Transistor and Circuit Technologies for Tomorrow

1
Transistor and Circuit Technologies for
Tomorrows Base Station Power Amplifiers

• Raymond S. Pengelly
• Cree Microwave
• Durham, NC 27703 USA

2002 IEEE Topical Workshop on Power Amplifiers
for Wireless Communications
2
More and More Power!

• Assuming GSM as the reference with 0 dB Peak
  to Average Ratio to cover a radius of X miles
• EDGE requires 2 x power for same coverage
• CDMA requires 4 x power for same coverage
• W-CDMA requires 8 x power for same coverage
• OFDM requires 15 x power for same coverage

3
There are No Free Lunches

• More data per unit time requires more bandwidth
  or clever modulation schemes
• Digital transmission techniques require more peak
  power for the same bit error rate for greater
  capacity
• In order to minimize spectral re-growth and
  interference transmitters have to be more linear

4
Competitive Power Transistor Technologies
5
New Devices

• High Power Density
• Reduced Size
• Higher Working Impedances
• Simpler Circuits
• Easier Manufacture
• Wide Bandgap transistors on 4H-SiC and AlGaN/GaN
  provide superior performance to GaAs or Si
  counterparts
• 4 to 6 Watts/mm for SiC MESFETs
• 10 to 12 Watts/mm for AlGaN/GaN HFETs

More inherent DC to RF Efficiency and Linearity
are Key
6
Envelope Distribution Functions

• Power capability is a direct function of where a
  power amplifier starts to saturate
• Average spectral re-growth is a function of
• Power capability
• Envelope statistics
• Clipping, even for short periods of time, is a
  serious issue

7

• Peak to Average Ratio is 15 dB

8
W-CDMA

• For the majority of the time (gt 90) the
  basestation transmitter delivers power at 1/8 its
  peak power capability (but it needs to be able to
  deliver any power level up to the peak)
• At peak power the overall base-station is
  typically 20 efficient but for most of the time
  it is only 6 efficient since the PAs become
  less efficient when backed-off
• 2.35 Kilowatts in for 140 watts out!

9
Typical Amplifier Line-up
10 30 125
Numbers are Peak Watts
10
Wide-Band Power Modules
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Typical Basestation Power Amplifier

• OLD - Lots of Silicon Power (400 watts)! - But
  physically LARGE
• Power Density of lt 10 watts per sq. inch

• NEW LDMOS
• Power FET Modules
• increase Power
• Density to
• 25 to 100 Watts
• per sq. inch

12
The Need for Smaller PAs
Macrocell Basestation Hut - Lots of Space!
Power Amplifiers with Fans

• Going to Microcell
• Higher powers in the same space
• Tower Top Arrays with no fans

13
.The Difference between an LDMOS Transistor and
a Silicon Carbide MESFET for 30 Watts Output Power
LD-MOSFET
SiC MESFET
14
GaN Amplifier- Comparison to GaAs pHEMT

• GaN based amplifier 6 W out to 50 W
• GaAs based amplifier 0.6 W out to 50 W
• -without impedance transformation

Device
Input
I
V
f
/f
Load
Power
max
max
t
max
Capacitance
mA/mm
(V)
GHz
impedance
(W)
Ohms
GaAs
3 pF
600
20
30/90
33
1
p-HEMT
GaN
3 pF
1200
60
30/90
50
6
HEMT
- 10x less impedance transformation - 5-10 x
Higher Bandwidth - Simpler, Smaller circuits,
High Yield, Low cost
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Thermal Conductivity is Critical for High Power

• Die size is constrained by wavelength
• - Y-dimension is limited by gate R and L
• - X-dimension is limited by phasing issues

• Key figure of merit is how much power the device
  can handle in terms of W/mm2 of die area

16
GaN on SiC The Thermal Advantage

• SiC has a very high thermal conductivity of 4.9
  W/cm-K
• - GaAs 0.4, Si 1.5, Sapphire 0.4

• SiC delivers higher power from given chip area gt
  SiC has higher W/mm2 gt reduces /W

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3 SiC Vs. 4 Si Wafer

• 100 W GaN HEMTs Die size on SiC 1 x 4 mm2, Die
  size on Si 2 x 6 mm2
• Fabrication (not Substrate) is the more
  expensive cost component

