A New Generation of Power Semiconductor Devices
A New Generation of Power Semiconductor
Devices
José Millán
Centro Nacional de Microelectrónica, CNM
CNM-CSIC, Campus Universitad Autónoma de Barcelona,
08193 Bellaterra, Barcelona, Spain
A New Generation of Power Semiconductor Devices
• Introduction
• Si Power Devices
• Si IGBTs
• Si Super-junctions
• SiC Power Devices
• SiC Power Rectifiers
• SiC Power Switches
• GaN Power Devices
• WBG Future Trends
Outline
A New Generation of Power Semiconductor Devices
Introduction
Power Electronics is:
“efficient processing of electrical energy through means of
electronic switching devices ”
40% of Energy
consumed as
electricity
A New Generation of Power Semiconductor Devices
Energy Distribution
Communications
Traction/Automotive
Introduction
A New Generation of Power Semiconductor Devices
Power Devices
Classification of High Voltage
Devices
A New Generation of Power Semiconductor Devices
Si Power Devices
Si Power Devices
A New Generation of Power Semiconductor Devices
Si Power Devices
GTO, Power MOSFET and Cool MOS
Voltage Range
Power MOSFET
Power
supplies
Motor
control
GTO Thyristor
Cool MOS
Electric
cars
Traction
& HVDC
A New Generation of Power Semiconductor Devices
Si IGBTs
IGBT Structure & Output Characteristics
Structure of ‘DMOS’ IGBT
Static Characteristics
Current x10 compared with power MOSFET
A New Generation of Power Semiconductor Devices
Si IGBTs
IGBT OFF-state
The p -base/n-base
junction blocks the voltage
while the device is in the
off-state
A New Generation of Power Semiconductor Devices
Si IGBTs
IGBT ON-state
-
When the device is in the
on-state the electron
current at the cathode flows
through the channel like in a
MOSFET and acts as the
base current for the pnp
transistor formed between
the p+ anode-(emitter), nbase & n+ buffer (base)
and p-base (collector).
Due to high level of
injection in the on-state the
entire n-base is modulated
by mobile carriers in
equilibrium with an
effective charge of few
orders of magnitude higher
than the original doping
A New Generation of Power Semiconductor Devices
Si IGBTs
The IGBT Equivalent Circuit
The IGBT has within its structure three MOS- bipolar devices:
(i) The cascade MOSFET - PIN diode
(ii) MOS base current controlled - wide base PNP transistor
(iii) Parasitic MOS turn-on thyristor - must be always
suppressed
Gate
Source/Cathode
Source/Cathode
n
+
p well
n
+
p well
p+
p+
n- drift
region
p
+
Anode
A New Generation of Power Semiconductor Devices
Si IGBTs
IGBT turn-off Characteristics
(2)
(1)
(3)
(4)
Examples of measured IGBT turn-off characteristics in inductive conditions.
The characteristics are plotted for different rail voltages. There are three
distinctive regions (1) voltage rise (2) electron current fall, (3) removal of
main charge stored in the drift region (4) current tail through recombination
A New Generation of Power Semiconductor Devices
Si IGBTs
Three concepts that led to major
advancements in IGBTs from one
generation to another
• Trench and thin wafer technologies – led to
~30 % cut in the on-state voltage drop
• PIN diode effect – Enhanced injection of
electrons at the top side (channel side) of the
drift region – led to a further 20% decrease in
the on-state voltage drop
• Field stop (Soft Punch Through) technology led
to ~20% cut in the turn-off losses and 10-20%
decrease in the on-state voltage drop
A New Generation of Power Semiconductor Devices
Si IGBTs
PT & NPT IGBT Structures
Ecr
Ecr
Punch-Through (PT IGBT)
Safety
distance
Non Punch-Through (NPT IGBT)
A New Generation of Power Semiconductor Devices
Si IGBTs
Trench IGBT Cross Sections
4μm
5μm
Schematic
SEM
A New Generation of Power Semiconductor Devices
Si IGBTs
Breakdown vs on-state in DMOS IGBT & Trench
IGBT
A New Generation of Power Semiconductor Devices
Si IGBTs
The ability to ‘engineer’ the PIN diode section in
the TIGBT can be used to optimise its performance
Channel
Electron
injector
Cathode
Cathode
p+
n+
p -well
n+
p+
p -well
Gate
n- drift region
n buffer
P anode
Anode
The heavily charged accumulation layer
serves as an electron injector forming a
PIN diode with n-drift region and p-anode
There are two paths for the current flow:
(i) the double sided injection path of the
PIN diode with increased plasma at both
injection ends (anode and cathode end),
and
(ii) the pnp path with increased plasma
only at the IGBT anode end.
