Gate Leakage Suppression and Contact Engineering in Nitride Heterostructures
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Gate Leakage Suppression and Contact Engineering in Nitride Heterostructures Yuh-Renn Wu, Madhusudan Singh, and Jasprit Singh Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI 48109, USA. We present a self-consistent approach to examine current flow in a general metalpolar heterostructure junction. The approach is applied to examine properties of two classes of junctions that are important in devices: (i) GaN/AlGaN/high-κ insulator structures for potential applications in very small gate devices to suppress gate tunneling current; (ii) GaN/AlGaN/LiNbO3 junctions for both n-type and p-type semiconductors with practical application for low source resistance regions. The physical parameters used for high-κ dielectrics and polarization charges reflect values typically found in ferroelectric materials. Our studies indicate that tailoring of junction properties is possible if polar oxides as thin as ∼ 20˚ A can be achieved.
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INTRODUCTION
GaN/AlGaN heterostructure field effect transistors (HFETs) have attracted considerable interest owing to their potential use in high power, high voltage and high temperature, electronic and optoelectronic devices[1, 2]. The nitride based structures are different from other semiconductor heterostructures since there is a large spontaneous polarization in the nitrides[3, 4]. Additionally, the piezoelectric effect is also very strong. As a result, there is a strong fixed polarization charge at the heterointerface[5], which can introduce very large electric fields (∼ 106 V· cm−1 ) and band bending, and induce a two dimensional electron gas (2DEG) (∼ 1013 cm−2 ) in the heterostructure interface grown on Ga-faced structures. For N-faced grown designs, a two dimensional hole gas (2DHG) can also be induced. Advances in nitride heterostructure technology have attracted careful studies into junction properties of metal-polar heterostructure combinations where dopants are replaced by built-in polar fixed charges. Very large band bending caused by interface fixed charges permits the tailoring of the current-voltage (I − V ) relations[6] by simply controlling the AlGaN layer thickness. It is important to develop a theoretical model that can predict the I − V relationship in a general metal-polar heterostructure system. In this paper, we will present such a model and apply it to two classes of junctions that are important for device technologies: (i) A GaN/AlGaN/high-κ dielectric insulator junction. We examine if such junctions can reduce gate tunneling without significantly influencing gate control of the channel; (ii) A GaN/AlGaN/LiNbO3 (or other similar polar materials) junction for application in very low resistance junctions. We will examine both n-type and p-type junctions. Such junctions could be useful as tunneling junctions or as junctions for very low source resistance in HFETs. In a previous work[6], we have demonstrated how the polar charge and the resultant band bending can affect tunneling probabilities and I − V relations in a
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