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3.3. POWER SEMICONDUCTOR DEVICES
3.3.1. Semiconductor devices [23,24,25,26,27]
(1) Diode
Most modern diodes are based on semiconductor p-n junctions. In the p-n diode,
current can flow from the p-type to the n-type side, but cannot flow in the opposite
direction. The Schottky diode, is formed by a contact between a metal and a
semiconductor rather than by a p-n junction. The basic application of the diode is as a
rectifier that converts alternating current (AC) into pulsating direct current (DC). Fig. 5
shows a full-wave bridge rectifier which consists of four diodes and a resistor. From the
aspect of switching, the diodes can be called as uncontrolled switches. No signals turn
the diode on or off.
M
D4
AC
D1
B
A
RL
D2
D3
N
During +ve half cycle
M is +ve and N is -ve
D1, D3 : forward
D2, D4 : reverse
During -ve half cycle
M is -ve and N is +ve
D2, D4 : forward
D1, D3 : reverse
Fig. 5. A full-wave bridge rectifier with four diodes and a resistor.
(2) Bipolar Junction Transistor (BJT)
The bipolar junction transistor is a three-terminal component composed of emitter
(E), collector (C), and base (B). The emitter and collector terminals are made of the
same type of semiconductor material, either p-type or n-type, while the base is made of
the other. There are two types of bipolar junction transistors, npn and pnp transistors,
and the npn transistor is shown in Fig. 6.
(heavily doped)
(lightly doped)
E
N
P
C
N
C
(moderately doped)
NPN-type
E
B
Fig. 6. Schematic diagram and symbol of an npn transistor.
The output current, voltage, and/or power of a transistor are controlled by its input
current. Thus, it is used as the primary component in the amplifier, a circuit that is used
1
to increase the strength of an ac signal. Also, the transistor is used as a high-speed
electronic switch that is capable of switching between two operating states (open and
closed) at a rate of several billions of times per second.
A small change in the base current causes a large change in both the emitter and
collector currents. Operation characteristics of a bipolar junction transistor are presented
in Fig. 7, and it shows the relationship between the emitter-collector voltage and the
collector current which is the output of a transistor. At a constant emitter-collector
voltage, the collector current is proportional to the base current which is the input
current. The maximum power is represented by a dashed parabola.
IC(mA)
Saturation region
8
90 A
80 A
7
70 A
6
60 A
50 A
5
40 A
4
Pcmax=VCEIC
3
30 A
Active region
20 A
2
10 A
1
IB=0 A
5
0
VCES
10
15
Cutoff region ICEO ~
= ICBO
20
VCE(V)
Fig. 7. Operation characteristics of a bipolar junction transistor.
As shown in the figure, the BJT has three operation modes, linear (active), cutoff,
and saturation regions, respectively. In the linear region, as the base current increases,
the collector current increases linearly. Thus, the amplification is possible. When the
transistor is used as a switch, it uses cutoff and saturation regions. Ideally, the collector
current (IC) is zero in the cutoff condition, but if the base current IB=0, then IC=ICEO
(collector emitter leakage current) and ICEO is very small. The collector current in the
saturation condition is set at the limited value. Therefore, transition between cutoff and
saturation is a switching of a transistor in a circuit. The modern transistor is usually
used as a switch.
(3) Silicon Controlled Rectifier (SCR)
The BJT transistor is widely used as a switch, but, its power rating is not so big.
To overcome the disadvantage of the BJT, 4-layer devices are used, and their family is
called thyristor. The thyristor is a four-layer, three-terminal semiconductor switch. A
representative device in this family is Silicon Controlled Rectifier (SCR). Only silicon
is used for the SCR, and rectification is its main duty. 3rd silicon layer of the SCR is
used for controlling the circuit opening or shorting. The configuration of SCR is the
same as a pnp transistor connected to an npn transistor as shown in Fig. 8. The anode
2
has (+) polarity for cathode in the forward conduction state. But, unless the sufficient
current supplies to the gate terminal, the device is maintained as an off state. An
equivalent circuit diagram is shown in Fig. 9.
Anode
+
IA
p
IGT
n
p
Cathode Anode
-+
Anode
+
n
p
p n p n
IGT
n
Cathode
--
IGT
Gate
Gate
-Cathode
Fig. 8. Two transistor equivalent SCR, symbol and basic construction.
