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A semi-superjunction MOSFET with P-type Bottom Assist Layer
Superlattices and Microstructures 83 (2015) 745–754
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
A semi-superjunction MOSFET with P-type Bottom Assist Layer Weizhong Chen a,⇑, Wei Wang a, Yong Liu b, Bo Zhang b, Chao Ma c a
College of Electronics Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 14 January 2015 Received in revised form 10 March 2015 Accepted 11 March 2015 Available online 23 April 2015 Keywords: Superjunction MOSFET Bottom Assist Layer Breakdown voltage On-state resistance
a b s t r a c t A new Semi-Superjunction Vertical Double-diffusion MOSFET (Semi-SJ MOSFET) with P-type Bottom Assist Layer (P-BAL) is proposed. The P-BAL is introduced upon the N-Stop substrate and embedded in the n-BAL, on the one hand, another reverse biasing PN junction JP-BAL/N-Stop at the bottom substrate is formed, which helps to deplete the n-BAL along the longitudinal direction and changes the shapes of the electric field distribution. On the other hand, another Semi-SJ structure is formed by the P-BAL and nBAL, the transverse electric field in n-BAL part is enhanced, and this indicates that the trade-off between breakdown voltage (BV) and on-state resistance (Ron,sp) has been optimized due to the improvement of BV. As results show, in the proposed MOSFET, the BV (i.e., 1097 V) is improved 25% higher than that of the conventional Semi-SJ structure (i.e., 877 V) and the proposed does negligible disadvantage to the Ron,sp (i.e., the proposed with a Ron,sp of 59 mX cm2 and the conventional MOSFET with a Ron,sp of 56 mX cm2). Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The power device is widely used in industry application such as motor driver, inverter and high frequency switch [1,2]. The superjunction MOSFET has become a dominate device in the low-medium ⇑ Corresponding author. Tel.: +86 28 83207790. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.spmi.2015.03.065 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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voltage application field, particularly in the voltage range of less than 900 V. The difficulty of the process to form the narrow and highly doped P/N Pillars and the hardness of the reverse-recovery of the body-diode are two key issues for the SJ MOSFET [3,4]. Then the Semi-SJ concept with an n-type Bottom Assist Layer (n-BAL) which is connected in series to the bottom of the SJ structure is proposed to overcome the issues [5–8]. From the aspect of structure, the Semi-SJ is composed of SJ part and nBAL part, thus the breakdown voltage BV for the Semi-SJ is the sum of the BVSJ and the BVn-BAL, the onstate resistance Ron,sp for the Semi-SJ is the sum of the RSJ and the Rn-BAL. As well known, the relationship between Rn-BAL and BVn-BAL for the n-BAL part which is formed by epitaxial process is still restrained by the Si limit. So the trade-off between Rn-BAL and BVn-BAL needs further optimized [9]. In this paper, by adopting the numerical simulation and based on the Semi-SJ MOSFET concept, another P-type Bottom Assist Layer (P-BAL) is introduced upon the N-Stop substrate and embedded in the n-BAL to breaks the trade-off between Ron,sp and BV of the Semi-SJ MOSFET .
2. Device structures and mechanism Fig. 1 shows the structures and electric distribution for comparison between the conventional Semi-SJ MOSFET and the proposed Semi-SJ MOSFET with a P-type Bottom Assist Layer (P-BAL). From Fig. 1(a), the Semi-SJ MOSFET is composed of SJ part and n-BAL part which is formed by epitaxial process and it is in series connected to the bottom of the SJ structure. So the breakdown voltage BV for the Semi-SJ is the sum of the BVSJ and the BVn-BAL, the on-state resistance Ron,sp for the Semi-SJ is the sum of the RSJ and the Rn-BAL. From Fig. 1(b), the proposed Semi-SJ MOSFET has the same structure except the P-BAL which is introduced upon the N-Stop substrate and embedded in the n-BAL. as the same as the conventional Semi-SJ MOSFET, the breakdown voltage BV for the proposed Semi-SJ is also the sum of the BVSJ and the BVn-BAL, and the on-state resistance Ron,sp is also the sum of the RSJ and the Rn-BAL.
2.1. The breakdown property (BV) When the Semi-SJ MOSFET is at the breakdown state, the gate and the source are zero biased; the drain is applied with high voltage. The breakdown voltage BV is determined by the electric field. As shown in Fig. 1(a) for the conventional Semi-SJ MOSFET, the electric field distribution of SJ part is
Fig. 1. Cross-sectional structures and electric field distributions for (a) Semi-SJ MOSFET (b) Semi-SJ MOSFET with P-BAL.
