Parasitic Resistance Effects on Mobility Extraction of Normally-off AlGaN/GaN Gate-recessed MISHFETs

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.1, FEBRUARY, 2018 ISSN(Print) 1598-1657 https://doi.org/10.5573/jsts.2018.18.1.078 ISSN(Online) 2233-4866 Parasitic Resistance Effects on Mobility Extraction of Normally-off AlGaN/GaN Gate-recessed MISHFETs Geunho Cho, Ho-young Cha, and Hyungtak Kim Abstract The extraction of channel mobility was performed on normally-off AlGaN/GaN gate-recessed metal-insulator-semiconductor heterostructure field effect transistors (MISHFETs) on GaN-on-Si substrate. Channel mobility was extracted from the unit device with 2 μm gate length by taking account of voltage drop across the gate-to-drain and the gate-tosource regions in order to minimize the effect of parasitic resistance. Extracted mobility became quite close to the mobility of 100 μm gate-length device suggesting that the parasitic resistance should be considered to provide accurate information on the channel under the recessed gate. Index Terms Nitride semiconductor, power device, parasitic resistance, mobility extraction, heterostructure, MISHFET, recessed-gate I. INTRODUCTION Field effect transistors (FETs) based on AlGaN/GaN heterostructures have been strong candidates for high frequency and high power applications owing to excellent material properties such as high electron mobility and large breakdown field [1-3]. Outstanding performance of high frequency operation with high output power was recently reported for broadband power amplification [4, 5]. Channel mobility in two-dimensional electron gas (2DEG) is a useful and critical parameter to evaluate Manuscript received May. 26, 2017; accepted Oct. 8, 2017 School of Electronic and Electrical Engineering, Hongik University, Seoul 121-791, Korea E-mail : hkim@hongik.ac.kr carrier transport property and device performance for high frequency and high power applications. Channel mobility extraction from GaN-based FETs utilizes the transfer characteristics, i.e. I D -V G characteristics with current-voltage relationship described in Si metal-oxidesemiconductor field effect transistor (MOSFET) [6, 7]. However, AlGaN/GaN high electron mobility transistors (HEMTs) or MISHFETs have an access region, i.e. the resistive region outside the gate with source-to-gate and gate-to-drain spacing as shown in Fig. 1, disparate from Si MOSFETs with gate-to-drain (or source) overlaps. Therefore, resistive components exist in source-to-gate and gate-to-drain channel region of MISHFETs and cause voltage drop which should be taken into consideration for accurate evaluation of channel mobility. In power electronics, normally-off devices are preferred to secure low power consumption and safe operation. 2DEG channel spontaneously formed at AlGaN/GaN heterostructure makes a HEMT depletion-mode, i.e. normally-on operation. Several types of device structures were developed to convert intrinsically normally-on devices to normally-off devices employing gate-recess etching, fluorine implantation, or p-gan gate structure [8-11]. Especially for devices with recessed gate structure, more accurate extraction of channel mobility under the recessed region is required since gate-recess etching can significantly modify carrier transport property in the channel under the etched area. This information will help optimize the gate-recess process. Fukui introduced a method to separate the source-todrain channel into field-effect (under the gate) and resistive (outside the gate) sections [12]. Applying this method to the GaN HEMTs showed that the extracted parameters under the gate was highly sensitive to the

