Studies of Electrical Characteristics of MESFET Using Wbg Iv-Iv SiC as Potential Substrate Material

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1 Studies of Electrical Characteristics of MESFET Using Wbg Iv-Iv SiC as Potential Substrate Material Maitreyi Ray Kanjilal 1, Damayanti Ghosh 2 Department of Electronics & Communication Engg. Narula Institute of Technology 81,Nilgunj Road,Agarpara,Kolkata ,India mr.kanjilal@gmail.com,ghosh.damayanti@rediffmail.com Moumita Mukherjee 3 Scientist- B (Senior Assistant Professor Grade) CMSDS, University of Calcutta, Kolkata, India mou_mita_m@yahoo.com Abstract The paper reports the intrinsic electrical characteristics such as Drain as well as Transfer characteristics and also Noise Power spectral density analysis of MESFET operating at ambient temperature. A generalized simulation technique has been developed by the authors for this purpose. The studies have been made on MESFET using different SiC polytypes, such as, 3C, 4H and 6H-Silicon Carbides (SiC) as substrate materials. To the best of authors knowledge, this is the first report on comparative analysis of electrical properties of all the cubic and hexagonal SiC based MESFETs. The higher band gap of SiC has enforced it to be used for high-field and high-current operation. The high current carrying capability of the devices over a wide range of drain-to-source voltage and significantly better noise performance over a range of frequency has been revealed as a result of using these widebandgap materials. Here low frequency noise is observed in MESFET with different types of substrate materials. stacking: It is commonly used in naming of 3C-SiC: the alphabet C denotes cubic symmetry and 3 means it has three bilayer periodicity. Similarly in 4H-SiC and 6H-SiC; H denotes hexagonal symmetry and 4, 6 denote four bilayer and six bilayer symmetry respectively. The following figures represent the structure of these polymorphs of SiC [5]. Keywords-3C-SiC, 4H-SiC and 6H-SiC, drain characteristics, wide band gap material, MESFET, Noise Power Spectral Density, Transfer Characteristics. Fig.1: Structure of 3C-SiC. I. INTRODUCTION Metal Semiconductor Field Effect Transistor (MESFET) becomes an emerging device applicable both at low and high frequency range. It attains high value of carrier mobility with high transconductance and transit frequency. High transit frequency of the device has enforced it to enter into the field of microwave, strategic and military application. MESFET with 1000µm gate width and 1µm gate length has been considered to observe the behavior [1]. The fundamental approach is to evaluate a comparative study on the different electrical characteristics of MESFET exhibited by these polymorphs of SiC. They show excellent response due to their high efficiency, wide bandwidth over conventional Si and GaAs [2],[3]. The higher band-gap materials are used where high power density and high frequency applications are needed [4].The notation used for the polytypes of SiC has certain physical significance depending upon the periodicity of Fig.2: Structure of 4H-SiC Fig.3: Structure of 6H-SiC 71

2 Now-a-days SiC is gaining acceptance in the field of high-power, high-frequency semiconductor devices due to its promising material, thermal and chemical properties over the conventional Si and GaAs [6]. The authors have chosen SiC as substrate material due to its several advantages over the conventional Si and GaAs. SiC possesses higher band gap (~3.3eV), high breakdown electric field (2-4x106 V/cm), high thermal conductivity [1]. It has also high electron saturation velocity (2.7x10 7 cm/s), high minority carrier lifetime. In this material the chemical bonding is more stable [3] where as the thermal expansion co-efficient of SiC is very low (4.0x10-6 /K) which causes no phase distortion[5]. II. THEORY The cross-sectional view of the MESFET [7] is shown in the Fig.4. Similarly, the drain current I d for the saturation region is given by [9]: } where K n = I d (V ds, Vgs) = K n for V ds > V gs V p (2) a= thickness of the channel V bi = built-in potential V p = pinch-off voltage N d is the channel doping concentration µ is the mobility of electrons in substrate material ε s is the permittivity of the substrate material used B. Transfer Characteristics The transfer characteristics can also be given by the following empirical relationship [8]: = (3) Where V sat,n = drift saturation velocity of electrons. Fig.4: Simplified cross-sectional structure of MESFET To study the noise power spectral density of MESFET for drain current fluctuation, the drain current I d is estimated for linear and saturation region using gradual channel approximation and constant mobility assumption[8]. A. Drain Characteristics For linear region: The drain current I d for the linear region is given by [9]: I d (V ds, Vgs) = K n For V ds V gs -V p.. (1) } C. Low Frequency Noise or 1/f Noise Using Hooge s relationship the noise power spectral density for drain current fluctuation can be obtained by the relation [8]: S ID =I d 2 (4) where N= no. of carriers in a given volume f = frequency of operation = dimensionless Hooge s constant The noise level in the semiconductor materials are characterized by this Hooge s constant. The noise spectra reveals the form of Flicker Noise [8]. III. RESULTS The results based on a generalized simulation analysis are discussed here to evaluate the drain characteristics, transfer characteristics, noise power spectral density over a range of frequencies. Various parameters used for the simulation are mentioned in the table I [10]. 72

