AlGaN/GaN high electron mobility transistors (HEMTs) on Silicon substrates grown by molecular beam epitaxy have been investigated using small-signal microwave measurements, to see performance Radio-frequency of components. Passivation of HEMT devices SiO /SiN a different pretreatment is used to reduce the effects of trapping and consequently has a large effect on these radio-frequency parameters. We used cold-FET and hot FET technique to extract the intrinsic and extrinsic parameters in order to show the effect passivation of parasitic elements; the parasitic capacitances, resistances and inductances. From this point we discover the extent of their impact on power and microwave performance. Keywords AlGaN/GaN/Si HEMTs · Passivation · Radio-frequency · Equivalent circuit parameters · Small signal modeling 1 Introduction 2 AlGaN/GaN/Si HEMT Structure AlGaN/GaN based high electron mobility transistors are The AlGaN/GaN HEMTs under investigation are grown on excellent candidates for high frequency and high power silicon (111) substrate by using molecular beam epitaxy (MBE) (present some high purity). The active layers consist applications [1–3]. The primary reason in the nitride gallium- based materials have wide band gaps, an efficient in a 500 nm thick of undoped AlN/AlGaN buffer, a carrier transport, large breakdown bias voltages and strong 1.8 μm undoped GaN channel, a 23 nm thick of undoped spontaneous and piezoelectric polarization fields . The Al Ga N barrier and a 1 nm n+-GaN cap layer. The 0.26 0.74 direct-current and radio-frequency parameters are found to device processing is made following conventional HEMT improve after passivation . Modeling of AlGaN/GaN/Si fabrication steps. The ohmic contact pads are patterned HEMTs for the subsequent design a low-noise amplifier is using e-beam lithography. Hereafter, the metallization by the objective of our study. means of evaporated 12/200/40/100 nm Ti/Al/Ni/Au is deposited at 900 C during 30 s. The Schottky gate is realized using 100/150 nm Mo/Au layers. On the other hand, the AlGaN/GaN HEMTs are passivated by 100/50 nm Malek Gassoumi SiO /SiN with pretreatment N OandNH . 2 2 3 email@example.com Laboratoire de Micro-Optoelectronique ´ et Nanostructures (LMON), Universite´ de Monastir, Faculte´ des Sciences, 3 Experimental Result Avenue de l’environnement, 5019 Monastir, Tunisie College of Science, Department of Physics, S-parameters measurement is an important step for estimat- Qassim University, Buraidah 51452, Saudi Arabia ing RF-performance of power component, and to extract the Institut d’Electronique de Microelectronique ´ various parameters of linear model. Measurements of these et de Nanotechnologie IEMN, parameters were performed under coplanar points up to 60 Universite´ des Sciences et Technologies de Lille, GHz using a vector network analyzer to characterize sam- Avenue Poincare, ´ Cedex 59652 Villeneuve d’Ascq, France Silicon Fig. 1 Mesured S-parameters before passivation of AlGaN/GaN/Si HEMT Fig. 2 Mesured S-parameters after passivation and with pretreatment of AlGaN/GaN/Si HEMT Silicon Table 1 Frequency characteristics H21 Sample F (GHz) F (GHz) F (GHz) T max MSG MSG Unpassivated 9,6 18,5 18,4 Passivated SiO /SiN with 32,2 41,9 41,9 NH pretreatment 15 Passivated SiO /SiN with 34,8 47,9 47,9 N O pretreatment From on wafer S-parameters measurements, Standard 1E9 1E10 1E11 extrapolations of frequency dependences of current gain Fréquence (GHz) (|H21|), the Mason gain (U) and maximum available gain (MSG) to higher frequencies in the shape of straight lines Fig. 3 Mesured current gain (H ), unilateral power gain (U) and with the slope of -20dB/decade were used to determine maximum available gain (MSG) before passivation of AlGaN/GaN/Si HEMT their points of intersection with the frequency axis. We have deduced the frequency parameters a function of frequency ranging from 250 MHz to 60 GHz. ples passivated SiO /SiN with N OandNH pretreatment Figures 3 and 4 shows the gains before and after 2 2 3 respectively. passivation for different pretreatment. The current gain and Figure 1 shows the experimental S-parameters of the maximum available gain to determine limits frequency AlGaN/GaN/Si HEMT before passivation on a frequency of the microwave component; cut-off and maximum range extends from 250 MHz to 67 GHz in 0.25 GHz step. frequency respectively. Table 1 summarizes the main for a bias V =− 1, 25 V and V = 15 V. characteristics microwave. gs ds Figure 2 shows the measured scattering parameters of We see from this table that passivation and surface treat- samples after passivation, we observe that the S-parameters ment improves the performance of different frequencies. spread more than before passivation Fig. 1; this change has The sample passivated SiO /SiN with N O pretreatment 2 2 a great influence on the performance frequency. corresponding largest frequencies; this can be explained by 45 45 H 2 1 H21 40 40 MS G 35 MSG 35 30 30 25 25 20 20 15 15 10 10 5 5 0 0 1E9 1E10 1E11 1E9 1E10 1E11 Fréquence (GHz) Fréquence (GHz) (a) (b) Fig. 4 Mesured current gain (H ), unilateral gain (U) and maximum available gain (MSG) after passivation of AlGaN/GaN/Si HEMT: (a), (b) Gain (dB) Gain (dB) Gain (dB) Silicon Fig. 5 Small-signal equivalent circuit of