Abstract
A novel enhancement-mode (E-mode) Metal-Insulator-Semiconductor- (MIS-) HEMT with selective fluorine ion (F−) treatment is proposed and its mechanism is investigated. The HEMT features the Selective F− treatment both in the AlGaN channel region and in the thick passivation layer between the gate and drain (SFCP-MIS-HEMT). First, the F− in the passivation layer not only extends the depletion region and thus enhances the average electric field (-field) between the gate and drain by the assisted depletion effect but also reduces the -field peak at the gate end, leading to a higher breakdown voltage (BV). Second, in the AlGaN channel region, the F− region realizes the E-mode and the region without F− maintains a high drain current (). Third, MIS structure suppresses the gate leakage current, increasing the gate swing voltage and the BV. Compared with a MIS-HEMT with F− treatment in whole channel (FC-MIS-HEMT), SFCP-MIS-HEMT increases the BV by 46% and the saturation drain current () by 28%.
1. Introduction
AlGaN/GaN HEMTs are promising candidates for next generation microwave power amplifiers and high-voltage switches owing to their superior properties [1, 2]. For such applications, a normally off or E-mode operation is desired, either for fail-safe operation or for simple gate drive circuit design. However, most AlGaN/GaN HEMTs show normally on operation for the high-density two-dimensional electron gas (2DEG). Normally off operation on AlGaN/GaN heterostructures has been obtained with several approaches, such as recessed gate structures [3], PN junction gate structures [4], thinned AlGaN barrier [5], InGaN cap layer [6], and F− treatment [7]. F− treatment in the AlGaN layer is used to realize normally off HEMT [8] but the gate leakage during on-state operation strongly limits the device performance, especially when high threshold voltage () and high current densities are required. By inserting a gate dielectric layer between the gate metal and the semiconductor, AlGaN/GaN MIS-HEMTs are expected to address the issues associated with the Schottky gate. The gate insulator material includes SiO2 [9], Si3N4, TaOxNy [10], and HfO2 [11].
The fluoride ions implanted into the AlGaN barrier layer are also employed to improve the BV [12]. However, F− treatment in the thin AlGaN barrier induces damage to the AlGaN/GaN heterojunction, and the F− easily gets into the channel and degrades the mobility of the 2DEG. Thus, the forward performance and reliability will be seriously affected.
In this paper, a novel E-mode MIS-HEMT with selective F− treatment both in the channel and in the passivation dielectric (SFCP-MIS-HEMT) is studied. The physical mechanisms of the F− on the device performance are investigated by the theoretical and simulated studies. The physical mechanism is analyzed based on 2D Sentaurus TCAD from Synopsys. The results indicate that the SFCP-MIS-HEMT can significantly improve the BV and the simultaneously.
2. Structure and Mechanism
Figure 1 is the schematic cross section of the proposed SFCP-MIS-HEMT. It features the selective F− treatment both in the AlGaN under the gate and in the passivation dielectric between the gate and the drain. and represent the F− concentration and length of the F− treatment region in the AlGaN layer, respectively; and are the F− concentration and lateral length of F− treatment in the passivation layer, respectively. When and , the structure is selective F− treated in channel (SFC-MIS-HEMT). When and , the structure is F− treated in the whole channel (FC-MIS-HEMT). Table 1 shows abbreviations and parameters of the three different HEMTs in this paper. The F− in the AlGaN under the gate is used in the three types of HEMTs to realize the E-mode. As F− in the passivation layer are immobile negative charges, electrons are depleted from the 2DEG channel in the implanted region of F− due to the repulsive force. The passivation layer implanted by F− acts like the field plate. The F− in the passivation layer not only enhances the -field at the drain side by the assisted depletion but also reduces the -field peak at the gate end, leading to a higher BV, with a little effect on the . All the three HEMTs are featured by and a gate-drain distance μm. The HEMTs also consist of a 1.5 μm thick GaN layer, a 25 nm thick AlGaN layer, and 200 nm thick Si3N4 passivation on the AlGaN. The and directions are given.
Figure 2 shows the 2D equipotential lines, the surface -field, and potential distribution of the SFCP-, SFC-, and FC-MIS-HEMT at off-state breakdown. Comparing Figure 2(a) with Figures 2(b) and 2(c), the F− in the passivation layer extends the depletion region and makes the potential lines more uniform between the gate and the drain. In Figure 2(d), owing to the modulation effect of the F− in the passivation layer, the -component of surface ( μm) -field distributions of the SFCP-MIS-HEMT is more uniform and higher than that of the FC-MIS-HEMT and SFC-MIS-HEMT, and the peak electric field at the gate edge is decreased, leading to a higher BV. The surface potential almost linearly increases from the gate to the drain. However, because the buffer layer leakage causes the device premature breakdown, the peak -field around the drain edge of SFCP-MIS-HEMT cannot continue improving.