3 TABLE I. Paramet er with symbol VALUES OF DIFFERENT PARAMETERS Different Parameter values for the polymorphs of SiC Value with Value with Value with unit unit unit 3C-SiC 6H-SiC 4H-SiC Mobility (µ n ) 800x10-4 Bandgap energy(e g) N c N v Hooge s Paramete r (α H) m 2 /V-s 600x10-4 m 2 /V-s 420x10-4 m 2 /V-s 2.36eV 3.23eV 3.0eV 1.5x10 17 /m 3 1.7x10 17 /m x10 17 /m 3 1.2x10 17 /m 3 2.5x10 17 /m 3 2.5x10 17 /m Fig.6: Drain characteristics of 4H-SiC MESFET Relative Permittiv ity(ε r) Drift Saturatio n velocity of electrons (v sat) x10 5 m/s 8x10 4 m/s 2x10 5 m/s Fig.5,6 and 7 show the drain characteristics of the MESFET for 3C-SiC,4H-SiC and 6H-SiC, respectively, used as substrate materials from the equation (1) and (2). The analysis is made on low frequency because the effect on the device capacitance is not taken into account. Fig.7: Drain characteristics of 6H-SiC MESFET All the drain characteristics for different SiC materials are compared in Fig.8. Fig.5: Drain characteristics of 3C-SiC MESFET 73

4 substrate materials with higher saturation drift velocity and mobility. The results of Power spectral density over a range of drain to source voltage, gate to source voltage and frequency are shown in the Fig.10, Fig.11 and Fig.12 respectively with N=10 17 /cm 3.Here drain to source voltage V ds is varied from 0 to 5 Volt and gate to source voltage V gs is varied from -1.5 to 0 Volt. Low frequency analysis is done to observe the form of Flicker Noise. Fig.8: Comparative analysis of Drain characteristics of MESFET The drain characteristics show the similar nature as expected. The high current carrying capability of the wide band gap materials has been established in the analysis. It is interesting to observe that among all these polytypes of SiC, 3C-SiC has the capability to introduce highest current in comparison with its 4H and 6H-SiC counterparts. The comparative analysis of transfer characteristics on MESFET is shown in Fig. 9 using different substrates from the equation (3). Here different drift saturation velocities are considered for different substrate materials as mentioned in the table 1. Fig.10: Comparative study on noise power spectral density characteristics of MESFET over a range of Vds Fig.9: Comparative study on transfer characteristics of MESFET for different semiconductor materials. Fig. 11.Comparative study on Noise Power Spectral Density of MESFET with respect to Vgs using Si, GaAs and SiC as substrate materials A comparative study of the transfer characteristics of MESFETs (Fig.9) based on different polytypes of IV-IV SiC reveals that the drain current (I d ) is increasing for 74

