinsic noise parameters indicate an excellent performance for this device (Fmin=0.3 dB@10 GHz) even if we confirm that the presence of the native oxide under the gate induces an significant decrease in fT and fmax of around 20%, together with an increase of noise figure and noise resistance.ROOTHz (FP7-243845

González Sánchez, Tomás

Grahn, Jan

Mateos López, Javier

Moschetti, Giuseppe

Rodilla, Helena

English

Gestion del Repositorio Documental de la Universidad de Salamanca

 1Monte Carlo study of the noise performance of isolated-gate InAs/AlSb HEMTs  H. Rodilla1, T. González2, G. Moschetti1, J. Grahn1 and J. Mateos2 1Microwave Electronics Laboratory, Dept. of Microtechnology and Nanoscience, Chalmers University of technology, Göteborg, Sweden  2Departamento de Física Aplicada, Universidad de Salamanca, Plaza de la Merced s/n, 37008 Salamanca, Spain  E-mail: rodilla@chalmers.se  Abstract— In this work, the extrinsic dynamic behaviour and noise performance of a 225 nm isolated-gate InAs/AlSb HEMT have been studied by means of Monte Carlo simulations. A very good agreement with experimental results has been achieved for fT. Discrepancies between experimental and simulated fmax have been observed and attributed to the experimental frequency dispersion of gd and Cds. The simulations of the intrinsic and extrinsic noise parameters indicate an excellent performance for this device (Fmin=0.3 dB@10 GHz) even if we confirm that the presence of the native oxide under the gate induces an significant decrease in fT and fmax of around 20%, together with an increase of noise figure and noise resistance.   2I. INTRODUCTION InGaAs/InAlAs High Electron Mobility Transistors (HEMTs) are for the moment the best option for low-noise high-frequency applications, providing higher cut off frequencies than any other devices [1,2]. Because of the excellent transport properties of InAs, its very high mobility and peak velocity, together with the very high offset in the conduction band between InAs and AlSb (ΔEc=1.35 eV), the InAs/AlSb material system has become a very promising substitute of InGaAs/InAlAs for improving the high-frequency, low-noise behavior of HEMTs [3]. One of the main drawbacks of this novel material system is the relatively high gate leakage current. This leakage has been reduced introducing a native oxide under the gate [4]. From now, we will call these devices isolated-gate AlSb/InAs HEMTs (IG Sb-HEMTs). In previous works, we have studied the static and intrinsic small signal equivalent circuit (SSEC) performance of these IG Sb-HEMTs by means of Monte Carlo (MC) simulations [5,6]. The validity of our model was verified by comparing with experimental data. Due to the high-frequency, low-noise target applications of IG Sb-HEMTs, the extrinsic high frequency performance and noise behavior warrant further study. In this work we present a study of the extrinsic dynamic performance, together with intrinsic and extrinsic noise behavior of IG Sb-HEMTs by means of MC simulations. The influence of the native oxide under the gate has also been studied by comparing the simulation results for the IG Sb-HEMT with those obtained by the simulation of a traditional Schottky-gate (SG) HEMT with the same topology as the IG but without the native oxide under the gate.  II. DEVICE DESCRIPTION To allow the comparison of simulation results with experimental data the simulated  3device has been modeled as similar as possible to the experimental IG Sb-HEMT [4,7]. In order to study the influence of the native oxide in the performance of the device, two transistors with the same epitaxial structure have been simulated: one IG and one standard SG Sb-HEMT (Fig. 1). The epitaxial stack for both devices was built up by an AlSb buffer of 800 nm followed by an InAs channel of 15 nm and a 15 nm thick AlSb barrier. An In0.5Al0.5As protection layer of 4 nm is located upon the barrier, followed by a 5 nm highly doped InAs cap