High Performance Flip-Structure Enhancement-Mode HEMT with Face-to-Face Double Gates – Nanoscale Research Letters

Figure 3 shows the electron concentration distributions and conduction band of the DFF HEMT. When V_gs = 0, the electron density along AA’ line (in Fig. 1) is just 5 × 10¹⁴ cm⁻³ in Fig. 3a, and the conduction band is above the Fermi level in Fig. 3b. The E-mode is thus realized. When V_gs = 3 V > V_th, the conduction band along AA’ line has been pulled below the Fermi level, and electron density is as high as 2 × 10¹⁹ cm⁻³. The DFF HEMT has been turned on. Additional, the electron density along AA’ line at V_gs = 3 V is higher than that along BB’ line at V_gs = 0 V in Fig. 3a. It indicates that 2DEG has been restored owing to high positive-voltage biased to the double gate.

a Electron concentration distributions and b conduction band of DFF HEMT at V_gs = 0 V and V_gs = 3 V

Figure 4a shows the electron concentration distributions along the 2DEG channel in the on-state. The 2DEG concentration of the gate region of the DFF HEMT is much higher than those of the MIS HEMT and the MIS-FP HEMT. On one hand, the potential in the gate region of the DFF HEMT is much higher because both the BG and TG are applied to 3 V. The conduction band of the DFF HEMT is thus much lower as shown in Fig. 4b, which contributes to the higher 2DEG concentration. On the other hand, the polarization effect under the gate of the MIS HEMT and the MIS-FP HEMT is weakened because the AlGaN layer is etched partly to realize E-mode. Additional, it is normal that the 2DEG density under the right side of the gate is low for the three devices, because there is a higher electric potential of the semiconductor than that of the gate, as a depletion region. The field plate of the DFF HEMT is far away from the 2DEG channel and has smaller depletion effect on the 2DEG concentration than that of the MIS-FP HEMT. Therefore, the 2DEG concentration under the field plate of the DFF HEMT is higher, which contributes to higher current and lower specific on-resistance (R_{on, sp}).

a 2DEG distributions and b conduction band along AA’ line for three devices in the on-state

Figure 5 shows the output characteristic and transfer characteristic of the three devices. It can be seen from Fig. 5a that the DFF HEMT has the largest I_{d, sat} and the smallest R_{on, sp}. The I_{d, sat} of the MIS-FP HEMT is the smallest because the field plate assists in depleting 2DEG along the drift region (see Fig. 4a). For the DFF HEMT, on one hand, the 2DEG is recovered, and on the other hand, the double gates introduce electron accumulation layer. Figure 5c shows the electron current density along AA’ line. The DFF HEMT has a large current density in electron accumulation layer, in addition to the 2DEG channel current. The 2DEG channel still plays a dominant role in the transport. It can be seen from Fig. 5d that DFF HEMT has the highest transconductance owing to the double gates. The V_th of three devices is designed as 0.8 V.

a output I-V characteristics, b electron current density, c electron current density distributions along AA’ line and d transfer characteristics for three devices

Figure 6 shows the lateral component of the E-field (E_x) distribution and I–V curves at breakdown. It indicates that the E_x for the DFF HEMT is effectively improved. Compared with the MIS HEMT, the field plate not only brings out new E-field peak, but also expands the depletion region for the MIS-FP and DFF HEMT. Particularly, for the DFF HEMT, the step field plate further uniforms the E-field distribution in the drift region. Meanwhile, the leakage current of the substrate has been suppressed by the poly-AlN substrate layer. Therefore, the DFF HEMT achieves the highest BV of 465 V (at I_d = 10^–6 mA/mm), and BV is 394 V for the MIS-FP HEMT, as shown in Fig. 6. Without the field plate and flip-structure for the MIS HEMT, the drift region cannot be completely depleted and leakage current is large, and thus the BV is just 75 V at the same drift region length.

