Vol. 36, No. 3
Journal of Semiconductors
March 2015
Backside optimization for improving avalanche breakdown behavior of 4.5 kV
IGBT
Tian Xiaoli(田晓丽)1; 2 , Lu Jiang(陆江)1 , Teng Yuan(滕渊)1 , Zhang Wenliang(张文亮)1 ,
Lu Shuojin(卢烁今)1; 2 , and Zhu Yangjun(朱阳军)1; 2; Ž
1 Institute
of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China
Lianxing Electronics Co., Ltd, Shanghai 201203, China
2 Shanghai
Abstract: The static avalanche breakdown behavior of 4.5 kV high-voltage IGBT is studied by theory analysis
and experiment. The avalanche breakdown behaviors of the 4.5 kV IGBTs with different backside structures are
investigated and compared by using the curve tracer. The results show that the snap back behavior of the breakdown
waveform is related to the bipolar PNP gain, which leads to the deterioration of the breakdown voltage. There are
two ways to optimize the backside structure, one is increasing the implant dose of the NC buffer layer, the other
is decreasing the implant dose of the PC collector layer. It is found that the optimized structure is effective in
suppressing the snap back behavior and improving the breakdown characteristic of high voltage IGBT.
Key words: avalanche breakdown; snap back; bipolar transistor gain; high voltage IGBT
DOI: 10.1088/1674-4926/36/3/034008
EEACC: 2560
two layers are mainly responsible for the avalanche breakdown
behavior of the high voltage IGBT.
1. Introduction
The insulated gate bipolar transistor (IGBT) is an important power device, which has been widely used in locomotive
traction, industrial control, automotive electronics and consumer electronicsŒ1 7 . IGBTs are the key components in such
power systems, they must have high avalanche ruggedness and
a large safe operating area to withstand all possible extreme
conditions that may occur during their lifetime. Generally, the
breakdown voltage of high voltage IGBT is determined by the
device structure and process technology. The backside structure of high voltage IGBT is one of the main factors affecting the device’s performanceŒ8 10 . This paper investigates the
static avalanche breakdown behavior of the homemade 4.5 kV
high voltage IGBT with different backside structures. The different backside structures are fabricated by various implant
processes. The static avalanche breakdown characteristic is
verified by a curve tracer and the physics mechanism of failure
is analyzed.
2. Device structure and experimental setup
2.1. Device structure
The typical structure for a high voltage IGBT and its equivalent circuit are illustrated in Figure 1. The IGBT structure consists of four (N–P–N–P) alternating semiconductor layers, and
it forms the coupled PNP and NPN transistors. There is an NC
buffer layer within the N-drift region. This layer is referred to
as the field-stop layer. It can sustain the high forward-blocking
voltage by a thinner N-drift layer resulting in reducing the onstate voltage drop. Typically, the NC buffer layer and PC collector layer are designed by fabrication technology and these
2.2. Experimental setup
The 4.5 kV IGBTs are designed and fabricated on the domestic process platform. In order to achieve higher performance, the trade-off is required by better device structure optimizationŒ6 . In this paper, different backside device structures
are fabricated for comparison. Different dose concentrations of
both the PC collector layer and the NC buffer layer are used to
form different backside structures.
The QT2 curve tracer is used to investigate the breakdown
waveform. The avalanche breakdown behavior of 4.5 kV IGBT
is analyzed under the standard test condition of breakdown
voltage, in which the gate and emitter are shorted to the ground.
3. Results and discussion
As shown in Figure 2, in the static blocking state, the electrical field in the N-drift region is stopped by the NC buffer
layer. The electrons generated in the space charge region are
swept to the collector of the device, and the holes are injected
from the PC collector layer. In the IGBT structure, the PNP
bipolar transistor is formed by the P-body, N-drift/NC buffer
and PC collector. The static breakdown behavior of IGBT is
influenced by the bipolar gain of the internal PNP transistor.
The key factor to optimize the backside structure is to select an
appropriate bipolar gain.
The bipolar transistor gain ˇpnp can be given byŒ8 :
ˇpnp D
DpB LnC NA n2ieB
;
DnC WB ND n2ieC
(1)
* Project supported by the National Major Science and Technology Special Project of China (No. 2011ZX02503-003).
† Corresponding author. Email: zhuyangjun@ime.ac.cn
Received 28 August 2014, revised manuscript received 11 September 2014
© 2015 Chinese Institute of Electronics
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Tian Xiaoli et al.
Figure 1. Structure and equivalent circuit of IGBT.
Figure 2. Cross-section of carrier distribution of high voltage IGBT in
the static blocking state.
Table 1. Process technology comparison of original structure and optimization structure.
