† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant Nos. 61974111, 11690042, and 61974115), the National Pre-research Foundation of China (Grant No. 31512050402), and the Fund of Innovation Center of Radiation Application, China (Grant No. KFZC2018040202).
Two types of enhancement-mode (E-mode) AlGaN/GaN metal–insulator–semiconductor high-electron-mobility transistors (MIS-HEMTs) with different gate insulators are fabricated on Si substrates. The HfO2 gate insulator and the Al2O3 gate insulator each with a thickness of 30 nm are grown by the plasma-enhanced atomic layer deposition (PEALD). The energy band diagrams of two types of dielectric MIS-HEMTs are compared. The breakdown voltage (VBR) of HfO2 dielectric layer and Al2O3 dielectric layer are 9.4 V and 15.9 V, respectively. With the same barrier thickness, the transconductance of MIS-HEMT with HfO2 is larger. The threshold voltage (Vth) of the HfO2 and Al2O3 MIS-HEMT are 2.0 V and 2.4 V, respectively, when the barrier layer thickness is 0 nm. The C–V characteristics are in good agreement with the Vth’s transfer characteristics. As the barrier layer becomes thinner, the drain current density decreases sharply. Due to the dielectric/AlGaN interface is very close to the channel, the scattering of interface states will lead the electron mobility to decrease. The current collapse and the Ron of Al2O3 MIS-HEMT are smaller at the maximum gate voltage. As Al2O3 has excellent thermal stability and chemical stability, the interface state density of Al2O3/AlGaN is less than that of HfO2/AlGaN.
GaN-based high electron mobility transistors (HEMTs) are well suitable for the applications in power switching devices.[1–4] Enhancement-mode (E-mode) HEMT is very important for the switch power supply which can reduce power loss by keeping the device closed at zero gate bias.[5–7] The gate-recessed metal–insulator–semiconductor (MIS) structure is considered as an important structure to realize the E-mode AlGaN/GaN power devices because of the high threshold voltage (Vth) and the high drain current density.[8]
Gate dielectric material is of vital importance for MIS-HEMTs.[9,10] The HfO2 has a relatively high dielectric constant of 22 and lower band gap of 6.0 eV as a new-type high-k material.[11] The Al2O3 has a relatively low dielectric constant of 9.3 and larger band gap of 8.8 eV as a frequently used gate dielectric. Difference in gate dielectric has a great influence on the characteristics of devices. The dielectric constant corresponds to the gate capacitance, and the gate can control the two-dimensional electron gas (2DEG) in the channel more easily when the gate dielectric constant is higher.[12] The band gap corresponds to the positive gate voltage capability of the gate dielectric. A larger band gap means a higher positive gate voltage capability.[13] Choi, et al. have reported the E-mode MIS-HEMT with dual gate dielectric of SiNx and HfO2.[14] However, few articles have reported the comparison of characteristics between the HfO2 and the Al2O3 gate dielectric for the gate recessed MIS-HEMT.
In this paper, two types of HEMTs are designed and fabricated, known as gate-recessed MIS-HEMTs with HfO2 and Al2O3 gate dielectric grown by plasma-enhanced atomic layer deposition (PEALD). There are three etching depths in the two types of MIS-HEMTs. The energy band diagrams of the two types of MIS-HEMTs are compared with each other. Moreover, the direct current (DC) characteristics and pulse characteristics are compared and analyzed.
The AlGaN/GaN heterojunction structure used in this paper was grown on a silicon (111) substrate by the metal organic chemical vapor deposition (MOCVD) method. The wafer consisted of an AlN nucleation layer, an AlGaN gradient layer in which the Al percentage ranges from 8% to 0, a 2-μm-thick C-doped GaN layer, a 160-nm-thick undoped GaN channel, and a 25-nm-thick undoped AlGaN barrier layer. Room temperature hall measurements of the epi-wafer yielded an electron sheet density of 9.0 × 1012 cm−2 and an electron mobility of 2000 cm2/V⋅s.
The mesa area was formed by using BCl3/Cl2 plasma etching in an inductively coupled plasma (ICP) system followed by the drain/source ohmic contact formation by using Ti/Al/Ni/Au (30 nm/180 nm/40 nm/60 nm) annealed at 840 °C for 30 s. A 60-nm-thick Si3N4 layer was deposited on a surface by the plasma-enhanced chemical vapor deposition (PECVD), and the Si3N4 of the gate area was removed by CF4 plasma etching. The gate-recessed MIS-HEMT was etched by using BCl3 and Cl2. The barrier layer thickness values were 6 nm, 3 nm, and 0 nm, respectively. The next step was high temperature (300 °C) N2 plasma treatment in the recessed-gate region by using the plasma enhanced atomic layer deposition (PEALD) with the treatment power of 150 W for 10 min. Then, the HfO2 and Al2O3 dielectric layer were deposited separately to a thickness of 30 nm. Then, Ni/Au E-beam evaporation and lift off were carried out to form the gate electrode. Finally, post gate annealing (PGA) treatment of 400 °C in ambient N2 for 5 min was implemented for reducing the interface state density.[15] The Lg, Lgd, and Lds of the devices were 1.0 μm, 3.5 μm, and 7.0 μm, respectively. The Wg of the device was 50 μm. Figure
Due to the fact that the band gap of HfO2 and Al2O3 are different, the energy band diagrams of the two types of MIS-HEMTs are different as shown in Fig.
Figure
Figure
For the MIS-HEMT with HfO2, The drain current density and transconductance increase greatly after post-gate-annealing (PGA, 400 °C, 5 min) treatment.[15] The threshold voltage (Vth) of the MIS-HEMT decreases after the PGA treatment. Figure
The C–V characteristics are shown in Fig.
In order to change the mobility values of the devices, the FAT-FETs are tested by measuring I–V and C–V characteristics. The gate width (WG) is 100 μm, the gate length (LG) is 50 μm, and the Vd is 0.1 V.[21] As the device is biased in the linear range, the channel drift mobility can be expressed by
A dual-pulse current collapse test is performed on each of the devices, and the results are shown in Fig.
The AlGaN/GaN MIS-HEMTs with three different etching depths by using HfO2 and Al2O3 gate insulators are fabricated on Si substrates. The barrier layer thickness values are 6 nm, 3 nm, and 0 nm respectively. The energy band diagrams of two types of dielectric MIS-HEMTs are compared. The VBR of the HfO2 and Al2O3 gate are 9.4 V and 15.9 V, respectively. At the same barrier thickness, the transconductance of MIS-HEMT with HfO2 is larger. The Vth of the HfO2 and Al2O3 MIS-HEMTs are 2.0 V and 2.4 V, respectively, when the barrier thickness is 0 nm. The C–V characteristics are in good agreement with the Vth’s transfer characteristics. When the barrier layer is thinner, the drain current density decreases sharply. The current collapse and Ron of Al2O3 MIS-HEMT are smaller at the maximum gate voltage. The interface states of Al2O3/AlGaN are less than those of HfO2/AlGaN, for the Al2O3 has excellent thermal stability and chemical stability and the Al2O3 and AlGaN both contain the Al element.
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