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Department of Electronic & Electrical Engineering
Wideband Gap Semiconductors and
New Trends in Power Electronics
Professor Peter R. Wilson
University of Bath
Where did we get to with Silicon Devices and Power Electronics?
SOME FUNDAMENTALS…GET
TO WITH SILICON DEVICES
AND POWER ELECTRONICS
Linear Power Supplies
• The “Old” method used a linear power
supply to rectify AC to DC
– Standby power of a few W
Diode Power ~0.5W
Transformer Losses ~3.5W
The “New Way” – switching
• By switching the power, we can take
advantage of reduced inductor sizes to
reduce the static power and “off” time
losses
Diode Power ~0.5W
Transformer Losses ~0.2W
Where can we go next?
– We can use higher voltage power devices
– GaN (600V) or SiC (1200V)
– Why Not use IGBTs?
– They are slower than MOSFETs
– Using GaN or SiC allows much faster switching
– Faster Switching Frequencies means
transformer can be much smaller
– Lower inductance L
– We can reduce standby power to almost zero by
using advanced controllers
What are the issues and sources of energy transfer and power loss
MANAGING POWER
Energy and Frequency
• The Energy transferred per cycle can be
defined using:
2
Lprimary ´ I peak
Energycycle =
2
• And the overall Energy by:
2
Frequency ´ L primary ´ I peak
Energytotal =
2
• Therefore as the Frequency goes UP, the
Inductance goes DOWN
Impact on Products
• The impact on circuit design and product
size can be dramatic
– Reduction in Inductor and Capacitor Size
– Simplification in Circuit Design
Losses
• There are two main Sources of Loss:
– Active
• Switching devices such as diodes & MOSFETs have
switching losses that create heat
– Passive
• These fall into two further types, component and layout
based losses.
• Component losses may be obvious such as resistor loss, but
also the resistive losses in inductors or capacitors
• Layout based losses occur when current flows through other
conductors, such as wires, PCB tracks, enclosures or other
physical structures
Dynamic switching losses
V
OFF
Switch control
OFF
ON
V,A
io
Vd
Vload
Iload
Von
Td(on) t t
ri
fv
t
Td(off) trv tfi
Switching times
determine the
switching Loss
W
Vload*Iload
Pon
Tc(on)
Tc(off)
t
On Resistance
determines the
On State Loss
Comparing new Power Devices with Silicon
WIDE BAND GAP DEVICES?
Standard HV MOSFET
• The Standard MOSFET has a BV of ~600V
• The drift Region dominates Ron
Source
Gate Conductor
Field Oxide
Gate Oxide
n+
``
``
n+
n+
p (body)
n+
p (body)
n- (drain drift)
n+
Drain
~40mm
Breakdown and Mobility
• In order to achieve sufficient breakdown
we need to calculate the required mobility
P
NE
dE qN d
=
dx
e si
N+
Emax =
Emax
qN d L
e si
2e siVr Emaxe si
L=
=
qN d
qN d
L
Vbr =
e si Ec2
2qNd
To get Vbr ≥ 700V => N ~ 3.5E14 cm-3 => 40 µm depletion region
Device Sizing and Ron
• In the Off state, In order to achieve the required
breakdown voltage we need the depletion region to be
an adequate size
1
V
Vbr µ
& Ldepletion µ br
Nd
Ec
• For example, to get Vbr ≥ 700V:
Vbr = 700V Þ Nd » 3.4e14cm-3 Þ Ldepl » 40mm
• In the ON state, the resistance will vary by:
Ron µVbr2
• Or more realistically:
Ron µVbr2.5
The “Silicon Limit”
• The relationship between the Breakdown Voltage and On
Resistance defines the effective limits of the device operation
– The “Silicon Limit”
Is this really the limit?
• IGBT: these have been around since the 1980s and essentially
consist of a MOSFET gate driver, and a PNP device
– High power is possible, but is relatively slow
• Lateral resurf (REduced SURface Field) devices
– Originally developed at Philips, these devices use the P substrate to
extend the depletion region and reduce Ron
• Superjunction devices: rotating a resurf device so the same principle
is implemented in a vertical device
– Invented by Prof. Xingbi Chen.