layer (ND=5×1018 cm−3). A δ–doping layer of 4.5×1012 cm−2 is located 5 nm far from the channel. In order to account for the interface states and reproduce the experimental Hall ns, a negative surface charge with a value σ/e=-2.8x1012 cm-2 has been considered. Due to the fabrication process, the negative surface charge at the top of the recessed part of the device can be different from the one in the top of the cap layer [8]. We have observed in previous works that in our simulations σrecess can be considered as a free parameter as it only introduces an offset in the threshold voltage [5]. We have adopted a value of σrecess/e=-1.2x1012 cm-2. The only difference between the IG and SG devices is that in the case of the IG transistor there is a native oxide under the gate. The oxidation of AlSb is a very complex process, making it difficult to predict the thickness and dielectric constant of the formed native oxide. After comparing the simulation results with experimental data for different values of the dielectric constant and the thickness of the oxide, we have considered a 2 nm thick oxide with dielectric constant of 2.2, corresponding to Al(OH)3[5].  III. SIMULATION MODEL The extrinsic dynamic performance and noise behavior of Sb-HEMTs has been  4studied by MC simulations. A semiclassical ensemble MC consistently coupled with a two dimensional Poisson solver has been used. This simulator has been proven to be a powerful and accurate simulation technique to study the noise performance of InP HEMTs [9]. The simulator has been modified to properly model this new InAs/AlSb material system [10], providing very good results for static behaviour and SSEC [5]. Impact ionization mechanisms are not considered in this version of the simulator since we will consider VDS=0.2 V, where noise figure is expected to be minimum [7] and impact ionization is negligible [6].  For the analysis of the extrinsic dynamic and noise performance of the device, experimental values of the extrinsic parameters have been included. We have considered the experimentally extracted values [7]: Ls=3 pH, Lg=3.3 pH, Ld=4.2 pH, Cpg=20.2 fF, Cpd=18.6 fF, Rg =1 Ω. For Rs and Rd we have considered the values previously used for comparing our intrinsic DC curves with the extrinsic experimental ones: Rs=0.13 Ω·mm and Rd=0.38 Ω·mm [5]. IV. EXTRINSIC DYNAMIC PERFORMANCE Once intrinsic dynamic performance has been studied [5], the extrinsic cutoff frequency fT and the maximum frequency of oscillation fmax have to be determined. Simulation results for fT and fmax have been obtained through current gain |Hfe|2 and unilateral power gain U. They are obtained by using the frequency dependent Y-parameters coming out from the MC simulations, not from any equivalent circuit approach, in other words, |Hfe|2 and U are direct results from the simulations [11].  From now, we will always consider VDS=0.2 V, where the noise figure is close to optimum. In our simulations we have not considered the mechanisms leading to gate leakage current, IG. So, to account for its influence in U, a gate-drain resistance and a gate-source resistance, rgd and rgs, in parallel with Cgd and Cgs, respectively, have been  5introduced externally in the SSEC [11]. We have used the experimental values of rgd and rgs, 12 KΩ and 14.6 KΩ [7]. The effect that the introduction of these resistances has in U and in the stability factor K at the lower frequencies is observed in Fig. 2, leading to a quite good agreement with the experimental behavior of U and K [7].  