a transistor HEMTs the larger current as well as the higher speed of the carri- plucking voltage when the conduction transistor channel ers in this structure . Indeed, as the structure is undoped, [10, 11]. The most common method can be categorized into investors are best confined to the interface AlGaN/GaN and analytical and optimization based. do not enter a low mobility layer. For a game of extrinsic parameters given, one can The extrinsic values Ft and Fmax increase respectively determine the intrinsic elements from a series of matrix from 9.6 GHz to 32,2 GHz for NH pretreatment and manipulation, comparable to a peeling process of the outer 34.8 GHz for N O pretreatment and 18.5 GHz to 41.9 GHz layers to reach the heart of the component [12–14]. for NH and 47.9 GHz for N O pretreatment. Also, after The comparison of the measured data (circles) with 3 2 passivation the F /F ratio is close to 1, it attests a good simulation results (lines) for the S-parameters variation of max T confinement of the charges in the channel and confirms the model before and after passivation is shown in Fig. 6. the absence of charge coupling effects with the substrate as The agreements between the measured and modeled data observed on conductive substrates . are excellent in a wide frequency range. The extracted values for all extrinsic and intrinsic parameters are listed in Table 2. 4 Small-Signal Modeling of AlGaN/GaN/Si One of the main factors affecting the gain of a FET HEMT is the feedback capacitance Cgd. Usually, the gate-drain capacitance is almost independent of the gate voltage it The equivalent circuit model used for microwave HEMTs is decreases with increasing drain voltage. This shows the presented in Fig. 5 . It is contains two parts; an intrinsic advantage of biasing the device at larger drain voltages for part corresponds to the active portion of the transistor and better gain. Therefore, these devices have improved MSG an extrinsic part outside the active region, which include the (Maximum Stable Gain) with increasing drain bias voltage RF contact pads and gate, drain, and source metallization. due to reduced feedback capacitance . The extraction method used is based on measurements In this context, we see that this capacity decreases of the small signal S-parameters, the most commonly used to 271,1 pF before passivation to reach 114,2 pF and technique, it uses two measures S-parameters for different 126,8 pF after passivation with N Oand NH pretreatment 2 3 measuring polarization conditions; measuring cold FET and respectively. From this point, we deduce that components measure hot FET [9, 10]. with O N pretreatment may give more power. The extrinsic equivalent circuit parameters can be We observe that the gate-resistance is reduced to 7.5 evaluated from S-parameters measurements under cold and before passivation to up nearly half after passivation pinched conditions; Vds = 0VandVgs <<Vp, Vp is the with N O pretreatment, this has led to improve the output 2 Silicon Fig. 6 Measured (mes) and modeled (mod) S-parameters of HEMT: A—before passivation, B—after passivation performance of AlGaN/GaN/Si HEMT devices. Because, The RF parameters of AlGaN/GaN/Si HEMTs are increasing gate resistance implies increasing the charging improving more after passivation by SiO /SiN with N O 2 2 delay time and this results in decreasing the speed of device pretreatment. Then, choose this passivation with this . treatment during elaboration for this type device. Silicon Table 2 Extracted parameters values before and efter passivation a References different pretreatement 1. 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As has been shown that as the passivation with N O 2 122 pretreatment gives better results, it makes the device more 13. Caddemi A, Crupi G, Donato N (2016) Microwave characteri- zation and modeling of packaged HEMTs by a direct extraction quickly and give more power. This leads to devices with procedure down to 30 k. IEEE Trans Microw Theory Technol greatly improved characteristics. 55:2006 14. Helali A, Nouira W, Gassoumi M, Gassoumi M, Gaquiere ` Acknowledgments The authors gratefully acknowledge Qassim Uni- C, Maaref H (2016) Small signal modeling of HEMTs versity, represented by the Deanship of Scientific Research, on the AlGaN/GaN/SiC for sensor and high-temperature applications. material support for this research under the number (3286). Optik 127:7881–7888 15. Therrien R, Singhal S, Johnson JW, Nagy W, Borges R, Open Access This article is distributed under the terms of the Chaudhari A, Hanson AW, Edwards A, Marquart J, Rajagopal Creative Commons Attribution 4.0 International License (http:// P, Park C, Kizilyalli IC, Linthicum KJ (2005) A 36mm gan- creativecommons.org/licenses/by/4.0/), which permits unrestricted on-si HFET producing 368w at 60v with 70% drain efficiency. use, distribution, and reproduction in any medium, provided you give In: IEEE Electron devices meeting, IEDM tech. dig., pp 568– appropriate credit to the original author(s) and the source, provide a 571 link to the Creative Commons license, and indicate if changes were 16. Golio JM (ed) (2003) RF And microwave semiconductor device made. handbook. CRC Press, Boca Raton
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