Novel trench gate field stop IGBT with trench shorted anode

Xudong Chen; Jianbing Cheng; Guobing Teng; Houdong Guo

doi:10.1088/1674-4926/37/5/054008

J. Semicond. > 2016, Volume 37 > Issue 5 > 054008

SEMICONDUCTOR DEVICES

Novel trench gate field stop IGBT with trench shorted anode

Xudong Chen^1,, Jianbing Cheng^{1, 2}, Guobing Teng¹ and Houdong Guo¹

+ Author Affiliations

Corresponding author: Corresponding author. Email: xdcnjupt@163.com

DOI: 10.1088/1674-4926/37/5/054008

Abstract: A novel trench field stop (FS) insulated gate bipolar transistor (IGBT) with a trench shorted anode (TSA) is proposed. By introducing a trench shorted anode, the TSA-FS-IGBT can obviously improve the breakdown voltage. As the simulation results show, the breakdown voltage is improved by a factor of 19.5% with a lower leakage current compared with the conventional FS-IGBT. The turn off time of the proposed structure is 50% lower than the conventional one with less than 9% voltage drop increased at a current density of 150 A/cm². Additionally, there is no snapback observed. As a result, the TSA-FS-IGBT has a better trade-off relationship between the turn off loss and forward drop.

Key words: FS-IGBT, trench shorted anode, breakdown voltage, turn off loss, tradeoff

1. Introduction

Insulated gate bipolar transistors (IGBTs) have been widely used to power electronic applications such as motor control and electrical drive circuits^{[1, 2]}. The major target of IGBT design is to improve the breakdown voltage and realize a better trade-off between forward drop and turn off loss. Great efforts have been made to optimize these performances of IGBTs. Field stop (FS) and trench gate structure have been used to significantly improve performances of IGBT^{[3, 4]}. To further lower the switching speed, various concepts including shorted anode^{[5, 6]}, striped anode^[7], segmented anode^[8], injection efficiency controlled IGBT^{[2, 9]}, dual-gates structure^{[10, 11, 12]}, n-region controlled^[13] and super junction IGBT^{[14, 15]} have also been introduced.

This paper proposes a novel trench shorted anode FS IGBT (TSA-FS-IGBT) structure. Extensive numerical simulations are carried out to analyze performances of the new structure. Comparison results between the TSA-FS-IGBT and conventional FS-IGBT are also discussed.

2. Device Structure and operation

A schematic cross section of the proposed TSA-FS-IGBT and a conventional FS-IGBT is shown in Figure 1. The major difference between the two devices is that the TSA-FS-IGBT has a trench shorted anode. The new anode design modulates the electric field in the drift region. Consequently, the breakdown voltage is improved compared with the conventional device.

Figure 1. Schematic cross-section of (a) the proposed TSA-FS-IGBT and (b) conventional FS-IGBT.

DownLoad: Full-Size Img PowerPoint

When the gate applied voltage is higher than the threshold voltage, the TSA-FS-IGBT turns on. If the anode bias voltage is small, it operates in unipolar mode because the n-drift is shorted to the anode. When the channel current is large enough, the PN junction (P $^{+}$ anode/shorted N-drift) is forward biased, holes start to inject into the drift region. Then the TSA-FS-IGBT operates into bipolar mode. During turn off, electrons are extracted faster than the conventional FS-IGBT due to the shorted electron extraction path.

3. Simulation results and discussions

The device parameters used in the simulation are listed in . Physical models used include the concentration dependent model, the perpendicular electric field model, the band gap narrowing model, the auger combination model, the Shockley-Read-Hall recombination model and the impact ionization model. The excess carrier lifetime is set to 1.2 $\mu$ s ( $\tau_{\rm n}$ $=$ 2 $\tau_{\rm p})$ for both structures.

Table 1. Device specification.