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.1, FEBRUARY, 2018 79 L SG L G L GD S G SiNx layer, 6 nm D GaN cap layer, 1.25 nm Al 0.25 Ga 0.75 N, 20 nm access region GaN buffer, 3~4 μm (a) Si(111) substrate Fig. 1. The schematic cross-sectional view of the fabricated normally-off AlGaN/GaN gate-recessed MISHFETs. threshold voltage [13]. Though Fukui s method was modified for GaN HEMTs to solve this problem in [13], this method yields average values of mobility and parasitic resistance over V GS. Average values are not suitable for process monitoring or physical understanding because they may provide misinformation on the conditions of fabricated devices. In this work, we fabricated normally-off AlGaN/GaN MISHFETs using gate recess process with SiN x gate insulator. Ungated devices, i.e. transmission lines, with different spacings were also fabricated to evaluate the resistance of the channel out of gate area. Using modified current equation of MOSFETs which reflects the effect of resistive access region, we calculated the field-effect mobility of AlGaN/GaN gate-recessed MISHFETs. The result suggested that this approach can provide proper estimation of the channel mobility under the recessed gate of AlGaN/GaN gate-recessed MISHFETs. II. DEVICE FABRICATION The AlGaN/GaN MISHFETs with recessed gate were fabricated on commercially available GaN-on-Si substrate. As shown in Fig. 1, the epitaxial structure consists of a 1.25 nm undoped GaN capping layer, a 20 nm undoped Al 0.25 Ga 0.75 N barrier, and 3~4 µm undoped- GaN buffer layer on n-type Si (111) substrate. For low resistive source and drain ohmic contacts, a Ti/Al/Ni/Au (20/120/25/50 nm) metal stack was alloyed at 830 C for 30 sec in nitrogen ambient after the ohmic recess process. After mesa etching defined active regions for device isolation, AlGaN barrier layer was recessed by using dry etching process (22 nm) and digital etching process (1 nm) to enable normally-off operation. Then a (b) Fig. 2. The optical microscopy image of (a) the unit MISHFETs with L GD = 5 μm, L GD = 10 μm and L GD = 15 μm, (b) the fat FET with L G =100 μm. Fig. 3. Equivalent circuit of the fabricated normally-off AlGaN/GaN gate-recessed MISHFETs for channel mobility extraction with parasitic resistance included. 6 nm SiN x dielectric layer was deposited and annealed at 400 C for 10 minutes in nitrogen ambient. Finally, the gate electrode was formed by Ni/Au (20/200 nm) evaporation. More detailed information can be found in [14-16]. Devices with the gate-to-drain distance (L GD ) of 5 μm, 10 μm and 15 μm were fabricated in order to investigate the influence of the parasitic series resistance of gate-drain access region. The source-to-gate distance (L SG ) and the gate length (L G ) were 1.5 μm and 2 μm, respectively. The fat FET with L G =100 μm was also fabricated to obtain channel mobility with minimizing the effect of series resistance. The optical microscopy images of the fabricated devices are shown in Fig. 2. III. EXTRACTION METHOD The equivalent resistance circuit of normally-off AlGaN/GaN MISHFETs is shown in Fig. 3 with contact

80 GEUNHO CHO et al : PARASITIC RESISTANCE EFFECTS ON MOBILITY EXTRACTION OF NORMALLY-OFF AlGaN/GaN resistance (R C ), source-to-gate resistance (R SG ), channel resistance under the gate (R gate ), and gate-to-drain resistance (R GD ). For small drain-to-source voltage (V DS < V G -V T ), MOSFET operates in linear region and drain current (I D ) is described in many textbooks as = {( ) 1 2 2 } (1) In Eq. (1), µ n and V T represent the electron mobility and the threshold voltage, respectively, and V DS and V G are bias voltages applied externally for the measurement. In case of Si MOSFET with source (or drain) region overlapped to gate, V DS is the lateral voltage drop between source and drain junction underneath the gate region controlled by gate bias. However, this is not the case for GaN-based power FETs with S-G or G-D spacing as shown in Fig. 2. Externally applied V DS and V G cannot describe current-voltage characteristics accurately because of parasitic resistance out