Increasing the PIN diode contribution
over that of the pnp transistor is the key
to enhance the device performance
This is equivalent to suppressing the
collection of holes by the p well to the
cathode short
A New Generation of Power Semiconductor Devices
Si IGBTs
On-state Characteristics of a TIGBT
A New Generation of Power Semiconductor Devices
Si IGBTs
The Field Stop (or Soft Punch-Through),
PT and NPT structures
PT - IGBT
Gate
Source/Cath
n
+
p well
120μ
NPT - IGBT
Source/Cath
Gate
n
+p well
n
+
p well
120μ
m
n- drift
region
m
15μm N-buffer
SPT - IGBT
Gate
Source/Cath
200μm
n- drift
region
1- 2 μm
1 μm
250μ
m
p+
(substrate)
1μm
P transparent
anode
n- buffer – field stop
P transparent anode
A New Generation of Power Semiconductor Devices
Si IGBTs
The Field Stop (or Soft Punch-Through),
PT and NPT comparison
Structure
PT -IGBT
NPT -IGBT
SPT - IGBT
thin
thick
thin
Epitaxial
Float zone (FZ)
Float Zone (FZ)
Thick and highly doped
N/A
Thin and lowly doped
Thick and highly doped
(whole substrate)
Thin and relatively
lowly doped
Thin and relatively
lowly doped
Lifetime killing
Injection efficiency
Injection efficiency
On-state losses
low
medium
low
Switching losses
high
medium
low
short
long
short
Voltage overshoot (in
some applications)
high
low
low
Temperature coefficient
negative (mostly)
positive
positive
SCSOA (short circuit
conditions)
medium
large
large
RBSOA (reverse bias
conditions)
narrow
large
large
Drift layer thickness
Wafer type (for 600 V
and 1.2 kV)
Buffer Layer
P+ anode injector
Bipolar gain control
Turn-off tail
A New Generation of Power Semiconductor Devices
Si IGBTs
1.2 kV IGBTs. SPT has a better carrier profile than the
PT and NPT structures with the Trench SPT showing the
most favorable result.
A New Generation of Power Semiconductor Devices
Si IGBTs
The trade-off between on-state voltage and turn-off energy
losses for 1.2 kV DMOS PT IGBT, the Trench IGBT and the
Trench SPT IGBT
A New Generation of Power Semiconductor Devices
Si IGBTs
The Reverse Conducting IGBT
n+
n+
M. Rahimo, 3.3 kV RC IGBT using SPT+
technology (ISPSD 2008)
H. Takahashi, 1.2 kV Reverse Conducting
IGBT (ISPSD 2004), Mitsubishi
A New Generation of Power Semiconductor Devices
Si IGBTs
The Reverse Blocking IGBT
• 600V RB-IGBT designed and fabricated at CNM
• Additional protection of IGBT periphery: trench isolation (patent pending)
• Applications: Current inverters, resonant converters, Matrix converters,
BDS
1,25
Al
SiO2
1,00
Poly Si
N
Body-P
0,75
+
Epitaxy -N
0,50
-
Substrate-P+
Junction supporting
forward bias
Junction supporting
reverse bias
IC (mA)
P
3328-RBI Wafer 11 Bidirectional Blocking Capability
0,25
0,00
-0,25
-0,50
-0,75
RB-IGBT
(G-E short)
-1,00
+
Substrate-P
-1,25
-800 -600 -400 -200
0
200 400 600 800
VCE (V)
A New Generation of Power Semiconductor Devices
Si Super-junctions
Super-Junction MOSFETS
COOLMOS
Rectangular e-field distribution
allows increasing Nepi doping.
RonxA below Si limit
A New Generation of Power Semiconductor Devices WBG Semiconductors
WBG Power Devices
A New Generation of Power Semiconductor Devices WBG Semiconductors
Why WBG Semiconductors?
• Si devices are limited to operation at
junction temperatures lower than 200 ºC
• Si power devices not suitable at very high
frequencies
• SiC and GaN offer the potential to overcome
both the temperature, frequency and power
management limitations of Si.
A New Generation of Power Semiconductor Devices WBG Semiconductors
Physical properties of WBG for Power Devices
Material
Si
4H - SiC
GaN
Diamond
Eg
μn
μp
Vsat
Ec
λ
(eV)
@300K
(cm²/Vs)
(cm²/Vs)
(cm/s)
(V/cm )
(W/cm.ºK)
εr
1.12
1450
450
107
3×105
1.3
11.7
3.2
950
115
2 × 107
3 × 106
5
10
3.39
1000
350
2 × 107
5 × 106
1.3
8.9
5.6
2200
1800
3 × 107
5 × 107
20
5.7
A New Generation of Power Semiconductor Devices
WBG Technology
• GaN & SiC process technologies are more mature
• At present, SiC is considered to have the best trade-off
between properties and commercial maturity
• GaN can offer better HF and HV performances, but the
lack of good quality large area substrates is a
disadvantage for vertical devices
• GaN presents a lower thermal conductivity than SiC
• GaN allows forming 2DEG heterojunctions (InAlGaN
alloys) grown on SiC or Si substrates
• Currently, it is a sort of competition SiC vs GaN, in a
battle of performance versus cost
• There is not a clear winner at the moment. They will
find their respective application niches with a
tremendous potential market
A New Generation of Power Semiconductor Devices
SiC Power Devices
SiC Power Devices
A New Generation of Power Semiconductor Devices
SiC Power Diodes
• SiC Power Rectifiers
•
Schottky barrier diodes (SBD): extremely high switching speed but
lower blocking voltage and high leakage current.