DownLoad: CSV | Show Table

Figure 2 shows the forward blocking characteristic of the TSA-FS-IGBT and conventional FS-IGBT. The two devices are designed to realize the blocking class of 1.2 kV. Figure 2 shows the breakdown voltage for the conventional FS-IGBT is 1316 V while the TSA-FS-IGBT is 1573 V, which is 257 V higher. In addition, the TSA-FS-IGBT has a lower leakage current because the introduction of the trench anode reduces the anode injection efficiency and the transport factor of the PNP transistor.

Figure 2. Blocking characteristics of different structures.

$T_{2}$

$=$ 0.7

$\mu$ m,

$L_{2}$

$=$ 2

$\mu$ m and

$T_{1}$

$=$ 5

$\mu$ m for the TSA-FS-IGBT.

DownLoad: Full-Size Img PowerPoint

shows the lateral electric field and the vertical electric field distribution at cut line BB' and AA'. The lateral electric field of the TSA-FS-IGBT is obviously higher than the conventional one from . An electric field peak appears at 2.5 $\mu$ m, because the electric field is concentrated at the trench oxide corner. Figure 3(b) shows the vertical electric field of the proposed structure is a little higher than that of the conventional FS-IGBT. An enhanced electron accumulation layer is formed around the anode trench oxide when the forward blocking voltage is biased at the shorted anode. Firstly, the electron accumulation layer stops the electric field just like the FS layer. Secondly, the electron accumulation layer around the trench oxide helps to compensate partial positive charges in the fully depleted FS layer^[16]. Thus, the electric field can be modulated to a higher level as seen from Figure 3. Therefore, the TSA-FS-IGBT can sustain a higher blocking voltage.

Figure 3. (a) Lateral electric field along BB' and (b) vertical electric field along AA' of TSA-FS-IGBT (T1 = 0.7 μm, L2 = 2 μm and T1 = 5 μm) and conventional FS-IGBT.

DownLoad: Full-Size Img PowerPoint

The $I$ - $V$ characteristic of the TSA-FS-IGBT is strongly dependent on the parameter $L_{2}$ and $T_{2}$ . The minimum voltage for generating the hole injection across the shorted drift region $V_{\rm sd}$ is expressed as:

${{\text{V}}_{{\text{sd}}}}{\text{ = }}\frac{{{{\text{I}}_{{\text{ch}}}}{{\text{L}}_{\text{2}}}}}{{{\text{q}}{{\text{N}}_{\text{d}}}{\mu _{\text{n}}}{{\text{T}}_{\text{2}}}{\text{w}}}},$

(1)

where

$I_{\rm ch}$ is the channel current,

$q$ is the electron charge,

$N_{\rm d}$ is the drift region doping concentration, and

$\mu_{\rm n}$ is the electron mobility. And

$w$ equals to 1

$\mu$ m, which is the device width in the third dimension. As for the TSA-FS-IGBT, because

$N_{\rm d}$ is much lower than the FS layer, the biased anode voltage during forward conduction mainly drops on the shorted drift region^[15]. Therefore, the PN junction (P

$^{+}$ anode/N

$^{-}$ drift) is forward biased under relatively low anode voltage leading to the conductivity modulation. As a result, snapback can be effectively suppressed.

shows the $I$ - $V$ characteristics of TSA-FS-IGBT and conventional FS-IGBT. There is no snapback observed from . Forward drop of TSA-FS-IGBT is a little higher (less than 0.13 V) because the existence of the shorted drift region reduces the anode injection efficiency. As $T_{2}$ increases, the injection efficiency decreases, leading to a higher voltage drop. The principle is similar to the variation of $T_{2}$ when $L_{2}$ varies according to Equation (1).

Figure 4. \caption{

$I$ -

$V$ characteristics of the TSA-FS-IGBT by varying

$T_{2}\, (L_{2}$

$=$ 3

$\mu$ m and

$T_{1}$

$=$ 5

$\mu$ m) and conventional FS-IGBT.

