Characterization of Deep Levels in AlGaNGaN HEMT by FT-DLTS and Current DLTS
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HYSICS OF SEMICONDUCTOR DEVICES
Characterization of Deep Levels in AlGaN|GaN HEMT by FT-DLTS and Current DLTS M. Gassoumia,b,* a
Research Unit Advanced Materials and Nanotechnologies, University of Kairouan, BP 471, Kasserine, 1200 Tunisia b Department of Physics, College of Sciences, Qassim University, P.O. 6644, Buryadh, 51452 Saudi Arabia *e-mail: [email protected] Received May 3, 2020; revised May 3, 2020; accepted June 17, 2020
Abstract—In this work, GaN|AlGaN high electron mobility transistor (HEMT) structures are investigated, grown on semi-insulating SiC substrates by molecular beam epitaxy and metal–organic chemical-vapor deposition techniques. This paper reports on the kink effect and hysteresis effect observed in AlGaN|GaN high electron mobility transistors (HEMTs) on SiC substrate. It is well known that trapping effects can limit the output power performance of microwave HEMTs, which is particularly true for the wide band gap devices. A detailed study is presented of FT-DLTS and CDLTS measurements performed on AlGaN|GaN HEMTs. It is demonstrated that the kink effect is directly correlated to shallow traps, and a remarkable correlation exists between deep levels observed by CDLTS and FT-DLTS and the presence of parasitic effects such as kink and hysteresis effects. Keywords: AlGaN, GaN (HEMT), kink effect, hysteresis effect, FT-DLTS, CDLTS, traps DOI: 10.1134/S1063782620100127
1. INTRODUCTION One of the key issues that limit the efficiency of AlGaN|GaN field effect transistors (HFETs) for highpower and high-frequency applications is the existence of traps on the AlGaN surface and in the GaN buffer layers [1, 2]. The structures of the III-nitrides, produced through either metal-organic chemicalvapour deposition (MOCVD) or molecular beam epitaxy (MBE), can behave as shallow or deep traps influencing the channel density of the two-dimensional electron gas (2D-EG) and causing consequences such as current collapse [1], transconductance frequency dispersion [3], and low-power output during switching behavior [4]. GaN is typically grown hetero-epitaxially on foreign substrates such as sapphire (Al2O3), silicon carbide (SiC), and silicon (Si). The substrate, on which the nitride layers are grown, determines the crystalline quality of the entire buffer-layer stack. In particular, the large lattice and thermal mismatch between the AlN nucleation layer and the Si substrate leads to a severe amount of defects and, in particular, of threading dislocations starting from the substrate and propagating through the entire buffer-layer stack. In previous works [5–7], the fact is discussed that at high voltage (HV) the leakage current flows from the ohmic contacts into the substrate and along the Si interface. Electrically-active threading dislocations can assist the vertical leakage path between the ohmic contacts
and the Si substrate. The high conductivity at the AlN|Si interface is due to several defects that create a conductive path, causing a degradation of the RF performance [8] and a premature breakdown [9].
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