•
PiN diodes: high-voltage operation and low leakage current, reverse
recovery charging during switching.
•
Junction Barrier Schottky (JBS) diodes: Schottky-like on-state and
switching characteristics, and PiN-like off-state characteristics.
A New Generation of Power Semiconductor Devices
SiC Power Diodes
State-of-the-Art
SiC rectifiers
• Schottky and now JBS diodes are commercially available up to
1.2 kV: CREE, Infineon basically.
• PiN diodes will be only relevant for BV over 3 kV.
- Need to overcome its reliability problem (forward
voltage drift) before commercialisation
A New Generation of Power Semiconductor Devices
SiC Power Switches
A New Generation of Power Semiconductor Devices
SiC Power Switches (unipolar)
•
•
•
Main problem: Normally on (?)
•
Very low Ron
Rugged Gate-structure
Excellent short-circuit
capability
High temperature possible
x
A New Generation of Power Semiconductor Devices
SiC Power Switches (unipolar)
Hybrid Si/SiC cascode electric switch
• Compared to a COOLMOS –
based converter, the SiCbased one offers the
highest efficiency (90%)
• All SiC sparse matix
converters
• CoolMOS + SiC
efficiency
higher than 96%
• All SiC sparse matrix converter: 100 KHz – 1.5 kW – efficiency
94% 1300V 4 A SiCED Cascodes + 1200 V 5 A CREE Schottky
diodes
• 3 phase PWM rectifier 10kW – 500KHz – 480V CoolMOS + SiC
Schottky diodes : efficiency higher than 96%
A New Generation of Power Semiconductor Devices
SiC Power Switches (unipolar)
MOSFET Advantages
Trench/DiMOSFET
• Simple planar structrure
• Voltage gate control
• Extensively used in Si
technology
• Normally-off
MOSFET main problems
x
• Low channel mobility in SiC
• High temperature operation ?
• Gate reliability ?
Lateral DMOFET
A New Generation of Power Semiconductor Devices
SiC Power Switches (unipolar)
• CREE: 2.3KV-5A Ron=0.48 Ω (25ºC) 13.5mΩ.cm2,
Ir=200uA. Cin=380pF, Cout=100pF, reverse transfer
C=19pF (Vgs=0,Vds=25V, 1MHz)
• Infineon: 1200V-10A, Ron=0.27 Ω (25ºC) 12mΩ.cm2
• Denso: 1200V-10A, 5 mΩ.cm2 (25ºC), 8.5mΩ.cm2
(150ºC)
A New Generation of Power Semiconductor Devices
SiC Power Switches (unipolar)
[M. Das et al. at ISPSD’2008, pp. 253-259]
10 kV MOSFET
(Cree)
A New Generation of Power Semiconductor Devices
SiC Power Switches (bipolar)
State-of-the-art
[S. Krishnaswami et al.,
ISPSD’2006, pp. 289-292]
•
•
•
• 3500 V - 6500 V range
•
•
•
• Unlike Si BJT, SiC BJT does not
suffer from a secondary breakdown
• ß is reduced (50%) under bias
stress (stacking faults base-emitter
region)
4 kV, 10 A BJT
βmax = 34
chip area = 4.24 mm × 4.24
mm
IR =50 µA @ 4.7 kV
turn-on time = 168 ns @ RT
turn-off time = 106 ns @ RT
A New Generation of Power Semiconductor Devices
SiC Power Switches (bipolar)
SiC IGBT?
• Problems of MOSFETS (Channel mobility,
reliability)
• Problems of Bipolar (current gain degradation,
stacking faults)
• Problems of highly doped P substrate growth
• May 2008 (ISPSD 2008): CREE 10kV n-channel IGBT
• 3V knee, 14.3 mΩcm2
• At 200ºC the n-IGBT operates at ×2 the current
density of the n-MOSFET
A New Generation of Power Semiconductor Devices
GaN Power Devices
GaN Power Devices
A New Generation of Power Semiconductor Devices
GaN Power Diodes
GaN Power Rectifiers
• Until recently, because of the lack of electrically
conducting GaN substrates, GaN Schottky diodes were
either lateral or quasi-vertical
• Breakdown voltages of lateral GaN rectifiers on
Sapphire substrates as high as 9.7 kV have been
reported
Zhang et al.
