Wang Hui, Wang Ning, Jiang Ling-Li, Lin Xin-Peng, Zhao Hai-Yue, Yu Hong-Yu. A novel enhancement mode AlGaN/GaN high electron mobility transistor with split floating gates. Chinese Physics B, 2017, 26(4): 047305
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A novel enhancement mode AlGaN/GaN high electron mobility transistor with split floating gates
Wang Hui1, 2, Wang Ning1, 2, Jiang Ling-Li1, 2, Lin Xin-Peng1, 2, Zhao Hai-Yue1, 2, Yu Hong-Yu1, 2, †
Department of Electrical and Electronic Engineering, Southern University of Science and Technology, Shenzhen 518055, China
Shenzhen Key Laborary of the Third Generation Semiconductor, Shenzhen 518055, China
† Corresponding author. E-mail: yuhy@sustc.edu.cn
Project supported by “Efficient and Energy-Saving GaN on Si Power Devices” Research Fund (Grant No. KQCX20140522151322946), the Research Fund of the Third Generation Semiconductor Key Laboratory of Shenzhen, China (Grant No. ZDSYS20140509142721434), the “Key Technology Research of GaN on Si Power Devices” Research Fund (Grant No. JSGG20140729145956266), and the “Research of Low Cost Fabrication of GaN Power Devices and System Integration” Research Fund (Grant No. JCYJ201602261926390).
Abstract
A novel enhancement-mode AlGaN/GaN high electron mobility transistor (HEMT) is proposed and studied. Specifically, several split floating gates (FGs) with negative charges are inserted to the conventional MIS structure. The simulation results revealed that the decreases with the increase of polarization sheet charge density and the tunnel dielectric (between FGs and AlGaN) thickness, while it increases with the increase of FGs sheet charge density and blocking dielectric (between FGs and control gate) thickness. In the case of the same gate length, the will left shift with decreasing FG length. More interestingly, the split FGs could significantly reduce the device failure probability in comparison with the single large area FG structure.
Enhancement-mode (E-mode) AlGaN/GaN HEMTs with a large positive threshold voltage ( V) are strongly desired for high voltage, high power and high frequency switching applications.[1] In the last few years, major efforts in the development of E-mode HEMTs have been witnessed.[2] These technologies mainly include fluorine[3, 4] or oxygen[5] ion implantation, P-type GaN or AlGaN gate,[6] recess gate etching,[7–9] and piezo neutralization,[10] etc. However, concerns associated with the above-mentioned technologies such as large gate leakage current owing to the process damage to the channel, difficulties of preparation process, and small resultant still have not been addressed completely.
Floating gates (FGs), usually applied in flash memory devices, have recently been proposed and experimentally confirmed to be a promising approach to obtain E-mode by way of charge storing without reducing the channel conductance.[11, 12] The main shortcomings are the requirement of large FG sheet charge density as well as charge retention limitation which affects the stability. Although Huang et al.[13] reported that the FG sheet charge density can be lowered by combining FG and gate recess etching, the charge retention still remains a problem. It is known that charge loss of a single large area FG due to a weak spot in the dielectric would result in dropping to a negative value and hence a catastrophic short-circuit can happen. To mitigate this problem, a novel HEMT with split FGs structure is proposed and studied by simulations in this work.
2. Simulation details
The simulations were carried out with software Silvaco. The key polarization and mobility models used were TEN.POLAR model and Albrecht model, respectively. The was studied as a function of polarization-induced sheet charge density (σ), FGs sheet charge density (, tunnel dielectric (TD), and blocking dielectric (BD) materials and thicknesses. The range of σ was from cm to cm. The ranges of negative were from cm to cm. SiO2, Si3N4, Al2O3, and HfO2 were applied as both TD and BD. The ranges of TD thickness ( and BD thickness ( were 5 nm 10 nm and 5 nm 30 nm, respectively. In the study of FG failure on the , 6 FGs when complete charge loss occurs in 0, 1, 2, 3, 4, 5 FGs were studied, the length of each FG ( was equal to and their spacing (, the horizontal distance between 2 adjacent FGs) was equal to . For figuring out the length of single FG on the , the FG length of was studied. In all the simulations of transfer characteristics, the gate metal work function was 4.6 eV and the drain voltage was 1 V. The gate length ( was kept at and the gate-to-source spacing ( and gate-to-drain spacing ( were kept at . The device schematic is shown in Fig. 1.
