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IEEE Press, New York City, NY, 2001. 15. H. Nishiyama and J. Nakazoe, Efficient calculation of interconnect capacitance and characteristic impedance for coupled pairs of micro- strip-like transmission lines, IEICE Trans Electron Commun (Part 2) (1997), 80. © 2007 Wiley Periodicals, Inc. ANALYTICAL PERFORMANCE EVALUATION OF AlGaN/GaN METAL INSULATOR SEMICONDUCTOR HETEROSTRUCTURE FIELD EFFECT TRANSISTOR AND ITS COMPARISON WITH CONVENTIONAL HFETs FOR HIGH POWER MICROWAVE APPLICATIONS Ruchika Aggarwal, 1 Anju Agrawal, 2 Mridula Gupta, 1 and R. S. Gupta 1 1 Semiconductor Devices Reasearch Laboratory, Department of Electronic Science, University of Delhi, South Campus, New Delhi 110021, India; Corresponding author: rsgu@bol.net.in 2 Department of Electronics, Acharya Narendra Dev College, University of Delhi, Kalkaji, New Delhi 110019, India Received 19 June 2007 ABSTRACT: In this work, a comprehensive analytical model for Al- GaN/GaN MISHFET has been presented to evaluate the drain current characteristics, transconductance, and cut-off frequency of the insulated device. The model takes into account polynomial dependence of sheet carrier density on position of quasi Fermi level to consider the quantum effects and to validate it from subthreshold to high conduction region. The effect of spontaneous and piezoelectric polarization at the AlGaN/ GaN interface and parasitic source/drain resistances have also been incorporated in the analysis. Its advantages over conventional HFET structure are discussed in detail. For a MISHFET with quarter micron gate length, the cut-off frequency is reported to be 52 GHz. The MISH- FET shows remarkable 36% increase in drain saturation current. The model has a broad utility as it is equally applicable to HFETs as well. The present model is based on closed form expression and does not in- volve any fitting parameter. The results obtained are compared with experimental data and show excellent agreement, thereby proving the validity of the model. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 331–338, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23073 Key words: AlGaN/GaN MISHFET; polarization; sheet carrier density; gate voltage swing; saturation drain current 1. INTRODUCTION The wide band gap AlGaN/GaN high electron mobility transistors (HEMTs) show great promise for applications such as high fre- quency wireless base stations and broad-band links, commercial and military radar and satellite communications [1-5]. The out- standing properties of nitride material system such as high electron mobility, high saturation velocity, low thermal impedance, and high breakdown field make them extremely promising devices for high power and high temperature microwave applications. GaN- based materials are usually grown in [0001] and [111] directions, and since these axes are polar, they cause GaN-based materials to exhibit strong lattice polarization effects. Because of the piezo- electric and spontaneous polarization fields, AlGaN/GaN-based HFETs have the ability to achieve two dimensional electron gas (2-DEG) with sheet carrier densities of the order of 10 12 – 10 13 cm -2 even without intentional doping. This mechanism of polar- ization leads to unprecedent high power densities and high current drive capability that are one order of magnitude higher than their silicon or GaAs counterparts [6, 7]. The development of new generations of AlGaN/GaN field-effect transistors (FETs) requires low gate leakage and superior pinch-off characteristics, specifi- cally at elevated temperatures for high temperature microwave power electronics [8]. These properties directly impact the device drain breakdown voltage, radio frequency (RF) performance, and noise figure. In the past, several groups have attempted to achieve gate leakage suppression and superior pinch-off characteristics by using the metal-insulator-semiconductor FETs (MISFETs) [9, 10] or metal-oxide-semiconductor FETs (MOSFETs) [11] device ap- proach. However, the performance level of all these insulated gate devices is well below that of the state-of-the-art AlGaN/GaN HFETs. Recently Khan et al. [12] reported the dc characterization results of AlGaN/GaN metal-insulator-semiconductor heterostruc- ture field-effect transistors (MISHFETs) on sapphire substrates. The built-in channel of MISHFET is formed by the high density 2-DEG at the AlGaN/GaN interface as in regular AlGaN/GaN HFETs. However, in contrast to HFETs, the metallic gate is isolated from AlGaN barrier layer by a thin Si 3 N 4 film. This insulator layer provides extremely low gate leakage current and allows for a large negative to positive gate voltage swing (GVS) [12]. Thus MISHFET combines the advantages of the MIS struc- ture that suppresses the gate leakage current and AlGaN/GaN heterointerface, which provides high-density high-mobility 2-DEG channel. Although piezoelectric polarization results in large values of sheet carrier density, it also gives rise to charged surface states within the device. These surface states are considered responsible for DC to RF current collapse or dispersion, because these electron traps act as a negatively charged virtual gate and limit maximum current available during microwave operation. Good insulator can passivate these surface states and also reduce gate leakage. Thus, the same dielectric can be used both as a gate insulator as well as the surface passivation layer [13]. The MISHFET approach also allows for application of high positive gate voltages to further increase the sheet carrier density in the 2-DEG channel and hence the device peak currents. These features make MISHFETs ex- tremely promising for high power microwave applications. How- ever, there are many milestones to be achieved and the work in this field is far from complete. Physics-based analytical modeling, which reflects the mechanism of device operation, is an essential requirement to fully explore the performance enhancements of MISHFET. To characterize and optimize the device performance, an accu- rate charge control relation between 2-DEG sheet carrier density n s and the controlling gate voltage V gs is desirable. Various models DOI 10.1002/mop MICROWAVE AND OPTICAL TECHNOLOGY LETTERS / Vol. 50, No. 2, February 2008 331