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rectangle, and the shape of electric field for n-BAL part is trapezoid, so the sustain voltage BV = BVSJ + BVn-BAL of the semi-SJ MOSFET can be expressed as:
BVSemi-SJ ¼ EV T SJ þ EV T n-BAL
qN n-BAL T 2n-BAL 2e
ð1Þ
where BVSJ and BVn-BAL are the sustain voltage of SJ and n-BAL part, respectively. EV is vertical peak electric field, TSJ and Tn-BAL are the thickness for SJ and n-BAL part, respectively, q is an electron charge, e is dielectric constant, Nn-BAL is the doping concentration of n-BAL part. In Fig. 1(b) for the proposed Semi-SJ MOSFET, as the introduction of the P-BAL, another reverse pffiffiffi biasing PN junction JP-BAL/N-Stop at the bottom is formed. A new critical electric field EC = 2EV [9] will occur at JP-BAL/N-Stop. It helps to deplete the n-BAL along the longitudinal direction and changes the shapes of the electric field distribution. Then assuming the EC equals to EV for simplicity, the sustain voltage BV = BVSJ + BVn-BAL + BVP-BAL for the semi-SJ MOSFET with P-BAL is given as:
BVSemi-SJðP-BALÞ ¼ EV T SJ þ EV T n-BAL þ
qNn-BAL T n-BAL ðT P-BAL T n-BAL Þ 2e
ð2Þ
where BVP-BAL and TP-BAL are the sustain voltage and length of P-BAL, respectively. From Eqs. (1) and (2) we can see that the BV is improved by the introduction of P-BAL. 2.2. The on-state resistance property (Ron,sp) When the Semi-SJ MOSFET is at the on-state of conduction, the source is zero biased, the gate and the drain are positive biased. As shown in Fig. 2(a) for the conventional Semi-SJ MOSFET, The electron will be emitted from the N+ source and vertically flow to the SJ part and n-BAL part through the electron channel. So the on-state resistance Ron,sp is determined by the SJ part and the n-BAL part and can be expressed as:
RSemi-SJ ¼ RSJ þ Rn-BAL ¼
T SJ þ qn-BAL ðT T SJ Þ qln N N-Pillar
ð3Þ
where RSJ and Rn-BAL are the resistance of SJ and n-BAL part, qn-BAL is the resistive rate of n-BAL.
Fig. 2. Cross-sectional structures and resistance distributions for (a) Semi-SJ MOSFET (b) Semi-SJ MOSFET with P-BAL.
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In Fig. 2(b) for the proposed Semi-SJ MOSFET, as the introduction of the P-BAL, the electron in the n-BAL part will bypass through the P-BAL, and another resistance between the P-BAL and the P-Pillar RJFET is introduced, so the on-state resistance Ron,sp is expressed as:
RSemi-SJðP-BALÞ ¼ RSJ þ Rn-BAL þ RJFET
T SJ q ðT SJ þ T d Þ þ qn-BAL ðT T SJ T P-BAL Þ þ n-BAL qln NN-Pillar Z L2M T d
ð4Þ
where RSJ and Rn-BAL are the resistance of SJ and n-BAL part, RJFET is the resistance between the P-BAL and the P-Pillar, qn-BAL is the resistive rate of n-BAL, Z is a constant, Td is the depletion layer extension bellow the P-Pillar. 3. Results and discussion From Figs. 1 and 2, the both MOSFETs have the same structure parameters except for the P-BAL. The main parameters of the MOSFETs are listed in Table 1. Moreover, the carrier lifetime for the electron and the ambient temperature are set as 10 ls and 300 K respectively. 3.1. The electric characteristics of BV Fig. 3 shows the numerical simulated breakdown property and equipotential contours for the conventional and proposed Semi-SJ MOSFET. In Fig. 3(a), the BV of proposed MOSFET is increased as the length TP-BAL increasing. When the TP-BAL = 25 lm, the BV (i.e., 1097 V) is improved 25% higher than that of the conventional Semi-SJ structure (i.e., 877 V). In Fig. 3(b), the equipotential contours are sparse for the conventional Semi-SJ at the n-BAL part. In Fig. 3(c), the equipotential contours in nBAL part are distributed uniformly due to the introduction of the P-BAL. Fig. 4 shows the 3-D electric field distribution under breakdown state for the conventional and proposed Semi-SJ MOSFET. It can be seen that the electric field distribution is almost the same and exhibits cube shape at the SJ part for both MOSFETs, but the electric field distribution is much different at n-BAL part. For the conventional Semi-SJ MOSFET, in order to obtain low Ron,sp, the concentration of the n-BAL part is set at much higher than the N-Pillar as show in Table 1, then the electric field slope is much steeper and the electric field will terminate at the n-BAL area. For the proposed Semi-SJ MOSFET due to the introduction of P-BAL, the electric field can be divided into n-BAL part and PBAL part, at the n-BAL part the electric field slope descend abruptly as the conventional MOSFET, but at the P-BAL part, the electric field slope ascend abruptly and another electric field peak value is appeared at the junction JP-BAL/N-Stop. Fig. 5 shows the 2-D vertical and lateral electric-field distributions under breakdown voltage. In Fig. 5(a), for the conventional Semi-SJ, the electric field gradient along the line C1C2 (as shown in
Table 1 Device specifications. Parameter
Proposed
Semi-Sj VDMOS
MOS cell size, LM Wafer thickness, T SJ thickness, TSJ SJ doping, Nsj (N-Pillar) SJ width, LN-column (N-Pillar) SJ doping, Nsj (P-Pillar) SJ width, LP-column (P-Pillar) n-BAL thickness, Tn-BAL n-BAL doping, Nn-BAL N-Stop thickness, TN-Stop N-Stop doping, NN-Stop P-BAL length, TP-BAL P-BAL doping, NP-BAL P-BAL width, WP-BAL
17 lm 75 lm 35 lm 3.3 1015cm3 11 lm 6.6 1015cm3 6 lm 35 lm 3.7 1014 cm3 5 lm 1 1017 cm3 25 lm 6.6 1015 cm3 2 lm
17 lm 75 lm 35 lm 3.3 1015 cm3 11 lm 6.6 1015 cm3 6 lm 35 lm 3.7 1014 cm3 5 lm 1 1017 cm3 – – –
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Fig. 3. The breakdown property and equipotential contours for the Semi-SJ MOSFETS (a) breakdown state I–V curves (b) equipotential contours for the conventional Semi-SJ MOSFET (c) equipotential contours for the Semi-SJ MOSFET with P-BAL (TP-BAL = 25 lm).