of gate area. In case of AlGaN/GaN MISHFET, V DS represents the sum of the voltages which include the voltage drop in contact resistance (V C ), the voltage drop across the 2DEG under the gate (V DS(gate) ) which corresponds to V DS for Si MOSFET, and the voltage drop in source-to-gate channel and gate-to-drain channel (V SG, V GD ). Therefore, V DS can be expressed as = 2 + + ( ) + = ( ) + (2 + + ) R SG and R GD can be calculated from the sheet resistance (R sh ) and R C which we can extract using transmission line measurement (TLM) as shown in Fig. 4. In Eq. (1), V DS should be replaced by V DS(gate) in order for intrinsic drain- source voltage in the gate region of MISHFET to be properly implemented. The voltage across the gate electrode and the 2DEG channel at the source end of the gate (V GC ) is equal to the difference between the applied gate voltage (V G ) and the voltage drop by source contact resistance and source-togate resistance as in Eq. (3). (2) = ( + ) (3) Fig. 4. Extraction of the contact resistance (R C ) and the sheet resistance of the ungated region in the channel from the TLM measurement. Finally, with Eqs. (2, 3), drain current of normally-off AlGaN/GaN gate-recessed MISHFETs can be described solely by intrinsic FET parameters as in = ( ) ( ) 1 2 2 ( ) = ( )( A ) with A = 2 R + R + R. C SG GD 1 2 ( A ) 2 From the first derivative of Eq. (4) with respect to V GC, we can eliminate V T to prevent possible errors introduced by V T determination and then obtain intrinsic transconductance (g m ). = {( ) ( ) = [( ) + ( )} (4) A + 1 ( ( ) 2 ) +A ( )] = ( ) + 1 2 ( ) (5) Field effect mobility of recessed channel can be calculated using

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.1, FEBRUARY, 2018 81 Fig. 5. The transfer characteristics of the normally-off AlGaN/GaN gate-recessed MISHFETs at V DS = 1 V with L G = 2 μm and different G-D spacing. Fig. 6. Field-effect mobility of the normally-off AlGaN/GaN gate-recessed MISHFETs extracted by conventional method based on Eq. (1). = ( 1 + 1 2 )( ) 2 (6) with I D, V DS, and g m obtained from the device measurement. IV. EXPERIMENTAL RESULTS In order to make fair comparison for two methods, we fabricated the devices with three different L GD of 5 μm, 10 μm, and 15 μm, and very long channel device with L G = 10 μm (fat FET). The fat FET was designed to exhibit high channel resistance so that the series parasitic drainend and source-end resistance could be neglected. The mobility extracted from the fat FETs will correspond to the intrinsic mobility of the 2DEG channel. Fig. 5 illustrates transfer current-voltage (I D -V G ) characteristics measured by using Agilent 4155A parameter analyzer. Fig. 6 shows the µ n extracted from the measured I D -V G data using conventional method based on Eq. (1). Clear discrepancy of the extracted µ n was observed between the unit FETs and fat FET. The peak µ n of the unit devices ranges from 110 to 130 cm 2 /V s, compared to 320 cm 2 /V s of the fat FET. In addition, the difference between µ n from the unit device and that from the fat FET magnifies with longer L GD. This result is attributed to the impact of resistance of the access region in S-G and G-D spacing which can be neglected in the fat FET. In other words, the µ n extracted from the unit device represents the mobility combining both channel mobility under the gate and the access Fig. 7. Intrinsic transconductance of the normally-off AlGaN/GaN gate-recessed MISHFETs at V DS = 1 V calculated from I D -V GC characteristics. region mobility together. Fig. 7 shows intrinsic g m from I D -V GC characteristics which was used to calculate field effect mobility. The µ n was extracted by using proposed method based on Eq. (6) and the result is shown in Fig. 8. In case that parasitic resistance was considered, extracted µ n was quite close to the value extracted from the fat FET, suggesting that this method can eliminate the effect of the parasitic voltage drop in the access region on the field effect mobility extraction. Channel mobility in Si MOSFETs is considered to be constant regardless of the gate length, i.e. source to drain spacing when the mobility extraction is based on Eq. (1). Because normally-off AlGaN/GaN gate-recessed MISHFETs can be regarded as MOSFETs with additional parasitic resistance, mobility of normally-off AlGaN/GaN MISHFETs should be also independent of