DownLoad: Full-Size Img PowerPoint

Figure 5 shows the current flow lines of the lower part of the TSA-FS-IGBT and conventional FS-IGBT in the forward conduction mode. There is no current non-uniformity in the TSA-FS-IGBT cell as seen from Figure 5. Figure 5 also shows there are less current flows at the very left part above the trench oxide of the TSA-FS-IGBT, because the electron current tends to choose the path that has smaller resistance.

Figure 5. Current flow lines of different structure in conduction mode. (a) Conventional FS-IGBT and (b) proposed with

$T_{1}$

$=$ 5

$\mu$ m,

$T_{2}$

$=$ 0.9

$\mu$ m and

$L_{2}$

$=$ 3

$\mu$ m.

DownLoad: Full-Size Img PowerPoint

shows the turn off times of the TSA-FS-IGBT with different $T_{2}$ . As seen from , the minimum and maximum turn off\, time of the TSA-FS-IGBT is 91.7 ns and 120 ns when $T_{2}$ varies, however, the turn off time of the conventional FS-IGBT is 184.6 ns. This can be explained as follows. During turn off, electrons can be extracted very fast by the path offered by the shorted anode. And holes can be extracted by the cathode in the TSA-FS-IGBT. However, removal of excess carriers in the conventional FS-IGBT mainly relies on recombination.

Figure 6. Turn off times of the TSA-FS-IGBT (

$L_{1}$

$=$ 3.5

$\mu$ m, and

$T_{1}$

$=$ 5

$\mu$ m) and the conventional FS-IGBT. The anode doping is 5 × 10

$^{18}$ cm

$^{-3}$ .

DownLoad: Full-Size Img PowerPoint

shows the trade-off relationships between the turn off loss and forward drop by changing the P $^{+}$ anode doping for both two structures. It can be seen that the trade off line of \, the TSA-FS-IGBT lies lower than the conventional FS-IGBT.

Figure 7. Forward drop and turn off loss trade-off curve of different structures by varying P

$^{+}$ doping. Simulated current density is 150 A/cm

$^{2}$ under resistive load and gate voltage is from 15 to 0 V.

DownLoad: Full-Size Img PowerPoint

4. Conclusion

A new TSA-FS-IGBT with a trench shorted anode is introduced in this paper. The blocking voltage of the TSA-FS-IGBT is 257 V higher than its conventional counterpart. Whereas the forward drop of the TSA-FS-IGBT is a little higher than that of a conventional FS-IGBT, there is no snapback observed. The turn off loss and forward drop trade-off relationship of the TSA-FS-IGBT is better than its conventional counterpart. As a result, the TSA-FS-IGBT is a novel device structure with enhanced performance.

References

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[16]

Fig. 1. Schematic cross-section of (a) the proposed TSA-FS-IGBT and (b) conventional FS-IGBT.

DownLoad: Full-Size Img PowerPoint

Fig. 2. Blocking characteristics of different structures.

$T_{2}$

$=$ 0.7

$\mu$ m,

$L_{2}$

$=$ 2

$\mu$ m and

$T_{1}$

$=$ 5

$\mu$ m for the TSA-FS-IGBT.

DownLoad: Full-Size Img PowerPoint

Fig. 3. (a) Lateral electric field along BB' and (b) vertical electric field along AA' of TSA-FS-IGBT (T1 = 0.7 μm, L2 = 2 μm and T1 = 5 μm) and conventional FS-IGBT.

DownLoad: Full-Size Img PowerPoint

Fig. 4. \caption{

$I$ -

$V$ characteristics of the TSA-FS-IGBT by varying

$T_{2}\, (L_{2}$

$=$ 3

$\mu$ m and

$T_{1}$

$=$ 5

$\mu$ m) and conventional FS-IGBT.

DownLoad: Full-Size Img PowerPoint

Fig. 5. Current flow lines of different structure in conduction mode. (a) Conventional FS-IGBT and (b) proposed with

$T_{1}$

$=$ 5

$\mu$ m,

$T_{2}$

$=$ 0.9

$\mu$ m and

$L_{2}$

$=$ 3

$\mu$ m.