Fig. 4. The 3-D electric field distributions under breakdown voltage for (a) the conventional Semi-SJ and (b) proposed Semi-SJ MOSFET with P-BAL.
Fig. 1) in n-BAL part is determined by the doping Nn-BAL of the n-BAL. with the increasing of Nn-BAL to realize lower Rn-BAL, the electric field will terminated in the n-BAL part, the shape of the electric field changes from trapezoid to triangle, thus the breakdown voltage will decrease. For the proposed SemiSJ due to the introduction of the P-BAL, another reverse biasing PN junction JP-BAL/N-Stop at the bottom is formed as shown in Fig. 1(b), a new critical electric field will occur at the JP-BAL/N-Stop. As the high doping concentration of the N-Stop substrate, the electric field in the N-Stop substrate will be terminated abruptly, and the depletion layer will reverse expand toward to the SJ part along the vertical direction, thus the electric field distribution is changed. Moreover, as the increase of the length TP-BAL of the P-
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Fig. 5. The electric field distributions under breakdown voltage for the conventional Semi-SJ and proposed Semi-SJ with P-BAL. (a) vertical electric field along the line C1C2 as shown in Fig. 1 (b) lateral electric field along the line B1B2 as shown in Fig. 1.
BAL, the maximum electric field will increase and further reverse expand to the SJ part. In Fig. 5(b), the lateral electric field along the line B1B2 (as shown in Fig. 1) is enhanced because another Semi-SJ structure is formed by the P-BAL and n-BAL. As the n-BAL is negative charged, according to the SJ theory [9], with the increase of the TP-BAL of the P-BAL, more positive charge will be introduced and more negative charge will be compensated by the P-BAL. The electric field will be further enhanced and this indicates that the trade-off of BV and Ron,sp has been optimized. Fig. 6 shows doping NP-BAL of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ MOSFET. It can be seen that as the increase of the NP-BAL of the P-BAL, more positive charge will be introduced and more negative charge will be compensated by the P-BAL, thus the electric field in the n-BAL part will be enhanced. But at the same time, with the increase of the NP-BAL, the electric field in the SJ part will decrease due to the shielding effect of the JFET formed between the P-BAL and the P-Pillar. Fig. 7 shows width WP-BAL of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ MOSFET. It can be seen that as the WP-BAL increases from 0 lm to 3 lm, the electric field in the n-BAL part is enhanced. But when the WP-BAL equals to 5 lm, the
Fig. 6. The doping of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ with P-BAL.
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Fig. 7. The width of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ with P-BAL.
Fig. 8. The comparison of forward conduction between the conventional Semi-SJ MOSFET and the proposed Semi-SJ MOSFET with P-BAL, the inset picture is the current flow lines for the conventional and proposed Semi-SJ with P-BAL (width = 3 lm).
space between the P-BAL and the P-Pillar is decreased and the JFET effect will be obvious, then the electric field in the SJ part will decrease abruptly due to the shielding effect of the JFET, so the WPBAL is a sensitive parameter to design the proposed Semi-SJ.
3.2. The electric characteristics of Ron,sp Fig. 8 shows the forward conduction property and current flow lines for the conventional and proposed Semi-SJ MOSFET. For the conventional Semi-SJ MOSFET at the on-state of conduction, as the source is zero biased, the gate is positive biased, the electron channel is formed and the electron will be emitted from the N+ source, at the same time, the drain is also positive biased and the electron will vertically flow to the SJ part and n-BAL part. For the proposed Semi-SJ MOSFET, as the introduction of the P-BAL, the electron in the n-BAL part will bypass through the P-BAL as shown in the inset picture,
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Fig. 9. The doping of the P-BAL influences on the BV and Ron,sp.
