A snapback-free and high-speed SOI LIGBT with double trenches and embedded fully NPN structure

Lateral insulated gate bipolar transistor (LIGBT) on silicon-on-insulator (SOI) is widely applied in power integrated circuits, due to its low on-state voltage, high input impedance and high current capability^[1-8]. However, LIGBTs exist the contradiction relationship between the turn-off loss (E_off) and the on-state voltage (V_on). The lateral injection-enhanced gate transistor (LIEGT) is proposed to improve the current capability and achieve a lower V_on^[9]. The U-shaped channel LIGBT with dual trenches is adopted to enhance carrier stored effect and obtain a low V_on^{[10, 11]}. However, the lower V_on usually results in a higher E_off. To reduce the E_off, the shorted-anode (SA) LIGBT provides an additional electron extracting path by introducing the N⁺ anode region, but it brings a snapback effect^[12]. The separated shorted anode (SSA) LIGBT needs a large cell pitch to solve this problem by increasing L_s (the length between N-buffer and N⁺ anode), leading to a low area efficiency^[13]. For the segmented (SEG) anode LIGBT, a very large geometry ratio of the P⁺ and N⁺ anode is needed to eliminate the snapback effect^[14]. The multi-segment anode (MSA) adopts multi-segmented P⁺ anode and P-buried layers in anode regions to increase the anode distribution resistance and thus eliminates the snapback effect, while its improvement on E_off is limited^[15]. The STA LIGBT suppresses the snapback effect by forming segmented deep trenches between P⁺ anode and N⁺ anode, but the path between the trenches should be small enough to eliminate the snapback effect, which will limit the improvement on E_off on the contrary^[16].

In this paper, a novel DT-NPN LIGBT featuring an NPN structure, double trenches and an N-CS layer is proposed to achieve a better tradeoff between the V_on and E_off. The proposed device eliminates the snapback effect within a small device dimension by adjusting the parameters of the embedded NPN structure. It achieves a low V_on due to the carrier storing effect enhanced by the double cathode trenches and N-CS layer. Meanwhile, the double cathode trenches enhance the latch-up ruggedness. This paper is implemented by TCAD Sentaurus Device simulation tools^[17], which includes the models of high field saturation mobility, Philips unified mobility, Auger recombination, Enormal mobility, Shockley–Read–Hall recombination, and Lackner avalanche generation.

Fig. 1 shows the schematic cross-sectional views of the proposed DT-NPN LIGBT, double trench segment anode LIGBT (named SEG LTIGBT), and double trench separated shorted anode LIGBT (named SSA LTIGBT). The proposed LIGBT features an NPN structure at the anode side, consisting of N⁺ anode, P-well and N-buffer. Moreover, there are cathode trench gate (CTG), cathode blocking trench (CBT) and an N-type carrier storage (N-CS) layer at the cathode side for the three devices mentioned above. The implantations for N-CS and N-buffer layer are implemented at the same time to achieve better performance without more cost. The CTG and CBT are carried out within the same process step. The key parameters used in simulations for the three devices are shown in Table 1. N_p and D_p are the doping concentration and depth of the P-well region at the anode side for DT–NPN LIGBT, as shown in Fig. 2. The carriers' lifetime is 1 μs for all devices.

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Fig. 2 shows the equivalent circuit of the DT-NPN LIGBT. At the initial forward conduction stage with CTG turned on, the device operates in unipolar mode and the electron current is dominant. The P-well acts as an electrons barrier and hinders the electron from flowing through the N⁺ anode directly. As the anode voltage increases, the P⁺ anode/N-buffer junction turns on and the LIGBT quickly transforms from unipolar mode to bipolar mode without snapback effect. Meanwhile, the double cathode trenches and N-CS layer prevent holes from flowing to the cathode directly. Then, the carrier storing effect is enhanced in the N-drift and the V_on decreases sharply. During the turn-off stage, the shallow P-well is fully depleted and the NPN structure is activated to provide a low-resistance path to extract the electrons rapidly. Hence, the turn-off time and the E_off are decreased. The vertical channel and the hole bypass along the CTG and CBT respectively could decrease the distributed resistance at the cathode side, and then the proposed device enhances the immunity of latch-up effect and widens the safe operation area (SOA).