Fig. 3 shows the dependence of |Hfe|2 and U on frequency. As observed, the effect of the gate leakage current modifies the -20dB slope of U for low frequency, but it does not affect the value of fmax. In the case of |Hfe|2, its value is not affected by gate leakage. For the higher frequencies, neither the SSEC nor the values of the extrinsic parameters are valid anymore, so fT has been obtained by linear interpolation of the low frequency -20dB slope [11]. The simulated values of fT and fmax as a function of the drain current for VDS=0.2 V have been compared with the experimental results in Fig. 3. While a very good agreement has been obtained for fT, strong discrepancies appear when trying to reproduce the experimental fmax. These discrepancies appears due to a previous disagreement already found when comparing the experimental and simulated gd and Cds due to presence of a frequency dispersion in the experimentally extracted values [5]. The value of fT is much lower to that obtained with InGaAs channels (around 200 GHz [12]), and the expected improvement with respect to that technology is not observed. We attribute the degradation to the non mature status of the Sb-HEMT technology. The influence of the presence of a native oxide in order to decrease the gate leakage current has been studied by comparing the simulation results for the IG Sb-HEMT with those obtained for the SG one, Fig 4. As can be observed, the presence of the oxide isolates the gate and deteriorates the high frequency performance of the device, thus lowering significantly the values of both fT and fmax (from about 230 and 130 GHz to 200 and 100 GHz).  6V. INTRINSIC NOISE To characterize the intrinsic noise, the frequency dependent spectral densities of drain and gate current fluctuations and its cross-correlation have been obtained directly from the MC simulations. Intrinsic dimensionless noise parameters P, R and C can be easily obtained from spectral densities and Y parameters [9]. We remind that P and R are a measure of the noise in the drain and gate current respectively, and C is the correlation between gate and drain current. The uncertainty in the calculation of the gate current noise leads to not very precise values for R and C. Fig. 5 shows the noise parameters as a function of drain current for the IG Sb-HEMT compared with the SG Sb-HEMT. P, R and C are practically frequency independent up to more than 50 GHz, as can be seen in the insets of Fig. 5.  The values of P, R and C in the 225 nm gate length IG Sb-HEMT (device under study) are similar to those obtained in InGaAs HEMTs with a much shorter gate of 100 nm [9], only with a slight degradation of R (due to the larger size of the gate). When comparing P, R and C between the IG and SG Sb-HEMTs, a degradation of the noise parameter P can be observed (Fig. 5) in the IG device due to the existence of the native oxide under the gate, which, on the other hand, reduces R in a wide range of drain currents. VI. EXTRINSIC NOISE From a practical viewpoint, it is much more useful to describe the noise behavior by the extrinsic noise parameters, like minimum noise figure, NFmin, noise resistance, Rn, and associated gain, Gass.  Fig. 6 shows the values of these parameters for the IG and SG Sb-HEMT for two different frequencies: 10 GHz and 94 GHz. Experimental data for NFmin and Gass for ID around 50 mA/mm at a frequency of 10 GHz [7] are also included in the same graph.   7A very good agreement with the experimental data has been obtained for NFmin [Fig.6 (a)], but an overestimation of the simulated Gass is observed [Fig. 6(b)]. This overestimation is related to the previous discrepancies due to experimental frequency dispersion of some parameters in the equivalent circuit, which also induced the discrepancies in fmax. It is to be noted that the noise performance of this 225 nm gate IG Sb-HEMT (Fig. 6) is comparable to that obtained for a InP HEMT with a much lower gate length, 100 nm [9], in spite of its much lower cutoff frequencies, thus showing the potential of this material system, InAs/AlSb, to replace InGaAs/AlInAs. When comparing the IG and the SG Sb-HEMTs, it can be noticed that the presence of the native oxide induces a small decrease in Gass of around 2dB and an increase in NFmin and Rn. However, as stated before, the mechanisms at the origin of the gate leakage current are not considered here and therefore this result may be different (mainly affecting at the lower frequencies) if those phenomena are included. In any case the interest of these MC simulations is that we have quantitatively determined the degradation that should be expected from the introduction of a gate oxide in order to check whether it is counteracted or not by the improvement of the dynamic and noise performances obtained by the reduction of the gate leakage current (that can only be determined experimentally).  