High Performance Enhancement-Mode AlGaN/GaN MIS-HEMT with Selective Fluorine Treatment

A novel enhancement-mode (E-mode)Metal-Insulator-Semiconductor(MIS-) HEMTwith 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 (E-field) between the gate and drain by the assisted depletion effect but also reduces the E-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 (I


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 ( TH ) 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 SiO 2 [9], Si 3 N 4 , TaO x N y [10], and HfO 2 [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  ,sat simultaneously.Electrostatic potential (V)  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  ,sat .All the three HEMTs are featured by   = 2 m and a gate-drain distance  gd = 3 m.The HEMTs also consist of a 1.5 m thick GaN layer, a 25 nm thick AlGaN layer, and 200 nm thick Si 3 N 4 passivation on the AlGaN.The  and  directions are given.

Structure and Mechanism
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 Conduction band (eV) 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 ( = 0.226 m) 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.Figure 3(a) shows the influence of  1 on conduction band under the gate.The  1 elevates the conduction band above the Fermi level (  ) at  1 ≥ 6 × 10 18 cm −3 , realizing the Emode.Figures 3(b  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  DS = BV, 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.

Results and Discussion
The influence of  2 and  2 on the off-state BV and  ,sat are shown in Figure 4(a).The BV increases with the increasing  2 owing to the spreading depletion region.As the  2 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  2 = 2.2 × 10 18 cm −3 .With the increase in the  2 and  2 , the  ,sat slightly decreases due to the enhanced assisted depletion on the 2DEG.The - curves for the SFCP-MIS-HEMT with different  2 and  2 are compared at   = 0 V 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 ( 2 ).Second, the peak -field around the gate edge is decreased by  2 , avoiding the premature breakdown herein (see Figures 4(c) and 2(d)).Figure 4(b) also shows that the BV increases with the increasing  2 in the range of 0-3 m owing to the spreading depletion region between the gate and the drain. 2 can increase the -field strength near the drain and thus improve the BV while the too high  2 (e.g.,  2 = 2.2 × 10 18 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  2 and  2 .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.Figure 5 indicates the dependence of the  TH and  ,sat on the  1 and  1 ( TH = 1 V). Figure 5(a) shows that the  TH increases with the increasing  1 at  1 = 7.7 × 10 18 cm −3 and  2 = 0 m.However, with the increases in the  1 , the  ,sat obviously decreases due to the enhanced depletion on the 2DEG.In Figure 5(b), the  TH increases with the increasing  1 at  1 = 0.5 m and  2 = 0 m.The inset in Figures 5(a) and 5(b) shows the transfer characteristic curves as a function of  1 and  1 .To make sure  TH = 1 V, the optimized  1 ( 1,opt ) decreases with the increasing  1 .When the  1 is too small, the corresponding  1,opt is extremely high (e.g.,  1 = 0.6 m and  1 = 8.7×10 18 cm −3 ) and the 2DEG density and mobility are very low, leading to a sharp decrease in  ,sat as shown in Figure 5(c).Finally, the minimum value of  1 is 0.8 m.
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  TH = 1V.In Figure 6(a), the  ,sat of the SFCP-,  When the   is small (e.g.,   = 3 V), the  ch accounts for the major part of the  on .The  ch 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   = 7V, 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  2 on the 2DEG.The degradation in the forward characteristics is slight nevertheless since the  2 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.
Figure 7 illustrates the specific on resistance ( on,sp ) 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 = BV 2 / on,sp ).The SFCP-MIS-HEMTs exhibit almost a linear  on,sp -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  gd .

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  ,sat .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  on,sp .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.

Figure 1 2 Advances
Figure1is 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. 1 and  1 represent the F − concentration and length of the F − treatment region in the AlGaN layer, respectively;  2 and  2 are the F − concentration and lateral length of F − treatment in the passivation layer, respectively.When  2 = 0 and  1 <   , the structure is selective F − treated in channel (SFC-MIS-HEMT).When  2 = 0 and  1 =   , the structure is F − treated in the whole
(a) with Figures 2(b) and 2(c), the F − in the passivation layer

3 )Figure 3 :
Figure 3: (a) Influence of  1 on conduction band; (b) 2DEG density of the three different MIS-HEMTs at on state; (c) 2DEG density of the three different MIS-HEMTs in the drift region at off state.
) and 3(c) show the 2DEG density at the on state and off state.In Figure3(b), for the SFCP-MIS-HEMT, the  1 region depletes the 2DEG and realizes the E-mode,

Figure 4 :
Figure 4: (a) Influences of  2 and  2 on BV and  ,sat ; (b) - curves for the different  2 and  2 values, where the BV is defined as the source-drain voltage at   = 1 mA/mm and   = 0 V; (c)   -field at AlGaN/GaN interface. (mA/mm) (mA/mm)

Figure 5 :
Figure 5: (a) Dependence of the  TH and  ,sat on the  1 ; (b) dependence of the  TH and  ,sat on the  1 ; (c) influence of the  1 on the  ,sat .

Figure 6 :
Figure 6: (a) Transfer characteristic curves at  DS = 10 V; (b) output characteristic curves for the three structures.

Table 1 :
Abbreviations and parameters of three HEMTs.