V
IA=ICO
VGT
VG
E1
E1
IB1
Q1
0V
t1
V
t2
t3
t4
t
VGT=0V
IB2=0
IC2=ICO
Q2
E2
VBE2
(a)
High
Z
open
circuit
E2
V
IA
V
E1
Q1
IC1
IB2
+
VGT=VG
--
E1
IB1
VBE2
IC2
Low
Z
short
circuit
Q2
E2
IK=IA E2
(b)
Fig. 9. An equivalent circuit diagram of the SCR and its operation.
When the gate signal is applied to the gate terminal of SCR, VGT is 0 before t1, thus,
IB2= 0. IC2, the base current of Q1 is about ICO. This current is too small to make Q1 an
on state. Therefore, two transistors are all off state, and status between E1 and E2 is an
open circuit. When VG is applied to the base terminal of Q2, VBE2 is equal to VG. So, the
enough current IB2 to make Q2 on state can flow. Then, IC2, the collector current of Q2,
increases, which can make Q1 on state. When Q1 becomes on state, IC1 is added to IB2,
then IC2 increases more. Finally, the circuit between emitters of two transistors becomes
short state. Although VGT becomes 0 at t2 as shown in Fig. 9, SCR is not turned off
because, once SCR turns on, all 4-layers of SCR are full of charge carriers. Therefore,
once SCR turns on, the gate does not function.
Fig. 10 shows an example of SCR applications. It is a circuit for half wave variable
resistance phase control. The gate current to SCR is limited by R and variable resistor
R1. Gate input current is the sinusoidal wave with the same phase as IL. As R1 decreases
and reaches a point, the gate current increases and reaches a value to trigger the SCR. At
this moment, the SCR turns on. By setting the proper resistor value, the phase when the
SCR is triggered can be adjusted. By this, electric power on a load can be controlled so
as required root-mean square current can be achieved.
3
RL
IL
R
A +
R1
IG
Trigger
level
G
K -90
Fig. 10. A circuit for half wave variable resistance phase control.
(4) Gate Turn-Off Thyristor (GTO)
The gate turn-off thyristor (GTO) is a power semiconductor device like the
thyristor. It has an added advantage that a negative gate-current pulse can turn off the
device. In fact, the GTO is a turn-off semiconductor switch with the highest current and
voltage ratings presently available. An electric-circuit symbol for the GTO is similar to
that of the thyristor except an addition to the gate electrode to represent the property of
gate turn-off as shown in Fig. 11. The structure of the GTO is essentially the same as
the conventional thyristor. As shown in Fig. 11, there are four silicon layers (pnpn),
three junctions, and three terminals (anode, cathode and gate).
Anode
+
Anode
+
p
n
Gate
p
Gate
n
-Cathode
-Cathode
Fig. 11. GTO composition and its electric-circuit symbol.
The most important characteristic of the GTO is its turn-off ability. If a negative
gate signal is injected while the GTO is in the on-state, the device will switch off. It will
remain off until the conditions of a positive anode and a positive gate with respect to the
cathode are met. The useful and economic power range of the GTO lies between the
bipolar power transistor (or IGBT) and the thyristor (SCR). It does not switch as fast as
the transistor but can handle higher voltage and current levels. It is faster than the phasecontrolled thyristor and can be turned off with a gate signal, but is not capable of
modulating such high powers as the thyristor. Accordingly by cost and ratings,
thyristors will not be replaced by GTOs at the highest power levels. The overall
switching speed of the GTO is faster than the thyristor with comparable voltage and
4
current, but the GTO has a higher on-state voltage drop. Since the GTO has
unidirectional current conduction and can be turned off at any time, its applications are
mainly in chopper circuits (dc-dc conversion) and in inverter circuits (dc-ac conversion)
at power levels above which MOSFETs, BJTs and IGBTs cannot be used.