DownLoad: Full-Size Img PowerPoint

Fig. 6. Turn off times of the TSA-FS-IGBT (

$L_{1}$

$=$ 3.5

$\mu$ m, and

$T_{1}$

$=$ 5

$\mu$ m) and the conventional FS-IGBT. The anode doping is 5 × 10

$^{18}$ cm

$^{-3}$ .

DownLoad: Full-Size Img PowerPoint

Fig. 7. Forward drop and turn off loss trade-off curve of different structures by varying P

$^{+}$ doping. Simulated current density is 150 A/cm

$^{2}$ under resistive load and gate voltage is from 15 to 0 V.

DownLoad: Full-Size Img PowerPoint

Table 1. Device specification.

DownLoad: CSV

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[4]
[5]
[6]
[7]
[8]
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[10]
[11]
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[15]
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Xudong Chen, Jianbing Cheng, Guobing Teng, Houdong Guo. Novel trench gate field stop IGBT with trench shorted anode[J]. Journal of Semiconductors, 2016, 37(5): 054008. doi: 10.1088/1674-4926/37/5/054008

X D Chen, J B Cheng, G B Teng, H D Guo. Novel trench gate field stop IGBT with trench shorted anode[J]. J. Semicond., 2016, 37(5): 054008. doi: 10.1088/1674-4926/37/5/054008.

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Received: 15 August 2015 Revised: Online: Published: 01 May 2016

Article Navigation > Journal of Semiconductors > 2016 > 37(5): 054008

Xudong Chen, Jianbing Cheng, Guobing Teng, Houdong Guo. Novel trench gate field stop IGBT with trench shorted anode[J]. Journal of Semiconductors, 2016, 37(5): 054008. doi: 10.1088/1674-4926/37/5/054008 ****X D Chen, J B Cheng, G B Teng, H D Guo. Novel trench gate field stop IGBT with trench shorted anode[J]. J. Semicond., 2016, 37(5): 054008. doi: 10.1088/1674-4926/37/5/054008.

Citation:

X D Chen, J B Cheng, G B Teng, H D Guo. Novel trench gate field stop IGBT with trench shorted anode[J]. J. Semicond., 2016, 37(5): 054008. doi: 10.1088/1674-4926/37/5/054008.

Citation:

X D Chen, J B Cheng, G B Teng, H D Guo. Novel trench gate field stop IGBT with trench shorted anode[J]. J. Semicond., 2016, 37(5): 054008. doi: 10.1088/1674-4926/37/5/054008.

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Novel trench gate field stop IGBT with trench shorted anode

DOI: 10.1088/1674-4926/37/5/054008

1.
College of Electronic Science and Engineering, Nanjing University of Posts and Telecommunications, Nanjing 210003, China
2.
National Application-Specific Integrated Circuit (ASIC) System Engineering Research Center, Southeast University, Nanjing 210096, China

Funds:

Project supported by the National Natural Science Foundation of China (No. 61274080) and the Postdoctoral Science Foundation of China (No. 2013M541585).

More Information

Corresponding author: Corresponding author. Email: xdcnjupt@163.com
Received Date: 2015-08-15
Accepted Date: 2015-09-25
Published Date: 2016-01-25

Abstract

Abstract

A novel trench field stop (FS) insulated gate bipolar transistor (IGBT) with a trench shorted anode (TSA) is proposed. By introducing a trench shorted anode, the TSA-FS-IGBT can obviously improve the breakdown voltage. As the simulation results show, the breakdown voltage is improved by a factor of 19.5% with a lower leakage current compared with the conventional FS-IGBT. The turn off time of the proposed structure is 50% lower than the conventional one with less than 9% voltage drop increased at a current density of 150 A/cm². Additionally, there is no snapback observed. As a result, the TSA-FS-IGBT has a better trade-off relationship between the turn off loss and forward drop.
- FS-IGBT,
- trench shorted anode,
- breakdown voltage,
- turn off loss,
- tradeoff

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