VII. CONCLUSIONS The extrinsic dynamic performance and noise behavior of a 225 nm gate InAs/AlSb HEMT has been studied by means of Monte Carlo simulations. The influence of the native oxide, whose appearance has been promoted in order to decrease gate leakage current, has been also analyzed. A degradation of the dynamic performance and noise behaviour has been observed because of the presence of the native oxide under the gate.  8While a very good agreement has been obtain when comparing the simulation results for fT with the experimental data, strong discrepancies appear for fmax due to the experimental frequency dispersion of gd and Cds. This problem is also present when trying to reproduce the experimental gain associated to the minimum noise figure conditions. A promising noise performance has been found for these 225 nm gate Sb-HEMT, better than that of InP devices with gate length of 100 nm, even if the cutoff frequencies are lower.  VIII. ACKNOWLEDGMENTS This work has been partially supported by the European Commission through the ROOTHz Project ICT-2009-243845 and the MC2ACCESS Project, by the Dirección General de Investigación (MEC) and by the Consejería de Educación, Junta de Castilla y León, through Project GR270.   9REFERENCES  [1] Kim D-H, del Alamo J A 2010 30-nm InAs PHEMTs With fT=644 GHz and fmax=681 GHz IEEE Electron Device Lett. 31 806 [2] Lai R, Mei X B, Deal W R, Yoshida W, Kim Y M,. Liu P H, Lee J 2007 Sub 50 nm InP HEMT Device with Fmax Greater than 1 THz Proceedings of IEDM 2007 609 [3] Kruppa W, Yang M J, Bennett B R and Boos J B 2006 Low-Frequency Noise in AlSb/InAs and Related HEMTs IEEE Trans. Electron Devices 54 1193 [4] Lefebvre E, Malmkvist M, Borg M, Desplanque L, Wallar X, Dambrine G, Bollaert S and Grahn J 2009 IEEE Trans. Electron Devices 56 1904  [5] Rodilla H, González T, Moschetty G, Grahn J and Mateos J 2010 Dynamic Monte Carlo study of isolated-gate InAs/AlSb HEMTs Semicond. Sci Technol. 26 025004 [6] Vasallo B G, Rodilla H, Gonzalez T. Moschetti G, Grahn J and Mateos J. 2010 Monte Carlo study of kink effect in isolated-gate InAs/AlSb high electron mobility transistors J. Appl. Phys. 108 094505. [7] Malmkvist M, Lefebvre E, Borg M, Desplanque L, Wallart X, Dambrine G, Bollaert S and Grahn J 2008 Electricalcharacterization and small-signal modelling of InAs/AlSb HEMTs for low-noise and high-frequency applications IEEE Trans. Microw. Theory Tech. 56 2685 [8] Mateos J, González T, Pardo D, Hoel V, Happy H and Cappy A 2000 Improved Monte Carlo algorithm for the simulation of δ-doped AlInAs/GaInAs HEMT’s IEEE Trans. Electron Devices 47 250  [9] Mateos J, González T, Pardo D, Höel V and Cappy A 2000 Monte Carlo Simulator for the design optimization of low-noise HEMTs IEEE Trans. Electron Devices 47 1050  10[10] Rodilla H, González T, Pardo D and Mateos J 2009 High-mobility heterostructures based on InAs and InSb: a Monte Carlo study J. Appl. Phys. 105 113705  [11] Mateos J, González T, Pardo D, Bollaert S, Parenty T, and Cappy A 2004 Design optimization of AlInAs-GaInAs HEMTs for high-frequency applications IEEE Trans. Electron Devices 51 521 [12] Schwierz F and Liou J J 2001 Semiconductor devices for RF applications: Evolution and current status Microelectron. Reliab. 41 1450   11FIGURE CAPTIONS  Figure 1. Scheme of the simulated devices.  Figure 2. MC results for the frequency dependence of (a) U and (b) K, with (red triangles) and without (green circles) considering rgd and rgs in the the IG Sb-HEMT. VDS=0.2 V and ID=50 mA/mm.   