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 (V_th) 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 SiN_x 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 L_g, L_gd, and L_ds of the devices were 1.0 μm, 3.5 μm, and 7.0 μm, respectively. The W_g of the device was 50 μm. Figure 1 shows the schematic cross-sectional structure of the gate-recessed MIS-HEMT. Figure 2 shows the photomicrograph of the MIS-HEMT and the focused ion beam (FIB) cross-sectional view of the device’s gate corner of the gate-recessed MIS-HEMT of HfO₂. The parameters of the devices were measured by Keithley 4200.

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. 3. The band gap of Al₂O₃ is 8.8 eV, much larger than 6.0 eV of HfO₂. However, the barrier height (ϕ_B) of Ni/Al₂O₃ is 3.5 eV, and the barrier height of Ni/HfO₂ is 3.25 eV for the Ni/Au gate.^[16] The ΔE_C1 is the conduction-band discontinuity between dielectric and AlGaN. The ΔE_C1 of the Al₂O₃/Al_0.25Ga_0.75N is 1.8 eV while the ΔE_C1 of the HfO₂/Al_0.25Ga_0.75N is 0.8 eV. Figure 3 shows that the barrier heights of the gate metals have almost the same depletion effect on the electrons in the channel. The capacitance of the dielectric mainly affects the electrons under the same gate voltage. The specific capacitance is shown in Fig. 6.

Figure 4 shows the molecular structure diagram of HfO₂/AlGaN and Al₂O₃/AlGaN interfaces. As the Hf atom is much larger than the Ga atom and N atom, there will be many hanging bonds on the N atoms at the HfO₂/AlGaN interface. On the other hand, the Al atoms are small and there are Al atoms in the AlGaN layer, so there are few hanging bonds on the N atoms at the HfO₂/AlGaN interface. The interface state density of the Al₂O₃/AlGaN is smaller than that of the HfO₂/AlGaN interface. This can be confirmed by the test results of current collapse in Fig. 9.

Figure 5 shows the curves of breakdown voltage (V_BR) between gate and source of the two types of MIS-HEMTs. Each structure of the three devices is tested. The average V_BR of HfO₂ dielectric layer and Al₂O₃ dielectric layer are 9.4 V and 15.9 V, respectively. As the band gap of HfO₂ and Al₂O₃ are 6.0 eV and 8.8 eV, respectively, the Al₂O₃ can withstand a larger gate voltage range. The gate voltage range of Al₂O₃ is closer to that of Si MOS device. The gate leakage of Al₂O₃ is smaller than that of HfO₂. As Al₂O₃ has a higher energy-band offset on the AlGaN layer, holes are more difficult to cross the barrier of Al₂O₃ and a low gate leakage current can be formed by weakening Fowler–Nordheim (FN) tunneling.^[17] On the other hand, the Al₂O₃/AlGaN interface is better than the HfO₂/AlGaN interface as shown in Fig. 4.

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 (V_th) of the MIS-HEMT decreases after the PGA treatment. Figure 6 shows the comparisons of transfer characteristics between the HfO₂ and Al₂O₃ MIS-HEMTs with different etching depths after the PGA treatment. To protect the devices, the maximum gate voltage for the HfO₂ and Al₂O₃ MIS-HEMT are set to be +8 V and +15 V, respectively. Figure 6(a) shows that the drain current density of the HfO₂ and Al₂O₃ MIS-HEMT of 6-nm barrier thickness are 898 mA/mm and 905 mA/mm, respectively. Their V_th values are 0.1 V and 1.1 V, respectively. The peak transconductance of the HfO₂ MIS-HEMT is 189 mS/mm, which is much larger than 102 mS/mm of the Al₂O₃ MIS-HEMT. Because the dielectric constant of HfO₂ is 22, which is much larger than 9.3 of Al₂O₃, the dielectric capacitance and the transconductance of HfO₂ are also much larger than those of the Al₂O₃ MIS-HEMT. For an E-mode device, a smaller gate capacitance requires a higher voltage to yield the same amount of electrons under the gate, so the V_th of Al₂O₃ MIS-HEMT is larger.^[18] Figures 6(b) and 6(c) show that the drain current density of the HfO₂ MIS-HEMT decreases to 610 mA/mm and 379 mA/mm for the barrier thickness of 3-nm and 0-nm barrier thickness, respectively. The current of Al₂O₃ MIS-HEMT also has a similar decreasing trend. The decrease of current is mainly due to the concentration and mobility of the 2DEG decreasing. When the barrier layer is thinner, the dielectric/AlGaN interface is very close to the channel, so the scattering of interface states will lead to the electron mobility to decrease.^[19] The V_th of the HfO₂ and Al₂O₃ MIS-HEMT are 2.0 V and 2.4 V, respectively when the barrier thickness is 0 nm. In this case there is no 2DEG (two-dimensional electron gas) in the channel and the structure of the device is similar to the MOS structure. As the gate voltage increases, the gate capacitance can attract the electrons in GaN to connect the channel, but the concentration and mobility of electrons are not so high as those of 2DEG. Therefore, the characteristics of the device greatly decrease. Figure 6(d) shows that the V_th of the Al₂O₃ MIS-HEMT is larger and the transconductance of HfO₂ MIS-HEMT is larger when the barrier layer thickness is the same.

