Current-Voltage Characteristics and Transconductance of Undoped Algan/Gan Mishfet’s

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  • Topic: MOSFET, Dielectric, Semiconductor
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  • Published : February 3, 2013
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Novel Analytical model for current-voltage characteristics and transconductance of undoped AlGaN/GaN MISHFET’s and performance comparison with different high-k dielectrics Rangababu Ganta1, Saptarsi Ghosh2 and Dhrubes Biswas1,2

1Department of Electrical & Electronics Communication Engineering, IIT Kharagpur, Kharagpur -721302, India 2Advance Technology Development Center, IIT Kharagpur, Kharagpur -721302, India E-mail: rangababu.ganta@gmail.com

Abstract. In this paper an improved charge controlled model of undoped ,lattice mismatched AlGaN / GaN MISHFET is presented .This model gives an accurate estimation of the 2-DEG sheet carrier concentration at the interface contributed entirely by spontaneous and piezoelectric polarization .The dependence of this sheet carrier density on Al composition and the insulator thickness is investigated in detail. Based on these developments an analytical current - voltage model incorporating Source/Drain parasitic resistance is presented. Also, the non-linear behavior of saturated Drain current is properly formulated and computation of transconductance is carried out for the same. Even without any intentional doping the proposed model exhibits fairly encouraging values of output current and conductance. Introduction

With the impulsive rise of mobile computing and communication sector, high power RF devices are destined to play significant role in consumer electronics. Wide band gap semiconductors such as GaN are strong contender for high power electronic and optoelectronic circuits. Larger direct band gap (3.4 eV), as compared to Silicon (1.1 eV) and GaAs (1.4 eV) leads to higher values of critical fields and breakdown voltages. Recently GaN/AlGaN based heterostructure devices are grabbing much attention of both material and device community due to excellent electron transport properties and high sheet carrier densities. In GaN/AlGaN based systems a significant amount of charge concentration is found to originate at the interface without any intentional doping in the AlGaN layer. This carrier charge confinement results from both piezoelectric and spontaneous polarization in the GaN/AlGaN layers. The omission of delta doping in the AlGaN layer leads to simpler and economic fabrication technologies. But such HFET’s are susceptible to high gate leakage due to the moderate Schottky barrier height of the gate metal (Au/Ni). For this purpose nowadays the device structure further modified , as an insulator layer with high permittivity is grown between the gate metal and the AlGaN layer[1,2]. This ensures lower gate leakage in terms of body currents which in turn reduces low power loss in OFF state and also serves the purpose of passivating the AlGaN layer. Now reliable and predictive analytical models of such metal-insulator-semiconductor heterostructure FET’s are needed along with their fast development in circuit implementation. But till date only a few of such models have been developed and there exists a great void to be filled. In this paper we present a threshold voltage based analytical model to compute the I-V characteristics of undoped AlGaN/GaN MISHFET structures. In our model we take into account the nonlinear relationship of carrier concentration with Fermi level. As natural continuation of our earlier work we have formulated the drain current in the saturation region also with non zero sloped behaviour. Therefore this present model accurately predicts the current in weak, moderate and strong inversion regions. Transconductance, which effectively determines the device cut-off frequency, is also found out analytically. All the calculations are repeated for different dielectric materials for device performance optimization. Compact MISHFET Model

In an undoped MISHFET structure the free charge concentration at the AlGaN/GaN interface is entirely comprised of those originating from polarization (both spontaneous & piezoelectric). A generalized layered structure of the same...
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