Fig. 3(a) shows the dependences of BV and V_on on the P⁺ anode doping concentration (N_A). For low N_A, the V_on of proposed DT-NPN LIGBT is smaller than that of the conventional (Conv.) LIEGT in Ref. [9] because the N-CS layer is dominant to increase the carrier density and decrease V_on. It is contrary for high N_A value, because the N⁺ anode results in the hole injection of the proposed device much smaller than that of Conv. LIEGT. Obviously, the BV of Conv. LIEGT decreases dramatically with the increasing N_A, while it almost remains stable for the DT-NPN LIGBT for different N_A. For N_A = 1 × 10¹⁸ cm^–3, the BV = 614 V of the proposed LIGBT is 7.3% higher than BV = 572 V of Conv. LIEGT. Fig. 3(b) compares the current component at breakdown. The Conv. LIEGT exhibits an open-base P–N–P transistor breakdown mechanism because the leakage current increases distinctly with the increasing N_A and the hole leakage current is higher than electron leakage current. In contrast, the DT-NPN LIGBT behaves MOS-like blocking mechanism as the electron leakage current is dominated and almost irrelevant to N_A. Because the embedded NPN structure suppresses the hole injection of the P⁺ anode/N-buffer junction. Particularly, the DT-NPN LIGBT relieves the conflict demand of N_A on high BV and low V_on, and then V_on and BV can be designed separately.

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Fig. 4 illustrates the snapback characteristics of different LIGBTs with the same N_A. The proposed LIGBT eliminates the snapback effect by the optimizing the N_p and D_p, without increasing length in the x-direction. However, the SSA LTIGBT still shows a little ΔV_SB even the length L_s = 40 μm. The SEG LTIGBT also has a snapback effect, even when the width W_P/ W_N of P⁺/ N⁺ anode reaches 79/1 μm. Fig. 5 shows the total current density distribution and the flowlines extracted from Fig. 4. In Fig. 5(a), one current flowline derives from the P⁺ anode for the DT-NPN LIGBT, which means the P⁺ anode/N-buffer junction is turned on and the device changes from unipolar mode to bipolar mode at a smaller V_A. Owing to the barrier effect induced by the P-well, the proposed LIGBT shows snapback-free effect with compact dimension in x-direction. However, the SSA and SEG LTIGBTs require large extra dimensions above 40 μm in x-direction and 79 μm in z-direction respectively to reduce the ΔV_SB, because all the current flowlines derive from the N⁺ anode and both devices are still in unipolar mode, as shown in Figs. 5(b) and 5(c).

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Fig. 6(a) shows the influences of N_p and D_p on the forward conduction characteristics. With the increasing N_p and D_p, the snapback effect is weakened and eventually eliminated, because larger N_p and D_p values increase the electron barrier height and width at P-well/N-buffer junction to hinder N⁺ anode from extracting electrons, as shown in Fig. 6(b). Therefore, the increasing N_p and D_p are beneficial to decrease the ΔV_SB and V_on, but increase the E_off as shown in Fig. 6(c). The snapback effect is eliminated with N_p = 1.5 × 10¹⁶ cm^–3 and D_p = 1 μm, or N_p = 3 × 10¹⁶ cm^–3 and D_p = 0.5 μm.

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Fig. 7 compares the forward conduction characteristics of four LIGBTs with different cathode structures. The proposed LIGBT shows the lowest V_on of 1.25 V because both the double trenches and the N-CS layer increase the hole density at the cathode side and enhance conduction modulation effect in the drift region. The V_on of the LIGBT with Type1 cathode structure increases to 1.37 V, owing to the deletion of N-CS layer^[18-20]. The LIGBT with Type2-cathode structure, without the CBT and N-CS layer, decreases the hole density at the cathode side and then V_on increases to 1.54 V. The LIGBT with a planar gate reaches the highest V_on of 1.94 V. The inserted hole density distribution verifies the current modulation effect induced by the N-CS, CBT and CTG, respectively.