Fig. 1. (color online) Schematic illustration of the HEMT structure with split floating gates (the number of FGs can be adjusted). The gate length ( is , the gate-to-source spacing ( and gate-to-drain spacing ( are both .
3. Threshold voltage calculation
For conventional MIS HEMTs, the schematic charge distribution and energy band diagram can be seen in Fig. 2(a). The can be calculated by Eq. (1)[14]
where 1 is the metal barrier height for gate/BD, q is the elementary charge, is the conduction band offset for gate dielectric/GaN, 2 is the conduction band distance from the Fermi-level in GaN, ε is the permittivity of gate dielectric, is the average bulk charge of gate dielectric (per unit volume), and σ are the sheet charge density at the gate dielectric/AlGaN interface and the AlGaN/GaN-channel interface, respectively.
Fig. 2. (color online) Schematic charge distribution and energy band diagram of (a) conventional MIS HEMT and (b) FG HEMT.
While for the FG MIS HEMTs, the schematic charge distribution and energy band diagram can be seen in Fig. 2(b). The will be shifted by the FG charge and the shift is derived in Eq. (2)[15]
Combining Eqs. (1) and (2), the of the FG MIS HEMTs can be calculated by Eq. (3)
where ε is the permittivity of TD, ε is the permittivity of BD. Since the TD and BD are the same material applied in the simulations, it is known that . It is also known that , , and σ are all positive, and the value of increases with the increase of the two-dimensional electron gas (2DEG) sheet charge density (.[14, 16] So the of the FG MIS HEMTs can be derived as Eq. (4)
4. Results and discussions
Figure 3 shows the as a function of σ. It is seen that the decreases linearly as the σ increases. In Eq. (4), it is seen that the increase of σ will lead to the linear decrease of . Figure 3 also shows that in the case of the same σ, the for is 1.5× 1013 cm−2 is larger than that for is 1× 1013 cm−2.
Fig. 3. (color online) The as a function of . The TD and BD were both SiO2. The and were 10 nm and 20 nm, respectively. Single FG with of (equal to the was applied.
Figure 4(a) shows that the increases linearly with increasing , the phenomenon of which is also found in Fig. 3. In Eq. (4), it is seen that when other factors remain unchanged, the has a positive linear dependence on . As the increases, the will increase accordingly. Figure 4(a) also shows that when other factors remain unchanged, the decreases when the dielectric changes from lowest-k SiO2 to highest-k HfO2. This variation results from the variation of 1, , , and for different dielectrics. Figure 44(b) shows that the decreases linearly with the increase of . It is due to the fact that as increases, the will decrease since in this situation it is just a function of . Figure 4(c) shows that the increases with the increase of . The variation is opposite in comparison with the conventional MIS HEMT in which the decreases with the increase gate dielectric (Eq. (1)). Figure 4(c) can also be explained by Eq. (4). It is seen that increases with the increase of . Furthermore, the is much larger than other sheet charge density, thus still resulting in the increase of . Figure 4(c) also reveals that the can be lowered moderately by increasing .
Fig. 4. (color online) (a) The as a function of . The and were 10 nm and 20 nm, respectively; (b) The as a function of . The and were 1× 1013 cm−2 and 20 nm, respectively; (c) The as a function of . The and were 1× 1013 cm−2 and 10 nm, respectively. The TD and BD were the same material and the is 5.3× 1012 cm−2. Single FG with of (equal to the was applied.