Fig. 10. The width of the P-BAL influences on the BV and Ron,sp.
and another resistance between the P-BAL and the P-Pillar RJFET is introduced, so with the increase of the WP-BAL, especially when WP-BAL = 5 lm, the Ron,sp increases abruptly. 3.3. The trade-off property between the BV and Ron,sp Fig. 9 shows the doping NP-BAL of the P-BAL influencing on BV and Ron,sp, for the proposed Semi-SJ. It is clear that the NP-BAL does little effect to the Ron,sp. But the BV firstly increases gradually and then decreases abruptly with the NP-BAL, the reason is that by the introduction of the P-BAL, the charge balance in the n-BAL part can be optimized by modulating the QP-BAL and the Qn-BAL, according to the SJ theory, when at condition of QP-BAL Qn-BAL, the BV will get maximum value at the achievement of the balance of the QP-BAL and Qn-BAL. Fig. 10 shows the doping WP-BAL of the P-BAL influencing on BV and Ron,sp, for the proposed Semi-SJ. It is clear that the WP-BAL does little effect to the BV and Ron,sp. But the BV decreases and the Ron,sp increases abruptly when the WP-BAL is more than 3 lm due to the JFET effect between the P-Pillar and the P-BAL. From the aspect of the Nn-BAL, the Ron,sp decreases with the increasing of Nn-BAL for both
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Fig. 11. Trade-off characteristics between on-resistance and breakdown voltage for the different MOSFETs.
Semi-SJ MOSFET. Differently, the BV decreases obviously with the increasing of Nn-BAL for the conventional Semi-SJ due to the electric field can no punch though the n-BAL part, but for the proposed, by the introduction of the P-BAL, the charge balance in the n-BAL part can be optimized by modulating the QP-BAL and the Qn-BAL, thus the BV increases gradually with the increasing of Nn-BAL, it also will get maximum value at the achievement of the balance of the QP-BAL and Qn-BAL. Fig. 11 shows the Ron,sp and BV trade-off characteristic for the demonstrated MOSFETs. In reference [5,7] it shows that the Semi-SJ achieved better trade-off property than the referenced SJ MOSFET when the SJ MOSFET is scaled with different breakdown voltage maintaining the aspect ratio, the Ron,sp is proportional to BV2.5. For the proposed MOSFET, based on the Semi-SJ, the P-BAL improves the BV greatly with a little loss of Ron,sp, so the trade-off relationship is enhanced obviously. At the same condition of TP-BAL, by increasing the Nn-BAL, an optimum point for the trade-off can be achieved. 4. Conclusions The novel Semi-SJ MOSFET with P-BAL which is introduced in the n-BAL part is proposed, The PN junction JP-BAL/N-Stop is formed to reverse depletes the n-BAL along the longitudinal direction. Another SJ part is formed between the P-BAL and n-BAL, which can deplete the n-BAL along the transverse direction at breakdown state, thus the electric field is optimized and the BV is increased from 877 V to 1097 V, so 25% higher BV is achieved with a little loss of Ron,sp, ultimately the trade-off characteristic between BV and Ron,sp has been improved. Acknowledgments This work was supported by the National Science and Technology Major Project (Grant no. 2011ZX02504-003), the National Natural Science Foundation of China (Grant no. 61076082), and the Fundamental Research Funds for the Central Universities (Grant no. ZYGX2011J024). References [1] W.Z. Chen, Z.H. Li, B. Zhang, M. Ren, Y. liu, Z.J. Li, A Snapback suppressed reverse-conducting IGBT with soft reverse recovery characteristic, Superlattices Microstruct. 61 (2013) 59–68. [2] W.Z. Chen, Z.H. Li, Y. liu, B. Zhang, P.F. Liao, Z.J. Li, Enhancing the robustness of the equipotential ring of edge termination for 4.5 KV IGBT by introducing a partial N layer, Superlattices Microstruct. 65 (2014) 124–133. [3] G. Deboy, M. März, J.-P. Stengl, H. Strack, J. Tihanyi, H. Weber, A new generation of high voltage MOSFET’s breaks the limit line of silicon, IEDM Tech. Dig. (1998) 683–685.