Implant dose of Implant dose of
Parameter
PC collector
NC buffer
Original structure
D
E
Optimization structure A 0.3D
E
Optimization structure B D
1.5E
where DpB and DnC are the minority carrier diffusion coefficients in the un-depleted buffer and collector region respectively. LnC is the diffusion length of electrons in the collector.
WB depends on the un-depleted base width in the NC buffer
layer, which is influenced by the implant dose of the NC buffer
layer. The ND and NA represent the background doping of the
un-depleted buffer layer and the collector region respectively.
The nieB and nieC are the intrinsic carrier concentrations in the
base and collector. According to Equation (1), the bipolar gain
can be optimized by changing the dose concentrations of the
PC collector layer and the NC buffer layer. In our actual technological process, the implant dose of the PC collector is decreased to one third of the original dose and the NC buffer layer
implant dose is increased to 1.5 times of the original dose, as
shown in Table 1.
The comparison results between the original backside
structure and the optimized design are illustrated in Figure 3. In
the waveform of the original backside structure, the snap back
behavior is emerging after the device reaches the avalanche
Figure 3. The breakdown waveform results of the 4.5 kV IGBT with
different backside designs. (a) Original backside structure. (b) The
optimized backside structure.
breakdown condition. Several chips with this original backside
condition are tested by a QT2 curve tracer. Most of the devices
have this snap back behavior. In this situation, the avalanche
breakdown curve is not so stable and the device is damaged
easily. In contrast, the avalanche breakdown curve of the optimized device shows notable change. As mentioned before, two
adjusting ways are used to compare the influence of backside
structures. It can be seen from the waveforms that either of the
two ways can improve the avalanche breakdown characteristic.
The final breakdown voltage reaches to 5.3 kV, which satisfies
the rated breakdown voltage requirement.
The experimental results show that the adjustment of back-
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Tian Xiaoli et al.
side structure is effective to optimize the high voltage IGBT
avalanche breakdown behavior. One way is to increase the
NC buffer layer carrier concentration, which enlarges the undepleted buffer layer WB . Another way is to decrease the PC
layer carrier concentration, which weakens the injection efficiency of holes. According to the bipolar gain expression, these
adjusting methods minimize the bipolar gain ˇpnp . Actually,
the static avalanche breakdown behavior of the IGBT is influenced by the PNP transistor inside the device, and the bipolar
PNP gain plays an important role in the avalanche breakdown
behavior.
In the ideal situation, the breakdown behavior of the PNP
transistor inside the IGBT is determined by the open-emitter
breakdown voltage (BVCBO /, which is mainly determined by
the doping concentration of the P-body/N-drift and the thickness of the lightly doped portion of the P-body. However, the
actual maximum blocking voltage capability for the power
bipolar transistor is decided by the open-base breakdown voltage (BVCEO /. The relationship of the BVCEO and BVCBO can
be given byŒ11 :
BVCEO
1
D
;
BVCBO
Œ1 C ˇpnp .0/1=n
n D 6:
(2)
When the high voltage IGBT is in the avalanche breakdown mode, the holes are injected from the PC collector region.
If the un-depleted NC buffer layer width WB is small or the
PC injection efficiency is large, the bipolar gain ˇpnp becomes
too large. From Equation (2), the open-base breakdown voltage (BVCEO / will decrease severely. It will cause the injection
current from the collector to increase quickly and the breakdown behavior display a negative resistance phenomenon, like
the snap back waveform in the curve tracer.
From the experimental comparison of the original structure
and the optimized structure, the static avalanche breakdown of
high voltage IGBT is severely influenced by the backside structure, which is in good agreement with the theory analysis. In
order to get an appropriate structure, the implant doses of the
NC buffer layer and the PC collector layer need to be carefully selected. To suppress the snap back behavior during the
static avalanche breakdown mode, the bipolar gain ˇpnp needs
to be adjusted so it is as small as possible; it becomes a major concern in the device structure design and the application
requirement. However, there is a trade-off between the static
avalanche breakdown voltage and the on-state voltage drop. To
achieve the best breakdown performance and suitable electrical performance requires better device structure optimization
and proper manufacture process selection.
4. Conclusion
In this paper, the static avalanche breakdown behavior of
a homemade 4.5 kV high voltage IGBT is studied. The results
show that the bipolar gain ˇpnp of the internal PNP transistor is
the main factor that affects the avalanche behavior of the high
voltage IGBT. The gain ˇpnp can be decreased by increasing
the implant dose of the NC buffer layer and decreasing the implant dose of the backside PC collector layer. The snap back
phenomenon of breakdown voltage is eliminated by the optimized backside structure. It is in good agreement with the experiment and the theory analysis.
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