Figure 3. (a) MC results for the frequency dependence of U (red circles) and |Hfe|2 (red triangles) for ID=50 mA/mm; the values of fT and fmax are also indicated. (b) Simulation results for fT (red triangles solid line) and fmax (blue circles dashed lines) for different ID compared with the experimental data (red solid line for fT, and blue dashed line for fmax). Everything for the IG Sb-HEMT for VDS=0.2 V  Figure 4. Comparison between MC simulation results of fT (circles) and fmax (triangles) for IG (red dashed lines) and SG Sb-HEMTs (green solid lines) for different values of ID and VDS=0.2 V.  Figure 5. MC results for the intrinsic noise parameters for 10GHz (a) P, (b) R and (c) C in the IG Sb-HEMT (red circles solid lines) compared with the SG Sb-HEMT (green triangles, dashed lines), all as function of ID at VDS=0.2 V. In the inset, frequency dependence of P, R and C around ID=50 mA/mm.   Figure 6. Comparison between the simulation results for the IG Sb-HEMT (red circles) and the SG ones (green triangles). (a) Minimum noise figure, NFmin,  (b) associated gain, Gass and (c) noise resistance, Rn, as a function of the drain current for two different frequencies 19 GHz (solid lines) and 94 GHz (dashed lines). The available experimental data at 10 GHz are also included (yellow stars) [7].    12  Figure 1. Scheme of the simulated devices.   13  Frequency (GHz)1 10 100U (db)01020304050with rgd=12K; rgs=14.6K rgd = rgs =  Frequency (GHz)0 10 20 30 40 50Stabillity factor K0.00.51.01.52.0(a)(b)   Figure 2. MC results for the frequency dependence of (a) U and (b) K, with (red triangles) and without (green circles) considering rgd and rgs in the the IG Sb-HEMT. VDS=0.2 V and ID=50 mA/mm.    14  frequency (GHz)1 10 100U, |Hfe|2 (dB)010203040 U|Hfe|2fmaxfTID (mA/mm)0 50 100 150 200 250f t, f max (GHz)050100150200250(a)(b)fT expfmax exp fmax MCfT  MC  Figure 3. (a) MC results for the frequency dependence of U (red circles) and |Hfe|2 (red triangles) for ID=50 mA/mm; the values of fT and fmax are also indicated. (b) Simulation results for fT (red triangles solid line) and fmax (blue circles dashed lines) for different ID compared with the experimental data (red solid line for fT, and blue dashed line for fmax). Everything for the IG Sb-HEMT for VDS=0.2 V   15 ID (mA/mm)0 50 100 150 200 250f t, f max (GHz)050100150200250fmax MC IGfT MC IGfmax MC SGfT MC SG   Figure 4. Comparison between MC simulation results of fT (circles) and fmax (triangles) for IG (red dashed lines) and SG Sb-HEMTs (green solid lines) for different values of ID and VDS=0.2 V.   16 ID (mA/mm)0 50 100 150 200 250C0.00.51.0P051015IG Sb-HEMTSG Sb-HEMTR0.00.51.01.5Frequency (GHz)1 10 100P012Frequency (GHz)1 10 100R012Frequency (GHz)1 10 100C0.00.51.0  Figure 5. MC results for the intrinsic noise parameters for 10GHz (a) P, (b) R and (c) C in the IG Sb-HEMT (red circles solid lines) compared with the SG Sb-HEMT (green triangles, dashed lines), all as function of ID at VDS=0.2 V. In the inset, frequency dependence of P, R and C around ID=50 mA/mm.    17  NFmin (dB)0246810        expID (mA/mm)0 50 100 150 200 250Noise resistance, Rn ()10100IG@10GHzIG@94GHzSG@10GHzSG@94GHzGass (dB)-505101594GHz10GHz94GHz10GHz  Figure 6. Comparison between the simulation results for the IG Sb-HEMT (red circles) and the SG ones (green triangles). (a) Minimum noise figure, NFmin,  (b) associated gain, Gass and (c) noise resistance, Rn, as a function of the drain current for two different frequencies 19 GHz (solid lines) and 94 GHz (dashed lines). The available experimental data at 10 GHz are also included (yellow stars) [7].  

Monte Carlo study of the noise performance of isolated-gate InAs/AlSb HEMTs

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Monte Carlo study of the noise performance of isolated-gate InAs/AlSb HEMTs

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