The C–V characteristics are shown in Fig. 7. The onset voltage (V_o) values of HfO₂ MIS-HEMTs are 0.2 V, 1.4 V, 1.9 V respectively corresponding to the different barrier thicknesses, which is in good agreement with the V_th’s transfer characteristic. Similarly, the V_o values of Al₂O₃ MIS-HEMTs are 0.8 V, 2.5 V, 3.0 V, respectively. A second slop exists when the barrier thickness is 6 nm.^[20] The electrons in the channel reach the dielectric/AlGaN interface when the gate voltage increases, so the gate capacitance increases due to the series connection of the dielectric layer capacitance and barrier layer capacitance to the only dielectric layer capacitance.^[15] For the MIS-HEMT with 3-nm and 0-nm barriers, the second slope is observed in none of the C–V curves. As the barrier layer is too thin, the electrons reach the dielectric/AlGaN interface directly. With the gate voltage increasing, the capacitance finally approaches to the dielectric layer capacitance which is 791 nF/cm⁻² and 308 nF/cm⁻² for the HfO₂ and Al₂O₃ MIS-HEMT, respectively. The dielectric constant of HfO₂ is 22 also much larger than 9.3 of Al₂O₃. There is also a good correspondence between gate capacitance and dielectric constant.

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 (W_G) is 100 μm, the gate length (L_G) is 50 μm, and the V_d is 0.1 V.^[21] As the device is biased in the linear range, the channel drift mobility can be expressed by

$$ \begin{eqnarray}\mu =\displaystyle \frac{{{\rm{G}}}_{{\rm{ch}}}{L}_{{\rm{G}}}}{{W}_{{\rm{G}}}C{V}_{{\rm{D}}}}.\end{eqnarray}$$ (1)

A dual-pulse current collapse test is performed on each of the devices, and the results are shown in Fig. 9. In the test, the pulse width is 5 × 10⁻⁷ s, with 1-ms period and the rise time and the decline time are both 1.5 × 10⁻⁷ s. In the current collapse test, selected are two static operating points, i.e., the (V_gs, V_ds) = (0, 0) state and the (V_gs, V_ds) = (−8, 10) state. According to the measurement, the current collapses of 6-nm barrier MIS-HEMT are 7.3% and 6.7% for the HfO₂ and Al₂O₃, respectively. Their current collapses increase to 29.7% and 20.8% when the barrier layer is 0 nm. The current collapse of HfO₂ MIS-HEMT is larger than that of Al₂O₃ MIS-HEMT, for there are more interface states at the HfO₂/AlGaN interface. When the barrier thickness is 0 nm, the trap state of HfO₂/GaN interface directly affects the electrons in the channel. The process of trap charge and discharge greatly influence the output current, so the collapse of HfO₂ MIS-HEMT reaches 29.7%. In addition, the specific on-resistance (R_on) can be obtained through the pulse output curve at (0, 0) state. The R_on values of the HfO₂ MIS-HEMT are 4.3 Ω⋅mm, 6.5 Ω⋅mm, and 9.2Ω⋅mm, respectively, which are larger than those of Al₂O₃ MIS-HEMT. At the maximum gate voltage, the mobility of HfO₂ MIS-HEMT is lower than that of Al₂O₃ MIS-HEMT, so the R_on of HfO₂ MIS-HEMT is larger. Figure 10 shows the comparisons of change trend among the devices. At the same barrier layer thickness, the current collapses of Al₂O₃ MIS-HEMT are smaller mainly due to the better interface of Al₂O₃/AlGaN as Al₂O₃ has excellent thermal stability (amorphous up to at least 1000 °C) and chemical stability.^[16] Also the Al element is contained in Al₂O₃ and there are many hanging bonds on the N atoms at the HfO₂/AlGaN interface, so the interface states of Al₂O₃/AlGaN are less than those of HfO₂/AlGaN. The interface states are in the form of trap states and fixed charges. The interface state density can be reduced effectively by filling the nitrogen vacancies through oxygen atoms into the HfO₂ and Al₂O₃ layer and reducing oxygen-related defects at the dielectric/AlGaN interface.^[15] It can be concluded that the interface states under the gate greatly influence the current collapse.

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 V_BR 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 V_th 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 V_th’s transfer characteristics. When the barrier layer is thinner, the drain current density decreases sharply. The current collapse and R_on 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.