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Citation for the original published paper (version of record):
Salemi, A., Elahipanah, H., Zetterling, C-M., Östling, M. (2015)
Optimal Emitter Cell Geometry in High Power 4H-SiC BJTs.
IEEE Electron Device Letters, 36(10): 1069-1072
http://dx.doi.org/10.1109/LED.2015.2470558
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Abstract— Three 4H-SiC BJT designs with different emitter cell
geometries (linear interdigitated fingers, square cell geometry,
and hexagon cell geometry) are fabricated, analyzed, and
compared with respect to current gain, on-resistance, current
density, and temperature performance for the first time. Emitter
size effect and surface recombination are investigated. Due to a
better utilization of the base area, optimal emitter cell geometry
significantly increases the current density (J
C
) about 42% and
reduces the on-resistance (R
ON
) about 21% at a given current
gain, thus making the device more efficient for high-power and
high-temperature applications.
Index Terms—Power 4H-SiC BJTs, current density, current
gain, on-resistance, surface recombination.
I. INTRODUCTION
ILICON carbide bipolar junction transistors (BJTs) with
high current density (J
C
) and low on-resistance (R
ON
) are
attractive candidates for power switching due to the absence of
a critical gate oxide, fast switching speed, and low loss [1].
However, to completely compete with metal oxide
semiconductor field effect transistors (MOSFETs) and
insulated gate bipolar transistors (IGBTs), it is important to
improve the BJT characteristics. In recent years, there has
been considerable interest to improve the breakdown voltage,
maximum current gain (β) and J
C
, meanwhile, decreasing the
R
ON
[2]-[12]. The progress has been achieved by using
continuous epitaxial growth [9], optimized device geometry
[10], and improved surface passivation [11], [12]. All 4H-SiC
BJTs; hitherto, employ the linear interdigitated fingers to
diminish the current crowding at the edge of the emitter. These
geometries mostly have an emitter width (W
E
) up to 20 µm.
In this work, we report a significant improvement in the J
C
and R
ON
at a given β by utilizing optimal emitter cell
geometry. We compare the β, J
C
, and R
ON
in three different
SiC BJT geometrical designs with a stable open-base
breakdown voltage of 5.65 kV as reported in [4]. The first and
reference design uses linear interdigitated fingers, whereas the
second and third designs, to the authors’ knowledge, square
and hexagon cell geometries are investigated for the first time
This work was supported in part by the Project STANDUP and the
Swedish Energy Agency.
The authors are with KTH Royal Institute of Technology, School of ICT,
SE-164 40 Kista, Sweden (e-mail: salemi@kth.se).
Fig. 1. Schematic cross-sectional view of the fabricated 4H-SiC BJTs. (Inset)
Micrograph top views of the fabricated BJTs with a W
E
= 20 µm and different
cell geometries: (a) linear interdigitated fingers, (b) square cell geometry, (c)
hexagon cell geometry. (d) Schematic top view of one cell for different cell
geometries.
for BJTs, which opens a new design space for improved
performance. The emitter size effect, the effect of surface
recombination, and also the temperature dependency are
investigated for all designs.
II. DEVICE FABRICATION
Fig. 1 shows a cross-sectional view of the fabricated BJTs
with five epitaxial layers grown in one continuous run on a
100-mm 4° off-axis 4H-SiC substrate. The emitter epilayer is
1.6 µm nitrogen doped to 1 × 10
19
cm
-3
, capped by 400-nm-
thick 3 × 10
19
cm
-3
to obtain optimum injection efficiency and
low-resistive ohmic contact, respectively. The base layer is
760 nm aluminum doped to 2.5 × 10
17
cm
-3
. The drift layer is
44 µm thick and nitrogen doped to 1.75 × 10
15
cm
-3
.
Inductively coupled plasma (ICP) etching with an oxide mask
was utilized to form the emitter mesa. Reactive ion etching
(RIE) with a photoresist mask was used to form the base mesa.
A dry sacrificial oxidation at 1100 °C for 1 hour was
employed to have the best performance according to [13]. To
minimize the interface charges at SiO
2
/SiC interface, a surface
passivation was deposited with 50-nm PECVD SiO
2
followed
by post oxide annealing in N
2
O ambient at 1100 °C for 3
hours according to [12]. A 110-nm and 140-nm Ni layer was
deposited for emitter and collector contacts respectively,
followed by rapid thermal annealing (RTA) at 950 °C for 1
min in Ar ambient to form the ohmic n-contact. A 110-nm
stack layer of Ni/Ti/Al with the thickness of 10/15/85 nm was
deposited for base p-contacts followed by RTA at 815 °C for 2
min in N
2
ambient. The emitter and base contact resistivities
were extracted to 1.4 × 10
-5
Ω.cm
2
and 1.5 × 10
-4
Ω.cm
2
Optimal Emitter Cell Geometry in High Power
4H-SiC BJTs
Arash Salemi, Student Member, IEEE, Hossein Elahipanah, Member, IEEE, Carl-Mikael Zetterling,
Senior Member, IEEE, and Mikael Östling, Fellow, IEEE
S
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Fig. 2. Maximum current gain for different emitter widths and cell
geometries.
respectively, utilizing the TLM structures. Two Al layers were
sputtered for current spreading and connecting the linear
interdigitated fingers, hexagon and square cells.
