1. TriQuint Semiconductor, Inc.
Increased channel temperature: the next challenge limiting RF performance on GaN on SiC FET technology and strategies to reduce it.
Sep 3, 2013
TriQuint Semiconductor, Inc.

2. TriQuint Semiconductor, Inc.
Outline
Introduction
The thermal limitation
Approaches
The obvious approach: A Diamond substrate
The not so obvious approach: A new circuit design approach
Summary
TriQuint Semiconductor, Inc.

3. TriQuint Semiconductor, Inc.
Disclosures
This is not a talk on Semiconductor Hetero-structures, but a talk on the limitations of one of such structures as a product.
This is not a talk with great detailed results, but a talk about an idea.
This is a talk on thermal improvements at the chip level. There is a parallel world on thermal improvements at the package level that I will not cover here.
Part of the work presented here is supported by TriQuint’s DARPA NJTT Program, Contract #W31P4Q-12-C-0067
TriQuint Semiconductor, Inc.

4. TriQuint Semiconductor, Inc.
Outline
Introduction
The thermal limitation
Approaches
The obvious approach: A Diamond substrate
The not so obvious approach: A new circuit design approach
Summary
TriQuint Semiconductor, Inc.

5. Barrier Discontinuity Substrate Thermal Conductivity
Introduction
2008 TriQuint’s Technology Roadmap
GaN: A high power density, high voltage technology
CREE demonstrates 40 W/mm on GaN on SiC
Nitronex demonstrates 10 W/mm on GaN on Si
TriQuint releases a 40V technology
TriQuint releases a 48V technology
GaAs
GaN on SiC
Band Gap
1.42 eV
3.50 eV
VSat
cm/s
cm/s
Barrier Discontinuity
~0.2 eV
0.5 eV
Substrate Thermal Conductivity
51.5 W/mC
400 W/mC
TriQuint Semiconductor Inc,

6. TriQuint Semiconductor Inc,
Introduction
2013 TriQuint’s Technology Reality
GaN: A high power density, high voltage technology
GaAs
GaN Products
In spite of good reliability, GaN on SiC is being used well below its performance potential
WHY?
TriQuint Semiconductor Inc,

7. THE NEW LIMITING FACTOR ON RF POWER DEVICES
Introduction
NOT SURPRISING !!!!
The typical heat flux at the GaN-SiC interface is ~ 100 times larger than the heat flux at the surface of the sun
The typical heat flux at the GaN on SiC – package is similar to the heat flux at the surface of the sun
HEAT DISSIPATION :
THE NEW LIMITING FACTOR ON RF POWER DEVICES
TriQuint Semiconductor, Inc.

8. TriQuint Semiconductor, Inc.
Introduction
Traditional chip level approaches to thermal management at the chip level
Thermal Vias
Thinning Substrates
In GaN on SiC, none of those approaches is effective
Thermal vias have lower thermal conductivity than SiC and do not spread the heat
Thinning substrates is only effective if the thermal conductivity of the package is higher than that of the substrate, which is not the case in GaN on SiC
TriQuint Semiconductor, Inc.

9. TriQuint Semiconductor, Inc.
Outline
Introduction
The thermal limitation
Approaches
The obvious approach: A Diamond substrate
The not so obvious approach: A new circuit design approach
Summary
TriQuint Semiconductor, Inc.

10. The obvious approach: RF Power limited by the substrate NOT by the FET
The Diamond Substrate
Gedanken Experiment. “The role of the substrate”
Performance-Reliability Space
GaN on GaAs
GaN on Si
GaN on SiC
Channel Temperature (C)
Dissipated Power (W/mm)
RF Power limited by the substrate NOT by the FET
TriQuint Semiconductor, Inc.