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[4] M. Saggio, D. Fagone, S. Musumeci, MDmesh: innovative technology for high voltage powerMOSFETs, Proc. ISPSD (2000) 65– 68. [5] W. Saito, I. Omura, S.Aida, S. Koduki, M. Izumisawa, T. Ogura, Semisuperjunction MOSFETs: new design concept for lower on-resistance and softer reverse-recovery body diode, IEEE Trans. Electron Dev. ED-50 (2003) 1801–1806. [6] W. Saito, I. Omura, S. Aida, S. Koduki, M. Izumisawa, H. Yoshioka, T. Ogura, A 20 mX cm 600 V-class superjunction MOSFET, Proc. ISPSD (2004) 459–462. [7] W. Saito, I. Omura, S.Aida, S. Koduki, M. Izumisawa, H. Yoshioka, T. Ogura, Over 1000 V semi-superjunction MOSFET with ultra-low on-resistance blow the Si-limit, Proc. ISPSD (2005) 1–4. [8] W. Saito, I. Omura, S.Aida, S. Koduki, M. Izumisawa, H. Yoshioka, T. Ogura, High breakdown voltage (>1000 V) semisuperjunction MOSFETs using 600-V class superjunction MOSFET process, IEEE Trans. Electron Dev. ED-50 (2005) 2317– 2322. [9] X.-B. Chen, J.K.O. Shin, Optimization of the specific on-resistance of the COOLMOS, IEEE Trans. Electron Dev. 48 (Jan. 2001) 344–348.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
A semi-superjunction MOSFET with P-type Bottom Assist Layer Weizhong Chen a,⇑, Wei Wang a, Yong Liu b, Bo Zhang b, Chao Ma c a
College of Electronics Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China c University of Chinese Academy of Sciences, Beijing 100049, China b
a r t i c l e
i n f o
Article history: Received 14 January 2015 Received in revised form 10 March 2015 Accepted 11 March 2015 Available online 23 April 2015 Keywords: Superjunction MOSFET Bottom Assist Layer Breakdown voltage On-state resistance
a b s t r a c t A new Semi-Superjunction Vertical Double-diffusion MOSFET (Semi-SJ MOSFET) with P-type Bottom Assist Layer (P-BAL) is proposed. The P-BAL is introduced upon the N-Stop substrate and embedded in the n-BAL, on the one hand, another reverse biasing PN junction JP-BAL/N-Stop at the bottom substrate is formed, which helps to deplete the n-BAL along the longitudinal direction and changes the shapes of the electric field distribution. On the other hand, another Semi-SJ structure is formed by the P-BAL and nBAL, the transverse electric field in n-BAL part is enhanced, and this indicates that the trade-off between breakdown voltage (BV) and on-state resistance (Ron,sp) has been optimized due to the improvement of BV. As results show, in the proposed MOSFET, the BV (i.e., 1097 V) is improved 25% higher than that of the conventional Semi-SJ structure (i.e., 877 V) and the proposed does negligible disadvantage to the Ron,sp (i.e., the proposed with a Ron,sp of 59 mX cm2 and the conventional MOSFET with a Ron,sp of 56 mX cm2). Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The power device is widely used in industry application such as motor driver, inverter and high frequency switch [1,2]. The superjunction MOSFET has become a dominate device in the low-medium ⇑ Corresponding author. Tel.: +86 28 83207790. E-mail address: [email protected] (W. Chen). http://dx.doi.org/10.1016/j.spmi.2015.03.065 0749-6036/Ó 2015 Elsevier Ltd. All rights reserved.
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voltage application field, particularly in the voltage range of less than 900 V. The difficulty of the process to form the narrow and highly doped P/N Pillars and the hardness of the reverse-recovery of the body-diode are two key issues for the SJ MOSFET [3,4]. Then the Semi-SJ concept with an n-type Bottom Assist Layer (n-BAL) which is connected in series to the bottom of the SJ structure is proposed to overcome the issues [5–8]. From the aspect of structure, the Semi-SJ is composed of SJ part and nBAL part, thus the breakdown voltage BV for the Semi-SJ is the sum of the BVSJ and the BVn-BAL, the onstate resistance Ron,sp for the Semi-SJ is the sum of the RSJ and the Rn-BAL. As well known, the relationship between Rn-BAL and BVn-BAL for the n-BAL part which is formed by epitaxial process is still restrained by the Si limit. So the trade-off between Rn-BAL and BVn-BAL needs further optimized [9]. In this paper, by adopting the numerical simulation and based on the Semi-SJ MOSFET concept, another P-type Bottom Assist Layer (P-BAL) is introduced upon the N-Stop substrate and embedded in the n-BAL to breaks the trade-off between Ron,sp and BV of the Semi-SJ MOSFET .
2. Device structures and mechanism Fig. 1 shows the structures and electric distribution for comparison between the conventional Semi-SJ MOSFET and the proposed Semi-SJ MOSFET with a P-type Bottom Assist Layer (P-BAL). From Fig. 1(a), the Semi-SJ MOSFET is composed of SJ part and n-BAL part which is formed by epitaxial process and it is in series connected to the bottom of the SJ structure. So the breakdown voltage BV for the Semi-SJ is the sum of the BVSJ and the BVn-BAL, the on-state resistance Ron,sp for the Semi-SJ is the sum of the RSJ and the Rn-BAL. From Fig. 1(b), the proposed Semi-SJ MOSFET has the same structure except the P-BAL which is introduced upon the N-Stop substrate and embedded in the n-BAL. as the same as the conventional Semi-SJ MOSFET, the breakdown voltage BV for the proposed Semi-SJ is also the sum of the BVSJ and the BVn-BAL, and the on-state resistance Ron,sp is also the sum of the RSJ and the Rn-BAL.
