Al mole fraction dependence of deep levels in AlGaN/GaN-HEMT structures estimated by CV profiling
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Al mole fraction dependence of deep levels in AlGaN/GaN-HEMT structures estimated by CV profiling Kikawa, Junjiroh 1; Imada, Katsuhiro 1; Yamada, Tomoyuki 1; Tsuchiya, Tadayoshi 1; Hiroyama, Yuichi 1; Iwami, Masayuki 1; Araki, Tsutomu 2; Suzuki, Akira 3, 1; Nanishi, Yasushi 2. 1. Advanced HF Devices R&D Center, R&A Association for Future Electron Devices, Kusatsu, Shiga, Japan. 2. Department of Photonics, Ritsumeikan University, Kusatsu, Shiga, Japan. 3. Research Organization of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan. ABSTRACT Current-voltage (IV) measurements and capacitance-voltage (CV) measurements have been carried out to investigate electrical properties of AlGaN/GaN-HEMT structures. By CV measurements of Schottky barrier diodes (SBDs) with large leak currents, we observed a distinct peak in CV profiling at low frequencies. The integral of this peak was found to have a correlation with a leak current. The behavior of this peak might be described by the Shockley-Read-Hall (SRH) model if we assume this peak is due to a phenomenon of an electron emission and capture by deep levels. Then Quasi-Fermi Level (Imref) at the bias point where this peak appears in CV profiling corresponds to energy depth of deep levels. That energy level can be approximated by Imref of two-dimensional (2D) electron gas. The result of our samples showed that the energy depth of deep levels from the conduction band is distributed from 320meV to 470meV for Al mole fraction from 0.19 to 0.30, respectively. INTRDUCTION GaN-based high mobility electron transistors (HEMTs) have emerged as very attractive candidates for high-power applications. But the gate leak current of those devices is as much as two orders magnitude more than that in GaAs-based HEMTs. The origin of this large leak current has been discussed by several institutes and universities, and deep levels play a major role in their models. To understand the behavior of gate leak current, we performed IV measurements and CV measurements. We observed a distinct peak in CV profiling near pinch-off at low frequencies in SBD with a large leak current. The integral of this peak was found to have a correlation with a leak current. In order to obtain excess capacitances, this additional capacitance must be parallel to the Schottky barrier and have an out of phase modulation. In the forward biase region, some authors explained the origin of this current from the modulation of the effective Schottky barrier height by interface charges[1] or an excessive minority carrier
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extraction at defective back contact[2]. But our case is in the reverse bias region. For a semiconductor with a wide energy gap, the generation of minority carriers in the deep depletion bias region would be extremely slow, and it is not necessary to think a minority carrier injection through Schottky barrier. Literatures that discussed the excess capacitance at the reverse bias region are few as long as we know. In this work we assume this peak is due to a phenom
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