11. The obvious approach: Diamond will allow GaN to operate up to 8 W/mm
The Diamond Substrate
Gedanken Experiment. “The role of the substrate”
Performance-Reliability Space
GaN on GaAs
GaN on Si
GaN on SiC
GaN on Diamond
Channel Temperature (C)
Dissipated Power (W/mm)
Diamond will allow GaN to operate up to 8 W/mm
TriQuint Semiconductor, Inc.

12. The obvious approach: GaN on Diamond process approach
The Diamond Substrate
GaN on Diamond process approach
Si (111)
GaN
Protection
GaN
GaN
Protection
GaN
Protection
Si (111)
Epoxy
Si (111)
Epoxy
Handle Wafer
Handle Wafer
Deposit Protective layer
GaN/Si(111)
Attach temporary support
Remove Si(111) substrate
Transition
Diamond
Protection
Diamond
Transition
GaN
GaN
Transition
GaN
Protection
GaN
Transition
Epoxy
Protection
Diamond
Handle Wafer
Epoxy
Handle wafer
Handle Wafer
Deposit
Transition Layer
Diamond
Remove handle wafer Remove protective layer
TriQuint Semiconductor, Inc.

13. TriQuint Semiconductor, Inc.
The obvious approach:
The Diamond Substrate
Critical properties and dimensions
Protection Layer
Removal should not damage the GaN surface
It should robust against temperature budget
It should withstand chemistry budget
Transition Layer
Thin to minimize thermal penalty
Mechanically strong to survive temperature cycling
Diamond
High Thermal conductivity
Protection
GaN
Transition
Diamond
TriQuint Semiconductor, Inc.

14. TriQuint Semiconductor, Inc.
The obvious approach:
The Diamond Substrate
Promising initial Results (Dumka et al.)
Electrical Results (GaN on Diamond)
DC I-V
40V 10Ghz Load Pull
Thermal Results
GaN on Diamond (50nm TL) GaN on SiC 50
100
150
200
0.8
1.6
2.4
3.2
P [W]
ΔT [K]
Peak T 50
100
150
200
250
300
0.8
1.6
2.4
3.2
P [W]
ΔT [K]
Peak T
DARPA’s NJTT Contract #W31P4Q-12-C-0067
TriQuint Semiconductor, Inc.

15. The obvious approach:
The Diamond Substrate
Summary
GaN on Diamond has already demonstrated its potential
By achieving similar RF power densities as GaN on SiC on equivalent geometries in spite of more challenging surface process
By lowering the temperature rise under the same power consumption
However, GaN on Diamond is an emerging technology
with several tough engineering problems still to solve
TriQuint Semiconductor, Inc.

16. TriQuint Semiconductor, Inc.
Outline
Introduction.
The thermal limitation
Approaches
The obvious approach: A Diamond substrate
The not so obvious approach: A new circuit design approach
Summary
TriQuint Semiconductor, Inc.

17. The not so obvious approach:
A new circuit design approach
2005 TriQuint’s Technology Roadmap
GaN: A high power density, high voltage technology
TriQuint Semiconductor, Inc

18. TriQuint Semiconductor, Inc.
The not so obvious approach:
A new circuit design approach
Examples of GaN Products
Product Features
• Frequency: DC to 3.5 GHz
• Linear Gain: >15 dB at 3.5 GHz
• Operating Voltage: 28 V
• Output Power (P3dB): 55 W at 3.5 GHz
•Lead-free and RoHS compliant
Product Features
Frequency: DC to 6 GHz
Output Power (P3dB): 18 W at 6 GHz
Linear Gain: >10 dB at 6 GHz
Operating Voltage: 28 V
Low thermal resistance package
Lead-free and RoHS compliant
If all we care about is power …..
Why is power density important?
TriQuint Semiconductor, Inc.

19. TriQuint Semiconductor, Inc.
The not so obvious approach:
A new circuit design approach
Why is power density so important?
Chip Size Cost
Reduce Combining
Losses
Bandwidth
Really???
TriQuint Semiconductor, Inc.