(a) SFCP-MIS-HEMT
(b) SFC-MIS-HEMT
(c) FC-MIS-HEMT
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Figure 3(a) shows the influence of on conduction band under the gate. The elevates the conduction band above the Fermi level at cm−3, realizing the E-mode. Figures 3(b) and 3(c) show the 2DEG density at the on state and off state. In Figure 3(b), for the SFCP-MIS-HEMT, the region depletes the 2DEG and realizes the E-mode, while the region without F− maintains a high 2DEG density under the gate so as to obtain a high on-state . The F− in the passivation layer has a little effect on the 2DEG density at on state. At the off-state , the 2DEG density in the drift region of the FC-MIS-HEMT and SFC-MIS-HEMT keeps a high value, providing a leakage path. With the assisted depletion caused by the F− in the passivation layer, the 2DEG density of the SFCP-MIS-HEMT in the drift region is decreased by five orders of magnitude as shown in Figure 3(c). Therefore, the buffer layer leakage current from the drain to the source is depressed and a higher BV is achieved.
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3. Results and Discussion
The influence of and on the off-state BV and are shown in Figure 4(a). The BV increases with the increasing owing to the spreading depletion region. As the increases, the BV first increases and then decreases because the -field at drain side steadily increases while the -field at gate side begins to decrease at cm−3. With the increase in the and , the slightly decreases due to the enhanced assisted depletion on the 2DEG. The - curves for the SFCP-MIS-HEMT with different and are compared at in Figure 4(b). In Figure 4(b), the SFCP-MIS-HEMT can realize the highest BV in these three HEMTs. First, the depletion width and the -field strength in the drift are enhanced due to the assisted depletion caused by the F− in the passivation layer (). Second, the peak -field around the gate edge is decreased by , avoiding the premature breakdown herein (see Figures 4(c) and 2(d)). Figure 4(b) also shows that the BV increases with the increasing in the range of 0–3 μm owing to the spreading depletion region between the gate and the drain. can increase the -field strength near the drain and thus improve the BV while the too high (e.g., cm−3) reduces the -field strength near the gate and leads to a decrease in the BV. Note that the premature breakdown caused by the BUFFER layer leakage prevents the -field peak at the drain end from further increasing with the increasing and . Figure 4(c) illustrates that the -field distributions of the SFCP-MIS-HEMT are more uniform and higher than those of the FC-MIS-HEMT and the SFC-MIS-HEMT.
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Figure 5 indicates the dependence of the and on the and (). Figure 5(a) shows that the increases with the increasing at cm−3 and μm. However, with the increases in the , the obviously decreases due to the enhanced depletion on the 2DEG. In Figure 5(b), the increases with the increasing at μm and μm. The inset in Figures 5(a) and 5(b) shows the transfer characteristic curves as a function of and . To make sure V, the optimized () decreases with the increasing . When the is too small, the corresponding is extremely high (e.g., μm and cm−3) and the 2DEG density and mobility are very low, leading to a sharp decrease in as shown in Figure 5(c). Finally, the minimum value of is 0.8 μm.
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The transfer characteristic and output characteristic are compared for the SFCP-, SFC-, and FC-MIS-HEMT in Figure 6. Three HEMTs have the same threshold voltage of V. In Figure 6(a), the of the SFCP-, SFC-, and FC-MIS-HEMT is 1540 mA/mm, 1730 mA/mm, and 1200 mA/mm, respectively. Compared with the SFC-MIS-HEMT, SFCP-MIS-HEMT has a small decrease in the because F− in the passivation layer causes the assisted depletion of the 2DEG in the drift region at large drain bias. In Figure 6(b), the drain current of the FC-MIS-HEMT is the lowest and that of the other two HEMTs are almost the same at V. The on resistance mainly consists of the channel resistance and the drift resistance (). When the is small (e.g., ), the accounts for the major part of the . The of the FC-MIS-HEMT is the highest and that of the other two HEMTs is almost the same. As the increases, the gets more and more important. At V, the of the SFCP-MIS-HEMT is smaller than that of the SFC-MIS-HEMT because of a higher caused by the assisted depletion effect of the on the 2DEG. The degradation in the forward characteristics is slight nevertheless since the is relatively low and the F− in the passivation layer induces no damage to the AlGaN/GaN heterojunction. As a result, an excellent trade-off between the off-state characteristics and the on-state characteristics is achieved.
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Figure 7 illustrates the specific on resistance and off-state BV of the SFCP-MIS-HEMT and recently reported normally off AlGaN/GaN HEMTs, together with the theoretical Si, SiC, and GaN limit. It indicates that SFCP-MIS-HEMT has a higher Figure of Merit (FOM = BV2/). The SFCP-MIS-HEMTs exhibit almost a linear -BV dependence as shown in Figure 7. The performance of the SFCP-MIS-HEMTs is better than that of the mentioned AlGaN/GaN HEMTs. It also implies that a higher BV can be expected in the proposed devices with a larger .
4. Conclusion
The off-state and on-state performances of the proposed AlGaN/GaN HEMT are improved by selective F− treatment in both the passivation layer and the AlGaN barrier layer. The F− in the passivation layer not only enhances the electric field near the drain side but also reduces the -field peak at the gate end, leading to a higher breakdown voltage, without the obvious effect on the . Moreover, as for as the AlGaN channel region, the F− region realizes the E-mode and the region without F− maintains a high . The proposed devices possess a better trade-off relationship between BV and . Therefore, the selective F− treatment technique is available to achieve power devices with the capacity of high voltage, high output power, and low power dissipation.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.