AlGaN/GaN HEMTs with 2DHG Back Gate Control
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AlGaN/GaN HEMTs with 2DHG Back Gate Control
Wei-Tse Lin, Wen-Chia Liao, Yi-Nan Zhong and Yue-ming Hsin Department of Electrical Engineering, National Central University
No. 300, Zhongda Rd., Zhongli District, Taoyuan City 32001, Taiwan
ABSTRACT
In this study, AlGaN/GaN high electron mobility transistors (HEMTs) with a two-dimensional hole gas (2DHG) were investigated. In addition to a two-dimensional electron gas (2DEG) formed at the interface of the AlGaN and GaN layers for being a channel, a 2DHG was designed and formed underneath the channel to be the back gate. The simulated results showed the operation of device can be depletion-mode and enhancement-mode by adjusting the back gate bias. The fabricated devices showed the feasibility of 2DHG back gate control.
INTRODUCTION AlGaN/GaN HEMTs have been shown to have low on-resistance and high breakdown characteristics because of the high electron mobility and high breakdown field characteristics [1-2]. The major applications include high-voltage power switching and high-frequency power amplifier [3-4]. Because of polarization effect, a two-dimensional electron gas (2DEG) can be formed at the interface of the AlGaN and GaN layers for being a channel in AlGaN/GaN HEMTs. A thin AlN layer is usually added between AlGaN and GaN layers to enhance electron concentration of 2DEG and avoid scattering for electron mobility. Moreover, a two-dimensional hole gas (2DHG) is possible to formed by using polarization to enhance GaN power device [5-8]. In the present study, an additional two-dimensional hole gas (2DHG) underneath the 2DEG channel by epitaxial layer design was proposed for being a bask-gate control in AlGaN/GaN HEMTs. It is similar to the back gate bias method of threshold voltage control for the design of field-effect transistors [9-11]. DEVICE STRUCTURE AND SIMULATION Figure 1(a) shows the cross-section of proposed device with 2DEG channel and 2DHG formed by AlGaN backbarrier layers design. The front barrier layer is a 20-nm Al0.3Ga0.7N on top of a 30-nm GaN channel layer. A 2DEG is formed at the interface of the Al0.3Ga0.7N and GaN layer as in a conventional AlGaN/GaN HEMT. Moreover, two different Al-mole fraction layers are implemented under GaN channel to form a 2DHG between GaN channel layer and Al0.3Ga0.7N back-barrier layer. A SILVACO Atlas device simulator was used to simulate 2D device and show the band diagram in Figure 2. The simulated peak concentrations of electron and hole were 1.041020 and 1.021020 cm-3, respectively, at thermal equilibrium. In order to have a good ohmic contact to 2DHG, it is important to maintain high concentration of 2DHG for a good p-type back gate contact while etching down to reach 2DHG. The simulated peak concentrations of 2DEG and 2DHG versus different etched depth from surface showed the peak concentration of 2DEG decreases to zero while etching down to 20-nm from surface because of removing the front barrier AlGaN, The peak concentration of 2DHG decreases significantly while etching down to 35-nm from surface, which mea
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