2.1. The breakdown property (BV) When the Semi-SJ MOSFET is at the breakdown state, the gate and the source are zero biased; the drain is applied with high voltage. The breakdown voltage BV is determined by the electric field. As shown in Fig. 1(a) for the conventional Semi-SJ MOSFET, the electric field distribution of SJ part is
Fig. 1. Cross-sectional structures and electric field distributions for (a) Semi-SJ MOSFET (b) Semi-SJ MOSFET with P-BAL.
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rectangle, and the shape of electric field for n-BAL part is trapezoid, so the sustain voltage BV = BVSJ + BVn-BAL of the semi-SJ MOSFET can be expressed as:
BVSemi-SJ ¼ EV T SJ þ EV T n-BAL
qN n-BAL T 2n-BAL 2e
ð1Þ
where BVSJ and BVn-BAL are the sustain voltage of SJ and n-BAL part, respectively. EV is vertical peak electric field, TSJ and Tn-BAL are the thickness for SJ and n-BAL part, respectively, q is an electron charge, e is dielectric constant, Nn-BAL is the doping concentration of n-BAL part. In Fig. 1(b) for the proposed Semi-SJ MOSFET, as the introduction of the P-BAL, another reverse pffiffiffi biasing PN junction JP-BAL/N-Stop at the bottom is formed. A new critical electric field EC = 2EV [9] will occur at JP-BAL/N-Stop. It helps to deplete the n-BAL along the longitudinal direction and changes the shapes of the electric field distribution. Then assuming the EC equals to EV for simplicity, the sustain voltage BV = BVSJ + BVn-BAL + BVP-BAL for the semi-SJ MOSFET with P-BAL is given as:
BVSemi-SJðP-BALÞ ¼ EV T SJ þ EV T n-BAL þ
qNn-BAL T n-BAL ðT P-BAL T n-BAL Þ 2e
ð2Þ
where BVP-BAL and TP-BAL are the sustain voltage and length of P-BAL, respectively. From Eqs. (1) and (2) we can see that the BV is improved by the introduction of P-BAL. 2.2. The on-state resistance property (Ron,sp) When the Semi-SJ MOSFET is at the on-state of conduction, the source is zero biased, the gate and the drain are positive biased. As shown in Fig. 2(a) for the conventional Semi-SJ MOSFET, The electron will be emitted from the N+ source and vertically flow to the SJ part and n-BAL part through the electron channel. So the on-state resistance Ron,sp is determined by the SJ part and the n-BAL part and can be expressed as:
RSemi-SJ ¼ RSJ þ Rn-BAL ¼
T SJ þ qn-BAL ðT T SJ Þ qln N N-Pillar
ð3Þ
where RSJ and Rn-BAL are the resistance of SJ and n-BAL part, qn-BAL is the resistive rate of n-BAL.
Fig. 2. Cross-sectional structures and resistance distributions for (a) Semi-SJ MOSFET (b) Semi-SJ MOSFET with P-BAL.
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In Fig. 2(b) for the proposed Semi-SJ MOSFET, as the introduction of the P-BAL, the electron in the n-BAL part will bypass through the P-BAL, and another resistance between the P-BAL and the P-Pillar RJFET is introduced, so the on-state resistance Ron,sp is expressed as:
RSemi-SJðP-BALÞ ¼ RSJ þ Rn-BAL þ RJFET
T SJ q ðT SJ þ T d Þ þ qn-BAL ðT T SJ T P-BAL Þ þ n-BAL qln NN-Pillar Z L2M T d
ð4Þ
where RSJ and Rn-BAL are the resistance of SJ and n-BAL part, RJFET is the resistance between the P-BAL and the P-Pillar, qn-BAL is the resistive rate of n-BAL, Z is a constant, Td is the depletion layer extension bellow the P-Pillar. 3. Results and discussion From Figs. 1 and 2, the both MOSFETs have the same structure parameters except for the P-BAL. The main parameters of the MOSFETs are listed in Table 1. Moreover, the carrier lifetime for the electron and the ambient temperature are set as 10 ls and 300 K respectively. 3.1. The electric characteristics of BV Fig. 3 shows the numerical simulated breakdown property and equipotential contours for the conventional and proposed Semi-SJ MOSFET. In Fig. 3(a), the BV of proposed MOSFET is increased as the length TP-BAL increasing. When the TP-BAL = 25 lm, the BV (i.e., 1097 V) is improved 25% higher than that of the conventional Semi-SJ structure (i.e., 877 V). In Fig. 3(b), the equipotential contours are sparse for the conventional Semi-SJ at the n-BAL part. In Fig. 3(c), the equipotential contours in nBAL part are distributed uniformly due to the introduction of the P-BAL. Fig. 4 shows the 3-D electric field distribution under breakdown state for the conventional and proposed Semi-SJ MOSFET. It can be seen that the electric field distribution is almost the same and exhibits cube shape at the SJ part for both MOSFETs, but the electric field distribution is much different at n-BAL part. For the conventional Semi-SJ MOSFET, in order to obtain low Ron,sp, the concentration of the n-BAL part is set at much higher than the N-Pillar as show in Table 1, then the electric field slope is much steeper and the electric field will terminate at the n-BAL area. For the proposed Semi-SJ MOSFET due to the introduction of P-BAL, the electric field can be divided into n-BAL part and PBAL part, at the n-BAL part the electric field slope descend abruptly as the conventional MOSFET, but at the P-BAL part, the electric field slope ascend abruptly and another electric field peak value is appeared at the junction JP-BAL/N-Stop. Fig. 5 shows the 2-D vertical and lateral electric-field distributions under breakdown voltage. In Fig. 5(a), for the conventional Semi-SJ, the electric field gradient along the line C1C2 (as shown in