The hexagon and square cells have different emitter widths
W
E
, i.e., 10, 20, 30, and 40 µm. Moreover, two different sets
of the finger cell geometries have been fabricated. The first
has a single finger cell with an emitter length of 500 µm and
different emitter widths W
E
, i.e., 10, 20, 30, and 40 µm; the
second has interdigitated fingers with a W
E
of 10 and 20 µm.
The optimum distance between the emitter mesa and base
contacts (3 µm) for all cell geometries, was chosen to obtain
the maximum value of β according to [14].
III. RESULTS AND DISCUSSION
Fig. 2 shows the emitter size effect on β for the different
designs. The β is decreasing as the W
E
reduces below 40 µm
for all emitter cell geometries. The wider emitter (> 40 µm)
does not result in higher current gain due to the saturation
behavior discussed in [15]. It also shows that the finger design
has the highest current gain, because of the smallest emitter
periphery over area (P
E
/A
E
) ratio. Decreasing of this ratio
lowers the surface recombination as discussed in [2], [16] -
[18]. The effect of surface recombination on the current gain
can be expressed as in [18]:
(1)
where, J
bulk
, J
Scr
, and J
Inj
are the bulk recombination, space-
charge recombination, and base-emitter back-injection,
respectively. K
b,surf
× P
E
is the emitter periphery recombination
current, and K
b,surf
(A/cm) is the normalized surface
recombination current. Fig. 3 represents the J
C
/β as a function
of P
E
/A
E
ratio for square cell geometry as an example. The
K
b,surf
can be calculated by the slope of the lines. It is apparent
that the K
b,surf
increases as the J
C
increases due to emitter
current crowding, resulting in a decline of the β. All
geometries show this behavior as the square cell geometry.
The extracted K
b,surf
are shown in Fig. 4 for all cell geometries.
Although the hexagon and square cell geometries have higher
K
b,surf
values leading to a lower β in Fig. 2, these cell
geometries can result in higher J
C
and lower R
ON
due to a
better utilization of the base area.
There has always been a difference between the estimated
values of J
C
and R
ON
for the small- and large-area devices.
Fig. 3. Emitter size effect for different current densities. W
E
varies from 10 to
40 µm. The slope in the graph is the K
b,surf
.
Fig. 4. Normalized periphery surface recombination current as a function of
J
C
for different cell geometries.
TABLE I. CALCULATED ACTIVE AREA FOR ALL CELL GEOMETRIES.
Emitter
width (µm)
Interdigitated fingers
(×10
-3
mm
2
)
Square
(×10
-3
mm
2
)
Hexagon
(×10
-3
mm
2
)
10
96
92
86
20
113
84
82
30
---
82
80
40
---
80
78
Therefore, to precisely estimate the J
C
and R
ON
, an actual
value for active area is needed. The active areas were
calculated by considering two aspects; Firstly, the current flow
in the thick collector layer which was simulated by Sentaurus
TCAD. Secondly, the calculation method based on the
simulation results were applied to the previous fabricated
BJTs [19], [20], and a good agreement was achieved for the
R
ON
and J
C
of the small- and large- area BJTs. Table I shows
the calculated active area for all cell geometries with different
emitter widths.
Fig. 5 and 6 illustrate the J
C
(at the maximum current gain)
and R
ON
(at collector current I
C
= 0.1 A) for all cell geometries
with different emitter widths as a function of β. At the
maximum current gain (~35), about 42% higher J
C
and about
21% lower R
ON
was resulted for the hexagon cell geometry,
and about 21% higher J
C
and about 23% lower R
ON
was
resulted for the square cell geometry compared to the linear
interdigitated finger geometry. It can be concluded that at a
given β, the hexagon and square cell geometries are more
suited for high-power SiC BJTs in order to obtain a higher J
C
and a lower R
ON
.
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Fig. 5. Collector current density as a function of maximum current gain for
the fabricated BJTs with different cell geometries.
Fig. 6. On-resistance as a function of maximum current gain for the
fabricated BJTs with different cell geometries.
Fig. 7 shows the high temperature performance of the
different fabricated BJTs with a W
E
= 40 µm. All cell
geometries show a negative temperature dependence of the β
with the same trend because of increasing acceptor ionization,
resulting in reduction of the emitter efficiency. This trend is in
a good agreement with the modeling in [21].
Fig. 7. Temperature dependency of the β for the different fabricated BJT cell
geometries.
IV. CONCLUSION
The influence of the cell geometrical design on the three
fabricated 4H-SiC BJTs (linear interdigitated fingers, square
cell geometry, and hexagon cell geometry) on the maximum
current gain (β), current density (J
C
), and on-resistance (R
ON
)
has been compared. All cell geometries showed a negative
temperature dependence of the β with the same trend. The
emitter size effect played a key role in determining the β for
all designs. The linear interdigitated fingers showed the
highest β, due to a lower periphery over area P
E
/A
E
ratio and
consequently a lower surface recombination. However, at a
given current gain, about 42% higher J
C
and about 21% lower
R
ON
was seen for the hexagon cell geometry, and about 21%
higher J
C
and about 23% lower R
ON
was observed for the
square cell geometry in comparison to the traditional linear
interdigitated finger geometry due to a better utilization of the
base area. This significant improvement in the J
C
and R
ON
makes the device more suited for high-power and high-
temperature applications.
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