3-inch SiC 4-inch Si Total die 860
532 Non-edge die 788 484
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High Power Density PAE from SiC MESFETs

• Pulsed on-wafer power densities of 5-6 W/mm
  consistently achieved on large FETs

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SiC MESFET with 7.2 W/mm

• Power density of 7.2 W/mm with 45 PAE at S-band
  demonstrates the capability of the technology

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Mobile Telephone Frequency Allocations
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20-Watt Broadband SiC MESFET Amplifier
Balanced Amplifier with 10-Watt, CRF22010 FETs

• 22 W at P1dB across a 400 MHz band
• Advantage of wide bandgap transistors
  power-bandwidth product greatly exceeding Si LDMOS

22
75-Watt SiC MESFET Amplifier
2 GHz test fixture for 60 W MESFET development

• 75 W CW, 11 dB gain demonstrated from a single
  SiC MESFET
• Currently 60-Watt Class A/B MESFET transistor
  being optimized, targeted for production release
  by the end of 2002
• REAL POWER!

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Broadband SiC MESFET Amplifier 200 MHz to 2200 MHz
Drain Bias
100 190 ohms
In Out
1.1 0.5
0.6 0.6 2.4
Gate Bias
All capacitor values in pF
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Ultra Broadband Amplifiers

• Broadband for Multi-Frequency and Multi-Mode

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GaN HEMTs for Power Amplifiers
Higher junction temperature, smaller pitch
higher device density, ease of packaging
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GaN HEMT with 12 W/mm

• Peak pulsed power density of 12 W/mm on a 0.5-mm
  HEMT
• CW power from same device of 9 W/mm

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Packaged GaN HEMTs

• P1dB of 12 W, 46 PAE at 4 GHz at 30 Volts

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Characterization of AlGaN/GaN HEMTs Using Fixed
Load at Varying Voltage Supply
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Cellular Base-Station Application
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10-Watt Broadband GaN HEMT Amplifier

• 11 W at P1dB across the 400 MHz to 2200 MHz band
• 17 dB gain with only 0.5 dB ripple
• Great for a generic driver amplifier

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GaN HEMTs for Infrastructure 2 GHz, CW Power
from a 24-mm HEMT

• Record Power exceeding 100 Watts

Peak Power 108 W CW Power Density 4.5 W/mm

• 103 W at 2.6 dB gain compression
• Peak Drain Efficiency of 54

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High Temperature Operation

• Demonstrated that at a TJ of 180OC (case
  temperature of 120OC) SiC MESFET has a mission
  life of gt 20 years (3 ? confidence level)
• Equivalent maximum junction temperature for Si
  LDMOS is 130 OC
• Equivalent maximum junction temperature for GaAs
  MESFET is 110 OC

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So, what does this mean for next generation
infrastructure power amplifiers?

• Easier and more tolerant designs
• Higher operating temperatures
• Removal of DC-DC converters (voltage versus
  current)
• Ruggedness
• Wider Band Designs
• Smaller Units

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Wide Bandgap is an Enabling Technology

• Wide Bandgap can provide a paradigm shift in the
  4G infrastructure sector
• Allows Tower Top Installations lowers power
  requirements by at least a factor of 2 by
  eliminating cable losses
• Fan-less Operation will be possible enabled by
  higher transistor operating temperatures
• Will make cost-effective Smart Antennas viable
• Integration of SiC with other technologies in
  standard Lego modules economies of scale

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Wide Bandgap enables Tower-Top Power Amplifiers
Antenna with Power amplifiers _at_ 48 volts ½ X watts
Antenna ½ X watts
Efficiency 50 x 85 x 30 12 If youre
really lucky!
3dB Loss in Cable
Efficiency 30
Power Amplifiers _at_ 28 volts (X watts)
Conventional
New Approach
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Summary of Features of Wide Bandgap Transistors

• High Power density and high operating voltage
• More convenient impedance levels than Si LDMOS or
  GaAs FET
• easier and more tolerant design
• broadband amplifiers
• High Temperature Operation
• High Voltage Operation
• allows drain modulation techniques (28 volts avg.
  48 volts peak) for increased efficiency
• Rugged

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Next Steps?

• Productization
• Customer Education
• Reliability Facts
• Acceptable Dollars/Watt
• Introduction of RFICs
• Nothing we havent been through with other
  technologies
• WATCH THIS SPACE!