(5) Field Effect Transistor (FET)
The most significant difference between BJT and FET is that the BJT is a current
controlled device in which output current is controlled by the current between base and
emitter, while FET is a voltage controlled device in which output current is controlled
by the voltage between gate and source (VGS). There are n- or p-channel FETs like the
npn or pnp types in BJTs. While the BJT is a bipolar device in which the conduction
state is dependent on the two charge carriers (electron, hole), the FET is an unipolar
device using one type of charge carrier. In the FET, the electric field is formed by the
charge carriers that control the conduction path. There are several types of FET
including:
JFET (Junction Field Effect Transistor)
MOSFET (Metal Oxide Semiconductor Field Effect Transistor):
Depletion-type MOSFET
Enhancement-type MOSFET
CMOS (Complementary MOSFET)
Fig. 12 shows a basic n-channel depletion type MOSFET. Three terminals in the
FET, source (S), gate (G) and drain (D) have metal contacts, and S and D are connected
to the n-channel. The gate is separated from the n-channel by a very thin SiO2 layer.
Since the SiO2 is a dielectric material, when electric field is applied to this layer,
opposite directional electric field is generated inside it. Since there is no electric contact
between gate and other terminals due to the SiO2 layer, input impedance of the
MOSFET is very large.
D-Drain
SiO2
D
n-channel
n
n
e
Metal contact
e
e
G-Gate
n
p
Substrate
Substrate
SS
e
n
e
G
p
SS
VDD
e
e
e
n
n
n-doped region
S-Source
(a)
S
ID=IS=IDSS
(b)
Fig. 12. An n-channel depletion type MOSFET and its operation with VGS=0 V and
applied voltage VDD.
5
When VGS=0 V, and a VDD is applied between source and drain, the current in nchannel can flow. If a negative voltage is applied to the gate, positive voltage is induced
at the gate side in the SiO2 layer and a negative voltage is induced at the n-channel side.
This negative voltage pushes the electrons in the n-channel, and attracts holes in the psubstrate. As a result, the recombination between electron and hole increases, and the
conduction current decreases. As the negative voltage at the gate increases, the pinchoff occurs. This mode is called a cutoff region. If a positive voltage is applied to the
gate, electrons which are the minority carriers in the p-substrate are accelerated to the nchannel. These electrons make other electrons become ionized, and make ID abruptly
increase. This mode is called an enhancement region. Thus, the current flow between
source and drain can be controlled by the gate signal
The basic structure of an enhancement-type MOSFET is shown in Fig. 13. A SiO2
layer separates the gate and the p-type substrate, therefore, in the case of VGS=0 V, there
is no current between D and S even though the voltage is applied. Fig. 14 shows the
basic operation of the enhancement-type MOSFET.
As shown in Fig. 14(a), when a positive voltage is applied to the VDS and VGS,
holes in the p-substrate are repelled by this positive voltage, and electrons which are the
minority carriers in the p-substrate are attracted by the voltage. Since SiO2 is an
insulator, electrons are not absorbed. As VGS increases, electrons form a channel, that
makes the current between D and S flow. According to the V GS, electron channel is
formed, so it is called the enhancement MOSFET. After fixing the VGS, increasing VDS
makes the gate less positive after pinch-off as shown in Fig. 14(b).
D-Drain
SiO2
No channel
n
Metal contact
G-Gate
p
Substrate
Substrate
SS
n
n-doped region
S-Source
Fig. 13. The basic structure of enhancement-type MOSFET.
6
Electrons attracted to + Gate
Region depleted of p-type
carriers(holes)
ID
n
D
G
VGS
S
Pinch-off(beginning)
+
+
+
+
+
+
n
D
+
+
+
+ p
+
+
+
+
e
e
e
e
e
e
e
e
Depletion region
ID
e
SS
VDS
n
p
e
VGS
S
SS
VDS
e
e
nn
Holes repelled by + Gate
IS=ID
(a)
IS=ID
(b)
Fig. 14. The basic operation of enhancement-type MOSFET, (a) Channel formation in
the n-channel enhancement type MOSFET, (b) Change in channel and depletion region.
(6) Insulated Gate Bipolar Transistor (IGBT)
GTO is available for high voltage and current applications but limited to a
relatively low frequency. Also it requires a relatively high power for gate control.
MOSFET can be applied to a very high frequency, and its control is very easy. But, it is
limited to relatively low power applications. IGBTs are used in the power and
frequency ranges between GTOs and MOSFETs.