82 GEUNHO CHO et al : PARASITIC RESISTANCE EFFECTS ON MOBILITY EXTRACTION OF NORMALLY-OFF AlGaN/GaN Fig. 8. Field-effect mobility of the normally-off AlGaN/GaN gate-recessed MISHFETs extracted by using modified drain current equation. L G and G-D spacing as shown in Fig. 8, compared with Fig. 6. V. CONCLUSIONS The effect of parasitic resistance on mobility extraction was evaluated in normally-off AlGaN/GaN gate-recessed MISHFETs. Series resistance outside the gate region was obtained from TLM measurement and excluded to determine the channel mobility under the recessed-gate. Compared to the mobility calculated by the traditional method, extracted mobility exhibited similar value to the mobility of the fat FETs in which parasitic series resistance hardly affects channel mobility extraction. ACKNOWLEDGMENTS This research was supported by Basic Science Research Program (NRF-2016R1A2B4010474, NRF- 2015R1A6A1A03031833) and was also supported under the framework of international cooperation program (NRF-2016K2A9A1A01952082) managed by National Research Foundation of Korea REFERENCES [1] O. Ambacher, B. Foutz, J. Smart, J. R. Shealy, N. G. Weimann, K. Chu, M. Murphy, A. J. Sierakowski, W. J. Schaff, and L. F. Eastman, Two- dimensional electron gases induced by spontaneous and piezoelectric polarization in undoped and doped AlGaN/GaN heterostructures, Journal of Applied Physics, Vol. 87, No. 1, pp. 334-344, Jan., 2000. [2] J.-G. Lee, B.-R. Park, H.-J. Lee, M. Lee, K.-S. Seo, and H.-Y. Cha, State-of-the-Art AlGaN/GaN-on- Si Heterojunction Field Effect Transistors with Dual Field Plates, Applied Physics Express, Vol. 5, No. 6, p. 066502, May, 2012. [3] S. Dimitrijev, J. Han, H. A. Moghadam, and A. Aminbeidokhti, Power-Switching Applications Beyond Silicon: The Status and Future Prospects of SiC and GaN Devices, MRS Bulletin, Vol. 40, pp. 399-405, May, 2015. [4] J. Kim, K. Choi, S. Lee, H. Park, and Y. Kwon, 6 18 GHz Reactive Matched GaN MMIC PowerAmplifiers with Distributed L-C Load Matching, Journal of Electromagnetic Engineering and Science, Vol. 16, No. 1, pp. 44-51, Jan., 2016. [5] H. Park, W. Lee, J. Jung, K. Choi, J. Kim, W. L. Lee, C. Lee, and Y. Kwon, A 6 16 GHz GaN Distributed Power Amplifier MMIC Using Selfbias, Journal of Electromagnetic Engineering and Science, Vol. 17, No. 2, pp. 105-107, Apr., 2017. [6] D. K. Schroder, Semiconductor Material and Device Characterization, John Wiley & Sons, 2006. [7] H. Kambayashi, Y. Satoh, T. Kokawa, N. Ikeda, T. Nomura, and S. Kato, High field-effect mobility normally-off AlGaN/GaN hybrid MOS-HFET on Si substrate by selective area growth technique, Solid-State Electronics, Vol. 56, pp. 163-167, Feb., 2011. [8] W. Saito, Y. Takada, M. Kuraguchi, and I. Omura, Recessed-gate structure approach toward normally off high-voltage AlGaN/GaN HEMT for power electronics application, IEEE Transactions on Eletron Devices, Vol. 53, pp. 356-362, Feb., 2006. [9] B.-R. Park, J.-G. Lee, W. Choi, H. kim, K.-S. Seo, and H.-Y. Cha, High-Quality ICPCVD SiO 2 for Normally Off AlGaN/GaN-on-Si Recessed MOSHFETs, IEEE Electron Device Letters, Vol. 34, pp. 354-356, Jan., 2013. [10] Y. Cai, Y. Zhou, K. M. Lau, and K. J. Chen, Control of Threshold Voltage of AlGaN/GaN HEMTs by Fluoride-Based Plasma Treatment: From Depletion Mode to Enhancement Mode, IEEE Transaction on Electron Devices, Vol. 53, pp. 2207-2215, Aug., 2006.

JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.18, NO.1, FEBRUARY, 2018 83 [11] Y. Uemoto, M. Hikita, H. Ueno, H. Matsuo, H. Ishida, M. Yanagihara, T. Ueda, T. Tanaka, and D. Ueda, Gate Injection Transistor (GIT)-A Normally-Off AlGaN/GaN Power Transistor Using Conductivity Modulation, IEEE Transaction on Electron Devices, Vol. 54, pp. 3393-3399, Nov., 2007. [12] H. Fukui, Determination of the Basic Device Parameters of a GaAs MESFET, The Bell System Technical Journal, Vol. 58, No. 3, Mar., 1979. [13] R. Menozzi et al., Temperature-dependent characterization of AlGaN/GaN HEMTs: Thermal and source/drain resistances, IEEE Transactions on Device Materials and Reliabilty, vol. 8, no. 2, pp. 255 264, Jun. 2008. [14] D. Buttari, A. Chini, G. Meneghesso, E. Zanoni, B. Moran, S. Heikman, N.Q. Zhang, L. Shen, R. Coffie, S. P. DenBaars, and U. K. Mishra, Systematic Characterization of Cl2 Reactive Ion Etching for Improved Ohmics in AlGaN/GaN HEMTs, IEEE Electron Device Letters, Vol. 23, pp. 76-78, Aug., 2002. [15] Y. Wang, M. Wang, B. Xie, C.P. Wen, J. Wang, Y. Hao, W. Wu, K.J. Chen, and B. Shen, High- Performance Normally-Off Al 2 O 3 /GaN MOSFET Using a Wet Etching-Based Gate Recess Technique, IEEE Electron Device Letters, Vol. 34, pp. 1370-1372, Sep., 2013. [16] R. Yamanaka, T. Kanazawa, E. Yagyu, and Y. Miyamoto, Normally-off AlGaN/GaN highelectron-mobility transistor using digital etching technique, Japanese Journal of Applied Physics, Vol. 54, No. 6S1, p. 06FG04, Apr., 2015. Geunho Cho was born in Seoul, Korea, on 1990. He received the B.S. degree in Electronic and Electrical Engineering from Hongik University, Seoul, Korea, in 2015. His research interests include fabrication of high power AlGaN/GaN heterostructure FET and the device characterization. Ho-young Cha received the B.S. and M.S. degrees in electrical engineering from Seoul National University, Seoul, Korea, in 1996 and 1999, respectively, and the Ph.D. degree in electrical and computer engineering from Cornell University, Ithaca, NY, in 2004. He was a Postdoctoral Research Associate with Cornell University until 2005, where he focused on the design and fabrication of SiC and GaN electronic devices and GaN nanowires. He was with the General Electric Global Research Center, Niskayuna, NY, from 2005 to 2007, developing wide-bandgap semiconductor sensors and high power devices. Since 2007, he has been with Hongik University, Seoul, where he is currently a Professor in the School of Electronic and Electrical Engineering. His research interests include wide bandgap semiconductor devices. He has authored over 60 publications in his research area. Hyungtak Kim received the B.S. degree in Electrical Engineering from Seoul National University, Seoul, Korea and the M.S./Ph.D. degree in Electrical and Computer Engineering from Cornell University, Ithaca, New York, U.S.A., in 1996 and 2003, respectively. He is currently an associate professor of the school of electronic and electrical engineering at Hongik University, Seoul, Korea. His research interests include wide bandgap semiconductor devices and harsh environment electronics. During his graduate program, he studied reliability physics of GaNbased heterostructure devices. Prior to joining with Hongik University, he spent 4 years developing CMOS devices and process integration for 60nm DRAM technology as a senior engineer in the semiconductor R&D center at Samsung Electronics, Co. Ltd.