IEEE T-ED,48, 407, 2001
SBD
PiN
A New Generation of Power Semiconductor Devices
GaN Power HEMTs
GaN Power HEMTs
• GaN HEMTs have attracted most attention with impressive
trade-off between Ron vs BV
• Power densities 1.1 W/mm in 1996 initially to microwave
power HEMTs with high output power capability as high as
40 W/mm recently
• A major obstacle trapping effects though drain-current
collapse
• Several solutions :
• (1) surface-charge-controlled n-GaN-cap structure
• (2) the recessed gate and field-modulating plate
structure
• (3) passivation of surface states via silicon nitride or
other dielectric.
A New Generation of Power Semiconductor Devices
GaN Power HEMTs
• High voltage AlGaN/GaN HEMTs over 1 kV were reported in
2006
S. YOSHIDA et al. ISPSD 2006
• It has been also demonstrated a GaN power switch for kW
power conversion.
• The switch shows a speed grater than 2 MHz with rise- and
fall-time of less than 25 ns, and turn-on/turn-off switching
losses of 11 µJ with a resistive load.
• Switching at 100 V/11 A and 40 V/23 A was achieved with
resistive and inductive loads, respectively.
A New Generation of Power Semiconductor Devices
GaN Power HEMTs
Via-holes through sapphire at the
drain electrodes enable very
efficient layout of the lateral HFET
array as well as better heat
dissipation
8.3 kV HEMT
(Panasonic)
Y. Uemoto et al. IEDM 2007
A New Generation of Power Semiconductor Devices
GaN Power HEMTs
GaN Power HEMTs
The state-of-the-art AlGaN/GaN HEMT
[T. Nomura et al., ISPSD 2006, pp. 313-316]
• Process technology based on a tri-metal Ti/AlSi/Mo layer →
very low contact resistance and an excellent surface
morphology.
• Mo (barrier metal) to improve the surface morphology
• AlSi results more efficient for a low contact resistance
than Al.
• Low stress, high-refractive index SiNx layer
→
Gate leakage current as low as 10-7 A/mm.
• Ron = 6.3 mΩ.cm2, VBR = 750 V.
• Turn-on time: 7.2 ns (1/10 of Si MOSFET).
• Switching operation no significant degraded at 225ºC.
A New Generation of Power Semiconductor Devices
GaN Power HEMTs
GaN Power normally-off AlGaN/GaN HEMTs
The state-of-the-art normally-off AlGaN/GaN HEMT
[N. Kaneko et al. , ISPSD 2009, pp. 25-28]
• Recess gate electrode and NiOx
as gate electrode
(NiOx operates as a p-type)
• Wgate= 157 mm, Vth = +0.8 V
• Ron ×A = 6.3 mΩ.cm2
• Ron = 72 mΩ
• VBR > 800 V
• IDmax > 20 A
The gate leakage current four orders of magnitude smaller than
the conventional normally-on HFETs.
A New Generation of Power Semiconductor Devices
GaN Power MOSFETs
Lateral GaN MOSFETs
•
Lateral MOSFETs have been fabricated on p-GaN epilayer
(MOCVD) on sapphire substrates
[W. Huang et al., ISPSD 2008, pp. 291].
- High quality SiO2/GaN interface
- 2.5 kV breakdown voltage
- High channel mobility (170 cm2/V.s)
• Lateral GaN MOSFETs can compete with SiC MOSFETs and GaN
HEMTs?
• Reduction of source/drain resistance is crucial to further
reduce the device on-resistance.
A New Generation of Power Semiconductor Devices
WBG Future Trends
WBG Future Trends
SiC Switches
• Successful demonstration of the cascode pair (a highvoltage, normally-on SiC JFET + a low-voltage Si
MOSFET).
• An industrial normally-off SiC switch is expected. It
could be the SiC MOSFET (<5kV) or the SiC IGBT
(>5kV).
• BJTs/Darlingtons are promising, they also suffer from
reliability problems.
• A normally-off SiC power transistor in the BV range of
600V-1200V available within next two years.
A New Generation of Power Semiconductor Devices
GaN Power HEMTs
WBG Future Trends
GaN Power Devices
• GaN is already commercialised in optoelectronics.
• Its applications in power switching still require
further work in materials, processing and device
design.
• GaN HEMT (5-10 A, 600-1200 V normally-off)
• It will be interesting to see if GaN power devices,
especially low cost Schottky diode, can overtake or
displace SiC diodes.
A New Generation of Power Semiconductor Devices
Thanks for your attention