The IGBT is a three-terminal power semiconductor device that combines the
simple gate-drive characteristic of the MOSFET with the high current and low
saturation voltage capability of bipolar transistors by combining an isolated gate FET
for the control input, and a bipolar power transistor as a switch, in a single device. The
IGBT is used in medium- to high-power applications such as switched-mode power
supply, traction motor control and induction heating, and its application field is being
extended. There are two types of IGBT, PT (punch through) and NPT (non-punch
through) types. The structures of the NPT and PT IGBTs are shown in Fig. 15. In an
actual case, many IGBT unit elements are connected in parallel, and it is called as IGBT
device or module.
7
SiO2
n+
Polysilicon
Gate
SiO2
p+
+
+
+
p-
n+
n-ch. formation
ee
e
ee
p+n- junction
R-modulation
+
+
+
Emitter
+
+
+
ee
e
ee
n+
p+
n-
p+
Polysilicon
Gate
Collctor
+
+
+
ee
ee
e
+
+
+
ee
ee
e
n+
pp+
(a) NPT IGBT
Emitter
p-
n+
buffer
n-
Collctor
p+
pp+
(b) PT IGBT
Fig. 15. The structures and operations of NPT and PT IGBTs.
The NPT IGBT consists of 4 layers of n+pn-p+, which comprises two transistors
like the thyristor. However, unlike the thyristor, the gate is separated by SiO2 oxide
layer. The gate terminal is connected to a polysilicon layer which covers n+ and nregions. The metal contact for the emitter terminal covers n+ and p- regions. If the
collector of an NPT IGBT is positively biased for the emitter, and if the gate potential is
0, there is no current flow from the collector to the emitter. If the gate is positively
biased against the emitter as shown in Fig. 15(a), the electrons in the p- region are
attracted and collected between n+ and n- regions due to the role of the SiO2 oxide layer.
Therefore, an n-channel between the emitter and the n+-p--n- is formed. Then, electrons
are entering from the emitter to the n- region and holes are injected from the collector to
the n- region. These excess electrons and holes reduce the resistivity of n- region. This is
called an R-modulation or a conductivity modulation, which reduces the on-state
resistance of IGBT. This allows the IGBT to be used in a higher power than the
MOSFET that has no R-modulation.
The PT type IGBT has an n+ buffer layer between the n- and p+ regions. Some of
holes injected from the collector to the p+ region are lost in this region due to the
recombination. This layer improves turn-off speed by reducing the minority carrier
injection quantity and by raising the recombination rate during the switching transition.
However, on-state voltage drop increases due to this layer. Therefore, PT IGBT has
trade-off characteristics when compared to the NPT type for the switching speed and
forward voltage drop or power dissipation.
In the PT IGBT fabrication procedure, an n- layer is grown by the epitaxial method
after growing an n+ buffer layer on the p+ substrate. But, the thickness of the epitaxial
layer that can be grown is limited. Therefore, the limited thickness of the n- layer makes
the blocking voltage of PT IGBT lower than the NPT type.
In fabricating the NPT IGBT, the base material is an n- type silicon wafer with an
extremely uniform dopant concentration, and NTD silicon can be applied for it. The p+
region in the collector side is formed by an implantation method. The emitter side and
MOSFET structure are achieved by a diffusion method.
IGBTs have the trade-off characteristics between a low conduction loss due to the
bipolar current and a high switching loss due to the tail current at turn-off. So,
8
technology for the minority carrier lifetime control has been necessarily developed. The
switching features were significantly improved by using the punch-through epitaxial
structure in comparison with NPT IGBT initially fabricated in the early 1980s. However,
since the thickness of the drift region should be large for a higher blocking voltage,
NPT structure fabricated using a bulk starting material has been preferred for high
voltage applications [28,29]. The short circuit capability of NPT type IGBT with
homogeneous base material is much more robust than the PT type based on the epitaxial
technology.
In the late 2000s, the competition of the IGBT device market from 600 to 3500V
has been between PT and NPT IGBT devices. PT IGBTs are dominant in the lowvoltage region less than 1000 V, whereas NPT-IGBTs are dominant in the region higher
than 2500 V. However, PT and NPT IGBTs have competed for some years in the midvoltage region [30].