Figure 5 shows the transfer characteristics of split FG HEMTs when FGs failure is considered. – curves when complete charge loss occurs in 0, 1, 2, 3, 4, 5 FGs of the 6 FG HEMTs are shown for comparison. It is seen that the decreases slightly when FGs failure happened, while the reduction (about 0.1 V per FG failure) is still moderate when even 5 FGs failed, implying that the split FGs HEMTs is feasible. It is also noteworthy that the increase of drain current is accompanied with the decrease of . Since the drain current increases rather than decreases, which may cause very little impact on the HEMT applications. Figure 5 thus reveals the advantages of the split FGs in comparison with the single large area of FG.
Fig. 5. (color online) Dependence of transfer characteristics on the failure of FGs. 6 FGs with equal and equal is applied. Other parameters are , , nm, and nm. The TD and BD are both SiO2. The is 1 V.
Figure 6 shows the influence of on the of single FG HEMTs, the was kept as . It is seen that as decreased from to , is left shifted gradually. As is less than , begins to decrease sharply. It is much like the short channel effect.[17] Figure 6 is able to explain the result in Fig. 5 why the decreases when FGs failure happened. Figure 6 also shows that the variation tendency was almost the same no matter if is 1× 1013 cm−2 or 1.3× 1013 cm−2.
Fig. 6. (color online) as a function of for the single FG. The , nm, and nm. The TD and BD are both SiO2.
According to the Weilbull distribution (see Eqs. (5) and (6)),[18] the failure probability of an MIS structure is strongly related to its area, it can also be used on charge loss of floating gates[19]
where F2 is the failure probability of an MIS structure with the area of A2, F1 is the failure probability of an MIS structure with the area of A1.
where t1 is the failure time of an MIS structure with the area of A1, t2 is the failure time of an MIS structure with the area of A2, β is the Weibull slope.
Equation (6) reveals that the smaller area corresponds to longer failure time, indicating that the split FGs HEMT has smaller failure probability in comparison with the HEMT with large area single FG. Furthermore, the control gate can be converted to a certain amount of connected multiple finger gates while each of the FG fingers is separated (Fig. 7). Under the same current rating, the failure probability of the single gate FGs HEMT is calculated to be about Y times higher than the multiple finger gates FGs HEMT according to Eq. (7), in most situations, the calculated result shows .
where n is the finger number, m is the number of FGs on each finger, p1 is the FG failure probability of the gate, p2 is the FG failure probability of the multiple gates.
Fig. 7. (color online) The schematic layout (top view) of the multiple finer gates FGs HEMT.
Assuming a 1-A HEMT under drain voltage of 2 V, the on-resistance R is 2 Ω. The R can be calculated by Eq. (8)
where ρ is the resistivity, w is the FG width, and μ is the mobility.
For AlGaN/GaN HEMT, the typical μ and are and 5× 1012 cm−2, respectively. If the is (with 6 FGs, each FG and is and , respectively), the gate width (nw) should be . So the gate can be divided to 25 multiple fingers with each finger area of 6 FGs with each area of can be prepared on each finger. In flash memory devices of 130-nm technology, the cell failure probability is within 3 years of retention.[20] Assuming that a cell area in the flash memory devices is typical , the FG (area of ) failure probability was calculated to be 0.025 according to Eq. (5). The failure probability of each finger is 0.0256=2.5× 10−10 and the device failure probability was calculated to be 6.3× 10−9 within 3 years. With the low failure probability, the split FGs structure is shown to be a viable route to E-mode GaN HEMT.
5. Conclusion
In this work, a charge storage-based E-mode AlGaN/GaN split FGs HEMT is proposed and studied by simulations. It is found that the decreases with the increase of 2DEG sheet charge density as well as the tunnel dielectric thickness, while it increases with the increase of FGs sheet charge density and blocking dielectric thickness. The short channel effect was found in the single FG HEMT. The reliability of the device is studied, too. It is found that for 6-FGs HEMT, the reduction of is still moderate when even 5 FGs failed. Moreover, the failure probability of the multiple gates FGs HEMT is calculated to be very low, suggesting that it is a viable route to E-mode GaN HEMT.
