Department of Electronic & Electrical Engineering Wideband Gap Semiconductors and New Trends in...

Department of Electronic & Electrical Engineering

Wideband Gap Semiconductors and New Trends in Power Electronics

Professor Peter R. Wilson

University of Bath

SOME FUNDAMENTALS…GET TO WITH SILICON DEVICES AND POWER ELECTRONICS

Where did we 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 ~0.5WTransformer 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 ~0.5WTransformer 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

MANAGING POWERWhat are the issues and sources of energy transfer and power loss

Energy and Frequency

• The Energy transferred per cycle can be defined using:

• And the overall Energy by:

• 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

VloadIload

t

V,A

VSwitch control

ONOFF OFF

Von

Td(on) Td(off)tri tfv trv tfi

t

W

Pon

Tc(on) Tc(off)

Vload*Iload

Vd

io

On Resistance determines the On State Loss

Switching times determine the switching Loss

WIDE BAND GAP DEVICES?Comparing new Power Devices with Silicon

n- (drain drift)

``

p (body)

``

p (body)

Standard HV MOSFET

• The Standard MOSFET has a BV of ~600V• The drift Region dominates Ron

n+

n+ n+n+n+

Source

Drain

Field OxideGate Conductor

Gate Oxide

~40mm

Breakdown and Mobility

• In order to achieve sufficient breakdown we need to calculate the required mobility

P N- N+

E Emax

L

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

• For example, to get Vbr ≥ 700V:

• In the ON state, the resistance will vary by:

• Or more realistically:

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.

A vertical power transistor needs a separate controller chip – either in a separate package, or as a hybrid (2-chip) package.

Controller

ResurfPower

MOSFET

Vertical SJ MOSController

(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.

Silicon, sapphire or SiC substrate

Lattice matching “Buffer” layer

GaN2DEG

(images courtesy Martin Manley, Power Integrations Inc.)

Material Properties

• How do the device materials compare?

• SiC and GaN have – much higher critical field => higher breakdown voltage– Electron saturation velocity twice as high– SiC has excellent Thermal Conductivity

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

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

Frequency

Silicon

SiC

500V

1000V

1500V

GaN

100 1k 10k 100k 1M 10M 100M 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

CONCLUSIONSWhat are the possibilities for Power Electronics?

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