3.3.2. NTD technology and power semiconductor devices
Power semiconductor devices or power devices are used to control the electricity
over several watts. They play a crucial role in regulation, distribution, transformation
and adaptation of electrical energy between suppliers and users. Consequently, the
performance of the power devices has a significant impact on the efficient use of
electricity. Power devices are usually used as switches or rectifiers in power electronic
circuits (switch mode power supplies for example).
Accommodation of higher current density, higher power dissipation and/or higher
reverse breakdown voltage is a basic property required for the power semiconductor
devices. Also in order to prevent the device disorder due to the heat, a power device is
usually attached to a heat sink to remove the heat generated by operation loss. Power
semiconductor devices have following typical characteristics:
(1) High breakdown voltage
The high breakdown voltage is required for their applications to high voltage. For
this, the drift region of the charge carriers must be thick with low doping density.
However, these make the on-resistance of this region in the state of current flowing high,
then a high power loss accompanied with heat cannot be avoided. So, there is a trade-off
between breakdown voltage rating and on-resistance.
(2) Low on-resistance
If a large number of parallel cells are used, the on-resistance can be lowered
enough for the circuit to be available for the high current rating. But this leads to a
higher overall capacitance and after all, the switching speed may be slowed down.
(3) Fast switching
In order for a device to be switched between on- and off-state as fast as possible,
rise and fall times must be short. It is practically very important in the aspect of the heat
treatment of a device since the switching loss is mainly responsible for the heat
generation.
(4) Wide safe operation area
9
In order for a device to avoid being out of ordered or destroyed, the safe operation
area from thermal dissipation must be as wide as possible.
After the invention of the transistor in the late 1940s, semiconductor devices were
first used in low power applications for communications, information processing, and
computers. In 1958, the first thyristor called SCR was developed [31], which is the era
of power electronics [32]. Since then, there has been a steady growth in the use of
power devices, and ratings of these components are also steadily increased owing to a
demand for them in various applications. Nowadays, various devices such as rectifier
diodes, bipolar transistor, thyristor, GTO, Diac, Triac, power MOSFET, IGBT,
IPM(Intelligent Power Module), etc. are widely used. Fig. 16 shows the
current/voltage/switching frequency of the main power electronic devices [33]. It is
found that the thyristor is the best for power applications, and the MOSFET is proper
for high frequency applications.
Fig. 16. The current/voltage/switching frequency of the main power electronic devices.
The power semiconductor diode operates on similar principles as its low power
counterpart. But, it is able to carry a high current and to support a high reverse bias
voltage in the off state. NTD technology can be applied to P-i-N rectifiers with n- drift
region of a low doping concentration.
Since the introduction of the metal-oxide-semiconductor (MOS) technology, the
power MOS-gated field effect transistors (MOSFETs) were developed in the 1970s.
Their performance was found to be powerful in the aspect of the operating frequency
but unsatisfactory for medium-power applications where the operating voltages exceed
300 V. Today, power MOSFETs are the most commonly used power switches below
200 V [34]. Thus, their utilities are limited to the low-voltage, high-frequency
applications. NTD technology is hardly applied to MOSFETs with p-type substrate.
The thyristor is a representative rectifier in high power region. Fig. 17 shows the
typical doping profile of a thyristor. The structure is usually constructed by starting with
10
a lightly doped n-type silicon wafer whose resistivity is chosen based upon the blocking
voltage rating for the device [35]. The n- drift layer is much wider than other regions,
and it has the lowest doping level about 1014 cm-3 for high breakdown voltage [36].
NTD technology is capable of making this low doping concentration and uniformity
throughout the whole region.
-3
Impurity concentration profile [cm ]
21
10
n+ diffusion
20
10
p+ diffusion
p diffusion
19
10
18
10
17
10
16
10
15
10
n- drift
14
10
13
10
Anode
p+
n-
p
n+
Cathode
Gate
Fig. 17. Typical doping profile of a thyristor.
Fig. 18 shows a typical thyristor fabrication process [37]. The anode p+ region is
formed by the diffusion of dopants from the backside of the wafer to a junction depth.
The p- base and n+ cathode regions are formed by the diffusion of dopants from the front
of the wafer, respectively. Electrodes are formed on the front and back sides of the
wafer. The electron or proton irradiation may be performed to reduce the carrier lifetime
as the final fabrication step.