5 produces higher drain current as compared to other SiC structure counterparts. Also its noise power spectral density analysis as compared to the counterparts reveals better performance. Moreover, its high breakdown electric field (5x10 6 V/cm) is preferable for reverse blocking voltage above 600V [10]. Fig.12: Comparative study on the noise power spectral density characteristics of MESFET over a range of frequency at Vds=5 Volt and Vgs= -0.5Volt IV. DISCUSSION From the drain characteristics curve it has been revealed that the higher mobility of carriers leads to higher drain current over a range of drain to source voltage and gate to source voltage. The transfer characteristics show that higher saturation drift velocity of electrons produces higher drain current. Thus, due to higher mobility of electrons and high saturation drift velocity of electrons 3C-SiC and 6H-SiC MESFET provide higher drain current. From the results of power spectral density variation it has been observed that higher drain current of the material offers higher noise power spectral density. If the gate to source voltage is varied towards positive it will increase the drain current, accordingly the noise power spectral density increases. Hence more the reverse bias of gate to source voltage lesser will be the value of noise power spectral density which is a figure of merit. Low frequency noise has significant impact on circuit performance. It occurs mainly due to interactions with the impurities or dislocations which lead to enhanced scattering of charge carriers [11]. The study shows that the noise power spectral density (S id ) is inversely proportional to the frequency. As the frequency increases, S id decreases, which is a figure of merit for the MESFET devices. Due to its highly promising material properties, the polymorphs of SiC such as 3C, 4H and 6H-SiC enforced better performance of MESFET when they are used as substrate material. Amidst the three polymorphs of SiC- 3C, i.e. cubic structure, exhibits best performance because of its interesting chemical, thermal and electronic property. In 3C-SiC mobility is high (800 cm 2 /V-s) due to the less phonon scattering [10],[12] which in turn ACKNOWLEDGMENT The authors would like to express our dearest thanks to Prof.Kaushik Sarkar, Asst. Professor of Narula Institute of Technology, Kolkata, West Bengal, India for his support in many aspects of the works. Moumita Mukherjee wishes to acknowledge Defence Research and Dev. Org, Ministry of Defence, Govt. of India and University Grants Commission, Govt. of India, for their support to do this work. REFERENCES [1] S.N.Chattopadhyay, P.Pandey, C.B.Overton, S.Krishnamoorthy and S.K.Leong, Simulation of 4H-SiC MESFET for High Power and High Frequency Response, Journal of Semiconductor Technology and Science,VOL.8,NO.3,SEPTEMBER, [2] Alicja Konczakowska, Jacek Cichosz, Dariusz Dokupil,Pawel Flisikowski, The Low Frequency Noise Behaviour of SiC MESFETs,21st International Conference on Noise and Fluctuation,IEEE,2011. [3] J.W.Milligan,J.Henning,S.T.Allen,A.Ward,P.Parikh,R.P.Smith, Tr ansition of SiC MESFET Technology from Discrete Transistors to High Performance MMIC Technology, Cree, Inc., 4600 Silicon Drive, Durham, NC [4] Stefan Steinhoff IXYS and Prof. Dr. Ing. Manfred Reddig, GaAs Schottly Diodes Allow Higher Power Density, Power System Design Europe,December 2004,pp [5] [6] Hoon Joo Na, Sang Yong Jung, Jeon Hyun Moon, Jeong Hyuk Yim,Ho Keun Song and Hyeong Joon Kim, 4H-SiC Planar MESFET for Microwave Power Device Applications, Journal of Semiconductor Technology and Science,VOL.5,NO.2,JUNE,2005. [7] [8] Sheil A Prasad, Herman Schumacher, Anand Gopinath, High Speed Electronics and Optoelectronics Devices and Circuits, Cambridge University Press, New York, 2009, pp [9] W.J. Choyke,H. Matsunami,G.Pensl, Eds., Silicon Carbide: Recent Major Advances,Springer,Berlin,Germany,2004. [10] [11] Martin Von Haartman, Low-Frequency Noise Characterization, Evaluation and Modeling of Advanced Si and SiGe Based CMOS Transistors,Stockholm,Sweden,2006. [12] 75

6 Dr. M. Ray. Kanjilal is at present Professor and Head in department of Electronics and Communication Engineering at Narula Institute of Technology, Kolkata. She has a wide teaching experience since 1998 in the colleges under C.U, Asansol engineering College, Narula Institute of Technology. She was also Head of the Department of ECE and EIE departments at Techno India College of Technology, Kolkata, India. She received her Ph.D from the University of Calcutta in the Year She is associated with different universities for various academic activities and also a life member of IETE, ISTE and Professional member of IEEE. Her field of interest is low and high frequency semiconductor devices and their applications, microelectronics and VLSI. published from UK. Dr. Mukherjee is the Technical- Reviewer of a number of reputed international journals in India and abroad. Dr. Mukherjee has received a number of international awards by Int. Biographical Centre, IBC, UK, American Biographical Centre, USA, and Marque s Who s Who, USA. She has received IEEE Best Paper Award two times in 2009 and Dr. Mukherjee is the member of IEEE (USA), IEEE-ED society (USA), IEEE- MTTs (USA) and Indian Science Congress. Damayanti Ghosh is associated with the department of Electronics and Communication Engineering at Narula Institute of Technology, Kolkata after completion of her M.Tech in Radio Physics and Electronics from University of Calcutta ( CU). She has published few papers on MESFET and her area of interest is high frequency devices like MESFET, HEMT, IMPATT etc. Dr. Moumita Mukherjee has received M.Sc. degree in Physics with specialization in Electronics and Communication and Ph.D. (Tech) in Radio Physics and Electronics from University of Calcutta, India. During June 2003 to June 2010, she worked first as a Senior Research Fellow (SRF) and then as a Research Associate of Defence Research and Development Organization (DRDO), Ministry of Defence, New Delhi, India. She is presently working as Scientist B (Sr. Assistant Prof.) at CMSDS, a centre of DRDO and University of Calcutta. Her research interest is focused on the modeling/simulation and fabrication of Millimeter and Sub-millimeter (Terahertz) wave high power homojunction/hetero-junction devices based on wide-band-gap semiconductors, photonics, nano-scale THz devices. She is the principal author of more than 100 peer-reviewed research papers on semiconductor devices in reputed International refereed journals and reviewed IEEE- Proceedings. She is the author of 8 invited international bookchapters and worked as a Chief Editor of 2 books 76