20. The not so obvious approach:
A new circuit design approach
Traditional Circuit design approach
Choose Unit Cell
Electrical Amplifier Design
Channel Temperature
Finish
Change Unit Cell
Increase Gate to Gate Spacing
Reduce Finger Width
The device is used at its full power density potential
Yes No
TriQuint Semiconductor, Inc.

21. The not so obvious approach:
Instead of increasing Gate to Gate pitch
We reduce Gate to Gate pitch
A new circuit design approach
What if:
ARE YOU CRAZY!!!
NO I AM NOT!!!
TriQuint Semiconductor, Inc.

22. The not so obvious approach:
A new circuit design approach
Instead of increasing gate to gate pitch
Reduce Gate to Gate pitch …
but keep the power density per area constant
Geometry: 4x100um. GtoG: 50um
PDissip: 5.0W/mm =2W
TBase: 25C. TPeak:80.62C
Geometry: 8x100um. GtoG: 25um
PDissip: 2.5W/mm =2W
TBase: 25C. TPeak:58.74C
That’s a 40% reduction of thermal resistance without increasing the chip .
TriQuint Semiconductor, Inc.

23. The not so obvious approach:
Instead of decreasing the finger width
Increase the finger width …
but keep the power density per area constant
A new circuit design approach
Geometry: 4x100um. GtoG: 50um
PDissip: 5.0W/mm =2W
TBase: 25C. TPeak:80.62C
Geometry: 4x200um. GtoG: 50um
PDissip: 2.5W/mm =2W
TBase: 25C. TPeak:54.02C
That’s a 48% reduction of thermal resistance without increasing the chip size.
TriQuint Semiconductor, Inc.

24. The not so obvious approach:
A new circuit design approach
What kind of channel temperature reduction can we expect?
2W 4x100um device equivalent
Depending on the bandwidth we are willing to sacrifice, the temperature increase can be limited by half.
TriQuint Semiconductor, Inc.

25. The not so obvious approach:
A new circuit design approach
What do we trade?
Chip Size
(Cost) No
But can be engineered even without touching the semiconductor
Smaller Gate Length
Individual source Vias
Bandwidth
Yes
Losses (Gain/Power) No
TriQuint Semiconductor, Inc

26. The not so obvious approach:
A new circuit design approach
AND THAT’S NOT ALL!!!
TriQuint Semiconductor, Inc.

27. The not so obvious approach:
A new circuit design approach
Relation between Power density and Mean Time to Failure
Normalized Power Density
0.85
0.9
0.95 1
1.05
1.1
1.15
100 40 10 4
0.4
0.1
0.04
0.01
Normalized Time to Failure
Lower Power Density
Lower Al in the Barrier
Lower Inverse Piezoelectric effect
Better Reliability
TriQuint Semiconductor, Inc.

28. The not so obvious approach:
A new circuit design approach
A circuit quoted to have a MTTF of 1Million hours at 85C base temperature using the old design strategy, will be able to be quoted to have a MTTF of 1Billion hours at the same 85C base temperature.
TriQuint Semiconductor, Inc.

29. TriQuint Semiconductor, Inc.
Outline
Introduction
The thermal limitation
Approaches
The obvious approach: A Diamond substrate
The not so obvious approach: A new circuit design approach
Summary
TriQuint Semiconductor, Inc.

30. TriQuint Semiconductor, Inc.
Summary
Today, RF power devices are limited by the substrate and not by the intrinsic capabilities of the GaN semiconductor.
The development of new hetero-substrates (GaN on diamond) or thick substrate circuitry and not the development of new barrier-channel configurations will push the technology to higher power densities.
In the meantime, we should use smarter design approaches and develop “less sexy support technologies” (e.g ISVs) to minimize the effect of temperature.
TriQuint Semiconductor, Inc.

31. TriQuint Semiconductor, Inc.
Engineering is about using science to solve the problems that really matter
TriQuint Semiconductor, Inc.