Table 1 Device specifications. Parameter
Proposed
Semi-Sj VDMOS
MOS cell size, LM Wafer thickness, T SJ thickness, TSJ SJ doping, Nsj (N-Pillar) SJ width, LN-column (N-Pillar) SJ doping, Nsj (P-Pillar) SJ width, LP-column (P-Pillar) n-BAL thickness, Tn-BAL n-BAL doping, Nn-BAL N-Stop thickness, TN-Stop N-Stop doping, NN-Stop P-BAL length, TP-BAL P-BAL doping, NP-BAL P-BAL width, WP-BAL
17 lm 75 lm 35 lm 3.3 1015cm3 11 lm 6.6 1015cm3 6 lm 35 lm 3.7 1014 cm3 5 lm 1 1017 cm3 25 lm 6.6 1015 cm3 2 lm
17 lm 75 lm 35 lm 3.3 1015 cm3 11 lm 6.6 1015 cm3 6 lm 35 lm 3.7 1014 cm3 5 lm 1 1017 cm3 – – –
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Fig. 3. The breakdown property and equipotential contours for the Semi-SJ MOSFETS (a) breakdown state I–V curves (b) equipotential contours for the conventional Semi-SJ MOSFET (c) equipotential contours for the Semi-SJ MOSFET with P-BAL (TP-BAL = 25 lm).
Fig. 4. The 3-D electric field distributions under breakdown voltage for (a) the conventional Semi-SJ and (b) proposed Semi-SJ MOSFET with P-BAL.
Fig. 1) in n-BAL part is determined by the doping Nn-BAL of the n-BAL. with the increasing of Nn-BAL to realize lower Rn-BAL, the electric field will terminated in the n-BAL part, the shape of the electric field changes from trapezoid to triangle, thus the breakdown voltage will decrease. For the proposed SemiSJ due to the introduction of the P-BAL, another reverse biasing PN junction JP-BAL/N-Stop at the bottom is formed as shown in Fig. 1(b), a new critical electric field will occur at the JP-BAL/N-Stop. As the high doping concentration of the N-Stop substrate, the electric field in the N-Stop substrate will be terminated abruptly, and the depletion layer will reverse expand toward to the SJ part along the vertical direction, thus the electric field distribution is changed. Moreover, as the increase of the length TP-BAL of the P-
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Fig. 5. The electric field distributions under breakdown voltage for the conventional Semi-SJ and proposed Semi-SJ with P-BAL. (a) vertical electric field along the line C1C2 as shown in Fig. 1 (b) lateral electric field along the line B1B2 as shown in Fig. 1.
BAL, the maximum electric field will increase and further reverse expand to the SJ part. In Fig. 5(b), the lateral electric field along the line B1B2 (as shown in Fig. 1) is enhanced because another Semi-SJ structure is formed by the P-BAL and n-BAL. As the n-BAL is negative charged, according to the SJ theory [9], with the increase of the TP-BAL of the P-BAL, more positive charge will be introduced and more negative charge will be compensated by the P-BAL. The electric field will be further enhanced and this indicates that the trade-off of BV and Ron,sp has been optimized. Fig. 6 shows doping NP-BAL of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ MOSFET. It can be seen that as the increase of the NP-BAL of the P-BAL, more positive charge will be introduced and more negative charge will be compensated by the P-BAL, thus the electric field in the n-BAL part will be enhanced. But at the same time, with the increase of the NP-BAL, the electric field in the SJ part will decrease due to the shielding effect of the JFET formed between the P-BAL and the P-Pillar. Fig. 7 shows width WP-BAL of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ MOSFET. It can be seen that as the WP-BAL increases from 0 lm to 3 lm, the electric field in the n-BAL part is enhanced. But when the WP-BAL equals to 5 lm, the
Fig. 6. The doping of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ with P-BAL.
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Fig. 7. The width of the P-BAL influences on the vertical electric field along the line C1C2 (as shown in Fig. 1) for the proposed Semi-SJ with P-BAL.
Fig. 8. The comparison of forward conduction between the conventional Semi-SJ MOSFET and the proposed Semi-SJ MOSFET with P-BAL, the inset picture is the current flow lines for the conventional and proposed Semi-SJ with P-BAL (width = 3 lm).
space between the P-BAL and the P-Pillar is decreased and the JFET effect will be obvious, then the electric field in the SJ part will decrease abruptly due to the shielding effect of the JFET, so the WPBAL is a sensitive parameter to design the proposed Semi-SJ.