11
n
Starting NTD-silicon
p
n
p
p
n
p
p
n
p
p-type diffusion
n+
n-type emitter diffusion
n+
n-type emitter structure
Metal
n+
p
n
Metal deposition
p
Metal
n+
Edge contour and passivation
Passivation
Electron or proton irradiation
Fig. 18. Typical thyristor fabrication process [37].
The growth of the power rating for the thyristors is shown in Fig. 19, and Professor
B. J. Baliga pointed out that it is much indebted to the development of NTD technology
[35].
12
5
Advent of NTD
8
4
6
3
4
2
2
0
1950
Current
Voltage
1960
1970
1980
1
1990
Thyristor current rating [kA]
Thyristor voltage rating [kV]
10
0
2000
Year
Fig. 19. Variation of applicable power to a thyristor device [35].
"Starting with a modest current of 100 A in the 1950s, the current rating has
been scaled to approach 5,000 A for a single device. These high-current levels are
required for power distribution systems such as high-voltage DC (HVDC)
transmission networks. From the figure, it can be observed that the most rapid
increase in the current-handling capability took place at the end of the 1970s. This
outcome can be traced to the development of the neutron transmutation doping
(NTD) process in the mid-1970 time frame. Using the NTD process, it became
possible to obtain larger diameter silicon wafers with uniform properties enabling
the observed scaling of the current-handling capability of thyristors. There was a
concomitant increase in the voltage-blocking capability for the thyristor as
illustrated in the chart. Beginning with devices capable of operating up to a few
hundred volts in the 1950s, the voltage rating for thyristors has been escalated to
8,000 V. The increase in the voltage rating had to be accomplished by the
availability of higher resistivity silicon wafers. This was initially achieved by the
development of the float-zone process. However, the resistivity variation produced
by this process was inadequate for utilization in the large diameter wafers desired
to increase the current ratings. The NTD process was instrumental in providing the
breakthrough required to create large diameter silicon wafers with low n-type
doping concentration and high uniformity in the resistivity across the wafers.
Consequently, a substantial gain in the voltage rating occurred in the late 1970s
after the commercial availability of NTD silicon as indicated in the chart."
The gate turn-off thyristor (GTO) was developed from a significant interest in the
control of DC-motors. It needs a large reverse gate drive current. In spite of the bulky
and expensive gate control circuits, GTO was widely used for the control of traction
motors so far. Like the thyristor, the GTO has been one of main parts of NTD wafer
consumption. Since the power-handling capability of the insulated gate bipolar
transistor (IGBT) becomes very high in the 2000s, the GTO has been replaced by the
IGBT.
Since the introduction of thyristors in the 1950s, many bipolar power devices were
developed. Among them, the power bipolar transistor extended the operating frequency
13
of power systems. However, since it was a current-controlled device using a small
current gain, the control circuits should be bulky and expensive. For this reason, the
power bipolar transistor has been replaced in the 1990s by the insulated gate bipolar
transistor (IGBT) which is a voltage-controlled device.
The IGBT was introduced in the 1980s to provide superior characteristics in the
medium-power applications. It combined the best features of the bipolar power
transistor and the power MOSFET. Nowadays, it has become the most powerful power
device in the medium-power range, and it has been widely accepted for various
applications including motor control systems.
As shown in Fig. 15, the p+ anode (collector) in PT IGBT is the low-resistivity
substrate material, and the n- layer is an epitaxial layer about 50 μm thick with a doping
concentration below 1014 cm-3 [38]. In the fabrication process of a punch through (PT)
IGBT, the starting material is high-purity, dislocation-free Czochralski (CZ) or float
zone (FZ) silicon having orientation <100>. They are usually heavily doped with p-type
dopant to form a p+ substrate. Thus, NTD technology is not capable of the fabrication of
the PT IGBT. The non-punch through (NPT) IGBT has a large n- substrate layer, and
neutron-transmutation doped (NTD) n-type float zone silicon is used for this layer
because the NTD provides tight tolerances for the dopant density fluctuations required
to achieve high manufacturing yields for large-area IGBT chips.
As shown in the above, the floating-zone NTD silicon wafer which has the best
quality has been mainly applied to the extremely high power range such as thyristors,
GTOs and a part of the IGBTs. Recently, the application of the NTD wafer is being
extended to the medium power range like IGBT modules for traction control of the
hybrid electric vehicle (HEV).