E_x distributions and I-V curves for three devices at breakdown at y = 0.051 μm

Figure 7 shows the electron concentration distributions and conduction band at V_gs = 0 V along AA’ line with different T values. As T decreases, the conduction band rises and the electron density decreases. When T ≤ 50 nm, the depletion effect of the MIS structure on electrons is enhanced and the polarization effect is weakened. The whole conduction band is raised above E_F. Consequently, E-mode is realized. When T > 50 nm, the depletion effect is weakened and the polarization effect is enhanced. Therefore, potential well of electrons is formed and the electron concentration rises to 10¹⁹ cm⁻³ order of magnitude, which is higher than the background charge concentration of 10¹⁸ cm⁻³. Figure 8 shows the transfer characteristic with different T values. The V_th increases as T decreases.

Electron concentration distributions and conduction band at V_gs = 0 V along AA’ line with the different T values

Transfer characteristics with the different T values

Figure 9 shows the output characteristic, electron concentration distributions and conduction band with the different T values at a constant of (V_gs− V_th). The I_{d, sat} increases and the R_{on, sp} decrease as T decreases because the electron concentration increases, as shown in Fig. 9a and b. As T decreases, the V_th increases (in Fig. 8) and thus V_gs accordingly increases to maintain a constant of (V_gs− V_th), leading to the increase in electron concentration. Meanwhile, the conduction band is lower and the 2DEG concentration is higher owing to the lower T and higher V_gs, as indicated in Fig. 9b. Different from the conventional GaN HEMT, the DFF HEMT break through the tradeoff relationship between the high V_th and high I_{d, sat}. As T decreases, both the I_d,sat and the V_th increase.

a output characteristics and b electron concentration distributions and conduction band (V_gs− V_th = 3 V and V_ds = 10 V along AA’ line) with the different T values

Figure 10 shows the influence of the L on the potential contours and E_x distribution of the DFF HEMT. The BV of the DFF HEMT firstly increases and then decreases as the L increases. When L = 0 μm means the DFF HEMT without the field plate, the BV is lower because the drift region has not been depleted. With the increase in L at L < 2.6 μm, the depletion region expands. The step field plate decreases the E-field peak at the gate edge (P1) and induces a new E-field peak below the end of the field plate (P2), so the BV increases as shown in Fig. 10b. P3 is the new E-field peak caused by the step field plate at y = 0.051 μm in Fig. 10b. At this time, the premature breakdown occurs at the right edge of the gate. When L = 2.6 μm, the E_x of the DFF HEMT is almost uniform and the BV of 465 V achieves the highest. When L = 3.0 μm, premature breakdown occurs owing to high P2, and BV decreases.

a Equipotential contours and b Ex distribution at breakdown at y = 0.051 μm with different L values

Figure 11 demonstrates the R_on,sp and BV values of the DFF HEMT and reported AlGaN/GaN HEMTs, the DFF-HEMT has a higher figure of merit (FOM = BV²/R_on,sp) than those of the AlGaN/GaN HEMTs in studies. As a result, when T = 40 nm, L = 2.6 μm, an excellent tradeoff between the on-state characteristics and the off-state characteristics is achieved.

R_on,sp and off-state BV in the DFF-HEMT and reported AlGaN/GaN HEMTs, together with the theoretical Si-, SiC- and GaN- limit

The fabrication process steps of the DFF HEMT are shown in Fig. 12, which is referred to the experiment in [16]. Key processes are given as follows: (a) Fabricate a conventional MIS HEMT on a Si (111)-based AlGaN/GaN heterojunction wafer. (b) form poly-AlN film on the device surface by physical vapor deposition (PVD), and bonding to the Si (100) wafer. (c) Remove the Si (111) substrate and part of the GaN epitaxial layer. (d) form the shallow trench of the field plate by inductively coupled plasma (ICP) etching. (e) ICP etch TG deep trench. (f) implement Al₂O₃ layer and the top gate metal. Compared with those of Ref. [16], the fabrication process of the DFF HEMT only add twice ICP etchings ((d) and (e)) to realize the E-mode and further improve the BV. The DFF HEMT simultaneously realizes E-mode, and achieves a higher BV and a smaller R_on.

Key fabrication process flows for the DFF HEMT