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Fig. 8 shows the switching characteristics of different LIGBTs at the same V_on without snapback effect. The devices start turning off at t₁ = 20 μs. The proposed LIGBT achieves the fastest switching speed of 54 ns because the embedded NPN structure provides a larger width of extra electron extraction path than the SEG LTIGBT and MSA LIGBT. However, the Conv. LIEGT exhibits the longest tail current and higher E_off, because its excess carriers stored in the drift region mainly depends on recombination to disappear without extra electron extraction path. Fig. 8(b) shows the electron concentration distributions of the proposed LIGBT and SEG LTIGBT from t₁ to t₅ period labeled in Fig. 8(a). It is quite obvious that the electron density of the proposed LIGBT is lower than that of the SEG LTIGBT at the same time, owing to the activated NPN with a wider rapidly electron extracting path. As shown in Fig. 8(c), a large number of current flowlines flow through the embedded NPN structure during the turn-off period of the proposed LIGBT.

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Fig. 9 shows the I_A–V_A characteristics for the DT-NPN LIGBT and MSA LIGBT at V_G = 10 V and 15 V. In the on-state, the cathode distributed resistance R_p-body should be small enough to avoid triggering the parasitic NPN transistor (N⁺ cathode/P-body/N-CS) at the cathode side. For the proposed LIGBT, the CBT and CTG force the hole current flowing vertically, which is conducive to decrease R_p-body. In contrast, the hole current inevitably flows under the N⁺ cathode and easily triggers the latch-up effect, even a P-buried layer is formed below the N⁺ cathode. Therefore, the DT-NPN LIGBT exhibits much better latch-up ruggedness than MSA LIGBT.

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Fig. 10 shows the E_off–V_on tradeoff of different LIGBTs at the load current density of 100 A/cm². Due to the contribution of double trenches at cathode side and embedded NPN structure at the anode side, the proposed IGBT shows better tradeoff than other counterparts. The E_off value of the proposed LIGBT is 55% lower than that of Conv. LIEGT at the same V_on of 1.22 V. Meanwhile, the proposed LIGBT decreases the V_on by 4.8%, 7.4% and 38.6% compared with SEG LTIGBT, Conv. LIEGT, and MSA LIGBT at the same E_off, respectively.

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Fig. 11 depicts the key fabrication steps of the proposed LIGBT. In Fig. 11(a), N-CS/N-buffer implantations are implemented at the same time. To ensure the reliability of double trenches and eliminate the snapback effect, the P-body/P-well implantations are implemented under different doses and masks in Fig. 11(b). The double trenches could be formed at the same time, as shown in Fig. 11(c). All the steps can be implemented by the feasible and mature trench BCD technology.

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A novel 600 V class DT-NPN LIGBT is proposed and investigated. Due to the electron barrier of P-well, the proposed LIGBT entirely avoids the snapback effect within small cell pitch, while it delivers an electron extracting path during the turn-off stage to decrease E_off. The double cathode trenches and N-CS layer enhance the carrier storing effect to achieve a lower V_on. Therefore, the proposed LIGBT acquires superior tradeoff characteristic between V_on and E_off compared with the Conv. LIEGT, MSA LIGBT and SSA LTIGBT. The proposed LIGBT reduces the E_off by 55% compared to the Conv. LIEGT at the same V_on, and decreases the V_on by 38.6% compared to the MSA LIGBT at the same E_off. Moreover, the DT-NPN LIGBT improves the BV by 7.3% and relieves the dependence of the BV on the N_A. In addition, the DT-NPN LIGBT enhances the latch-up ruggedness, because the double trenches form vertical channels and provide a hole by-pass path to reduce the cathode distributed resistance.

This work is supported by Postdoctoral Innovative Talent Support Program under Grant BX20190059, the China Postdoctoral Science Foundation under Grant 2019M660235, the Sichuan Science and Technology Program under Project 2018JY0555 and the Science and Technology on Analog Integrated Circuit Laboratory under Project 6142802180509.