– Taken to market by Infineon
• Despite these excellent developments, all have some limitations and
only move some way from the Silicon Limit
Lateral Resurf devices
• Lateral power devices can easily be integrated with an on-chip
controller. Can sense temperature of power device directly.
– Less costly packaging.
• Vertical power device has high-voltage of back of die which in
normally connected to the tab of the package  handling issues.
Controller
Vertical SJ MOS
Controller
Resurf
Power
MOSFET
A vertical power transistor needs a
separate controller chip – either in a
separate package, or as a hybrid (2chip) package.
(images courtesy Martin Manley, Power Integrations Inc.)
GaN HEMT (High Electron Mobility Transistor)
• Structure is quite different from conventional Si and SiC devices.
• They are Hetero-junction devices – the current is carried in a
2-dimensional electron gas (2DEG), rather than an inversion layer.
• 2DEG is created by spontaneous polarization at AlGaN/GaN interface
• Conduction is modulated by a gate electrode that overlays the 2DEG.
GaN
Lattice matching “Buffer” layer
Silicon, sapphire or SiC
substrate
(images courtesy Martin Manley, Power Integrations Inc.)
2DEG
Material Properties
esi Ec2
Vbr =
• How do the device materials compare?
2qNd
Property
Silicon (Si)
Silicon Carbide (SiC)
Gallium Nitride (GaN)
Band Gap (eV)
1.1
3.2
3.4
Critical Field (106V/cm)
0.3
3.0
3.5
Electron Mobility (cm2/Vs)
1450
900
2000
Electron Saturation velocity (106cm/s)
10
22
25
Thermal Conductivity (W/cm2K)
1.5
5
1.3
• SiC and GaN have
– much higher critical field => higher breakdown voltage
– Electron saturation velocity twice as high
– SiC has excellent Thermal Conductivity
Extending the “Silicon Limit”
• The characteristics of the Wide Band Gap devices (especially SiC
and GaN) lead to an extending of the “Silicon Limit”
Advantages of WBG
• SiC has a similar structure to Si devices
– Easier transition from Si to SiC for processing
• SiC has MOSFETs and Diodes in production
– Available NOW for Power Electronics Designers
– Wafers more expensive, but in the same order of magnitude
• SiC thermal Conductivity is excellent and is therefore
easier to extract excess heat
• SiC tolerates higher voltages (1200V is routine)
– GaN only 600V
• GaN limit is higher than SiC
• GaN can operate at higher speeds than SiC
• GaN reliability is a question for large scale deployment
Voltage/Frequency Context
Voltage
1500V
1000V
SiC
500V
Silicon
GaN
100 1k 10k 100k 1M 10M 100M Frequency
1G 10G
Challenges for designers
• Compact Models (Challenge for researchers/Synopsys?)
– We need new compact models for SiC and GaN
– We need accurate parameters for components
• Long Term Behaviour (challenge for vendors?)
– Stress, Reliability and Thermal behaviour
• Packaging
– How do we model modules not just devices?
• Gate Drivers
– A specific and difficult issue with the devices possibly able to
switch at 1MHz and beyond – inductive effects are critical
– SiC devices need a larger voltage (~20V) than Si MOSFETs to
switch properly and dedicated gate drivers are needed
What are the possibilities for Power Electronics?
CONCLUSIONS
Conclusions
• Wide Band Gap devices offer the possibility of lower On
resistances for higher voltages
• High Speeds of operation leading to smaller designs,
smaller passives
• Thermal tolerance is exceptional for SiC devices in
particular
• Issues remain with gate drivers and speed of operation –
packaging and layout
• Higher speeds, Higher Temperatures, More Reliable,
Lighter, Smaller Circuits
– an exciting time for Power Electronics!
Acknowledgments
• Martin Manley, Power Integrations Inc
– Some figures
– Interesting and Stimulating Discussions on
Wide Ban Gap devices
• IEEE Power Electronics Society
– Funding a study into the reliability of SiC
MOSFETs – results to be published in 2016