3.2. The electric characteristics of Ron,sp Fig. 8 shows the forward conduction property and current flow lines for the conventional and proposed Semi-SJ MOSFET. For the conventional Semi-SJ MOSFET at the on-state of conduction, as the source is zero biased, the gate is positive biased, the electron channel is formed and the electron will be emitted from the N+ source, at the same time, the drain is also positive biased and the electron will vertically flow to the SJ part and n-BAL part. For the proposed Semi-SJ MOSFET, as the introduction of the P-BAL, the electron in the n-BAL part will bypass through the P-BAL as shown in the inset picture,
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Fig. 9. The doping of the P-BAL influences on the BV and Ron,sp.
Fig. 10. The width of the P-BAL influences on the BV and Ron,sp.
and another resistance between the P-BAL and the P-Pillar RJFET is introduced, so with the increase of the WP-BAL, especially when WP-BAL = 5 lm, the Ron,sp increases abruptly. 3.3. The trade-off property between the BV and Ron,sp Fig. 9 shows the doping NP-BAL of the P-BAL influencing on BV and Ron,sp, for the proposed Semi-SJ. It is clear that the NP-BAL does little effect to the Ron,sp. But the BV firstly increases gradually and then decreases abruptly with the NP-BAL, the reason is that by the introduction of the P-BAL, the charge balance in the n-BAL part can be optimized by modulating the QP-BAL and the Qn-BAL, according to the SJ theory, when at condition of QP-BAL Qn-BAL, the BV will get maximum value at the achievement of the balance of the QP-BAL and Qn-BAL. Fig. 10 shows the doping WP-BAL of the P-BAL influencing on BV and Ron,sp, for the proposed Semi-SJ. It is clear that the WP-BAL does little effect to the BV and Ron,sp. But the BV decreases and the Ron,sp increases abruptly when the WP-BAL is more than 3 lm due to the JFET effect between the P-Pillar and the P-BAL. From the aspect of the Nn-BAL, the Ron,sp decreases with the increasing of Nn-BAL for both
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Fig. 11. Trade-off characteristics between on-resistance and breakdown voltage for the different MOSFETs.
Semi-SJ MOSFET. Differently, the BV decreases obviously with the increasing of Nn-BAL for the conventional Semi-SJ due to the electric field can no punch though the n-BAL part, but for the proposed, by the introduction of the P-BAL, the charge balance in the n-BAL part can be optimized by modulating the QP-BAL and the Qn-BAL, thus the BV increases gradually with the increasing of Nn-BAL, it also will get maximum value at the achievement of the balance of the QP-BAL and Qn-BAL. Fig. 11 shows the Ron,sp and BV trade-off characteristic for the demonstrated MOSFETs. In reference [5,7] it shows that the Semi-SJ achieved better trade-off property than the referenced SJ MOSFET when the SJ MOSFET is scaled with different breakdown voltage maintaining the aspect ratio, the Ron,sp is proportional to BV2.5. For the proposed MOSFET, based on the Semi-SJ, the P-BAL improves the BV greatly with a little loss of Ron,sp, so the trade-off relationship is enhanced obviously. At the same condition of TP-BAL, by increasing the Nn-BAL, an optimum point for the trade-off can be achieved. 4. Conclusions The novel Semi-SJ MOSFET with P-BAL which is introduced in the n-BAL part is proposed, The PN junction JP-BAL/N-Stop is formed to reverse depletes the n-BAL along the longitudinal direction. Another SJ part is formed between the P-BAL and n-BAL, which can deplete the n-BAL along the transverse direction at breakdown state, thus the electric field is optimized and the BV is increased from 877 V to 1097 V, so 25% higher BV is achieved with a little loss of Ron,sp, ultimately the trade-off characteristic between BV and Ron,sp has been improved. Acknowledgments This work was supported by the National Science and Technology Major Project (Grant no. 2011ZX02504-003), the National Natural Science Foundation of China (Grant no. 61076082), and the Fundamental Research Funds for the Central Universities (Grant no. ZYGX2011J024). References [1] W.Z. Chen, Z.H. Li, B. Zhang, M. Ren, Y. liu, Z.J. Li, A Snapback suppressed reverse-conducting IGBT with soft reverse recovery characteristic, Superlattices Microstruct. 61 (2013) 59–68. [2] W.Z. Chen, Z.H. Li, Y. liu, B. Zhang, P.F. Liao, Z.J. Li, Enhancing the robustness of the equipotential ring of edge termination for 4.5 KV IGBT by introducing a partial N layer, Superlattices Microstruct. 65 (2014) 124–133. [3] G. Deboy, M. März, J.-P. Stengl, H. Strack, J. Tihanyi, H. Weber, A new generation of high voltage MOSFET’s breaks the limit line of silicon, IEDM Tech. Dig. (1998) 683–685.
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