A HEV equipped with an electric power source like electric motors and batteries in
addition to the conventional propulsion system became available to the public from late
1990s. The inverter is the most important component for controlling the electricity flow
between the motor, the generator and the battery for the traction of an HEV, and the
power devices are key components for their performance. Power devices used in the
HEV should be small to be equipped in a limited space, and they should have additional
features like ruggedness under the rather severe environment. Most HEVs adopt IGBTs
as power devices for controlling the traction.
A great improvement in the IGBT properties for the HEV has been brought by the
field-stop (FS) IGBT fabricated by a thin wafer processing technology using FZ-wafers
[39]. The switching loss of FS-IGBT is much lower than typical NPT IGBT. FS IGBT
has greatly contributed to the shrink of the chip size and the thickness [40]. Table 2
compares three different IGBTs [41,42]. Prius II by Toyota, which is the most popular
HEV model nowadays, adopted the conventional planar FS IGBT concept for the main
inverter. For the better performance of the inverter, the use of the NTD wafer with the
most uniform dopant distribution seems to be crucial. The HEV market has been
growing very fast, and it can be easily expected that the demand for NTD wafer would
be growing according to the development of the HEV. Infineon, a power device
company, estimated that a future hybrid electric vehicle would consume approximately
one 6 inch wafer of semiconductor [43].
Table 2. Comparison of the different IGBTs.
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PT-IGBT
NPT-IGBT
FS-IGBT
Structure
n+ buffer layer
between p+ substrate
and n- drift region
n- drift region on
implanted p+ layer
(no buffer layer)
Fabrication
Epitaxial growth
FZ wafer process
Doping
concentration of
additional n+ layer
Paralleling for chips
or modules
NTD wafer
applicability
More than 1017 cm-3
n+ field stop layer
between implanted
p+ layer and n- drift
region
FZ thin wafer
process
1015 to 1016 cm-3
Difficult
Easy
Easy
No
Yes
Yes
3.4. RESISTIVITY PROFILE
As described above, the neutron transmutation doping (NTD) for silicon is a
method for producing an n-type silicon with highly uniform phosphorous distribution.
In general, the uniform p-type doping with a p-type material such as boron and
aluminium is easily achieved by non-nuclear technologies because dopants are
relatively stable in the molten silicon. However, there are several problems in
conventional technologies for the n-type doping due to the volatility of the group 15
elements, such as small distribution coefficient, etc. Since silicon is very transparent to
neutrons, if an irradiation device is carefully designed for the uniform and accurate
neutron irradiation, the NTD can give very high doping uniformity compared to other
technologies.
Fig. 20 compares the radial resistivity profiles of an NTD silicon with another one
doped by conventional method [44]. Local resistivity depends on the charge carrier
density at that position which is determined by the dopant concentration. As shown in
Fig. 20, the NTD method results in an extremely uniform concentration of dopant. The
local resistivity variation of the NTD silicon is usually less than 5 %. But, for the
conventional method, the variation is over 80 ~ 90 %. In addition to larger resistivity
variation, the concentric distribution pattern like an annual ring is represented in the
radial resistivity profile due to the striation of the dopant distribution.
Table 3 compares typical radial resistivity uniformity of the floating zone (FZ)
silicon crystals doped by gas doping and NTD. The FZ silicons are classified, according
to the uniformity of the resistivity, into premium class, super-premium class and NTD
FZs. Premium class and super-premium class FZ silicon crystals are obtained by the gas
doping method. The FZ silicon doped by the NTD method shows a much more uniform
resistivity than those by gas doping.
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Fig. 20. Radial resistivity profiles of the neutron transmutation doped silicon and that by
conventional doping method.
Table 3. Resistivity variations of floating zone (FZ) silicon crystals doped by gas
doping and NTD methods.
Material
Doping method
Premium FZ
Super premium FZ
NTD FZ
Gas doping
Gas doping
NTD
Radial Resistivity
Gradient (RRG)
8 ~ 25 %
5 ~ 15 %
1.5 ~ 5 %
Local uniformity in
very small range
3%
2.5 %
0.8 %
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