Transistor and Circuit
Technologies for
Tomorrow’s Base
Station Power
Amplifiers
Raymond S. Pengelly
Cree Microwave
Durham, NC 27703 USA
2002 IEEE Topical Workshop on
Power Amplifiers for Wireless Communications
Slide 1
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
Slide 2
“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
Slide 3
Competitive Power Transistor
Technologies
Price/Watt
POWER
DENSITY
SUPPLY
VOLTAGE
LINEARITY
FREQ.
PAE
Si BJT
LOW COST
MEDIUM
26 V
POOR
< 2 GHz
LOW
SiGe BJT
LOW COST
MEDIUM
< 20 V
GOOD
> 2 GHz
HIGH
Si LDMOS
LOW COST
LOW
26 V
V. GOOD
< 3 GHz
MEDIUM
GaAs MESFET
COMPETITIVE
MEDIUM
12 V
GOOD
> 2 GHz
MEDIUM
GaAs PHEMT
MEDIUM
MEDIUM
8 V to 12 V
V. GOOD
> 2 GHz
HIGH
GaAs HBT
COMPETITIVE
HIGH
8 V to 26 V
GOOD
> 2 GHz
HIGH
SiC MESFET
COMPETITIVE
VERY HIGH
48 V
GOOD
> 4 GHz
MEDIUM
N/A
VERY HIGH
48 V
PROMISING
> 12 GHz
TECHNOLOGY
GaN HEMT
Slide 4
HIGH
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
Slide 5
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
Slide 6
• Peak to Average Ratio is 15 dB
Slide 7
W-CDMA
• For the majority of the time (> 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 PA’s become
less efficient when backed-off
2.35 Kilowatts in for 140 watts out!
Slide 8
Typical Amplifier Line-up
10
30
125
Numbers are Peak Watts
750 watt peak amplifier
contains 30 LDMOSFETs
@ a total price of
$3,000
Slide 9
Wide-Band Power Modules
Parameter
PFM21020
PFM21020WB
RF Band
Typical Gain
Gain Flatness
2110-2170 MHz
28 dB
0.25 dB
Phase Linearity
Time Delay
In/Out VSWR
Typical P1dB
2 Tone IMDs
1.5 degree
4.1 nanosec
1.7/1.4
20 W
2080-2200 MHz
26 dB
0.10 dB
(allows 0.4 dB linear slope)
1.2 degree
3.6 nanosec
1.3/1.3
20 W
-37 dBc
-42 dBc
-45 dBc
-42 dBc
-46 dBc
-50 dBc
(+24<Pave<36 dBm)
a) 3rd order IMs
b) 5th order IMs
c) 7th order IMs
0
32
IM3L
IM3U
-10
28
IM5L
IMD Rejection (dBc)
24
IM7L
P.A.E.
IM7U
-30
20
SPEC
PAE
-40
16
3rd Order IM Specification
3rd Order IMs
-50
12
5th Order IMs
-60
8
7th Order IMs
-70
4
-80
0
20
22
24
26
28
30
32
34
Average Output Power (dBm)
Slide 10
36
38
40
Power Added Efficiency (%)
IM5U
-20
Typical Basestation Power Amplifier
• OLD - Lots of Silicon Power (400 watts)! - But physically LARGE
• Power Density of < 10 watts per sq. inch
• NEW LDMOS
Power FET Modules
increase Power
Density to
25 to 100 Watts
per sq. inch
Slide 11
The Need for Smaller PA’s
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
Slide 12
….The Difference between an LDMOS
Transistor and a Silicon Carbide MESFET
for 30 Watts Output Power
LD-MOSFET
SiC MESFET
Slide 13
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
GaAs
p-HEMT
GaN
HEMT
Input
I max
Vmax f t /fmax Load
Power
Capacitance mA/mm (V)
GHz impedance (W)
Ohms
3 pF
600
20
30/90
33
~1
3 pF
1200
60
30/90
50
~6
- 10x less impedance transformation
- 5-10 x Higher Bandwidth
- Simpler, Smaller circuits, High Yield, Low cost
Slide 14
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
Gate Width
Power Transistor
< l/4
 Key figure of merit is how much power the device
can handle in terms of W/mm2 of die area
Slide 15
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
Gate pitch with Silicon
Gate pitch with SiC
Gate pitch with Silicon
Gate pitch with SiC
SiC delivers higher power from given chip area => SiC
has higher W/mm2 => reduces $/W
Slide 16
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
4” Si
3” SiC
Total die
Non-edge die
3-inch SiC
860
788
4-inch Si
532
484
Slide 17
High Power Density & PAE from SiC MESFETs
34
P2dB = 5.2 W/mm
GAssoc = 11.1 dB
PAE = 63%
PAE
Output Power (dBm)
40
30
28
20
Gain
24
14
40
Gain = 10 dB
40
30
35
20
W G = 8 mm
30
10
0.25-mm SiC MESFET
Freq. = 3.5 GHz
VDS = 50 V, VGS = -8 V
12
PAE = 45%
45
0
16
18
Input Power (dBm)
20
22
PAE (% )
POUT
26
P 3dB = 48 W (6 W/mm)
PAE (%) or Gain (dB)
50
30
50
60
Output Power (dBm )
32
50
70
10
Freq. = 3 GHz
V DS = 60 V
25
10
15
20
25
30
35
0
40
Input Power (dBm)
• Pulsed on-wafer power densities of 5-6 W/mm consistently achieved on
large FETs
Slide 18
SiC MESFET with 7.2 W/mm
8.0
Power Density = 7.2 W /m m
Power Density (W /m m )
7.0
Efficiency = 48%
6.0
5.0
4.0
3.0
2.0
W G = 0.25 m m
Freq. = 3 G Hz
1.0
0.0
0.00
V DS = 70 V
0.05
0.10
Input Power (W )
0.15
0.20
• Power density of 7.2 W/mm with 45% PAE at S-band demonstrates
the capability of the technology
Slide 19
Mobile Telephone Frequency Allocations
Slide 20
20-Watt Broadband SiC MESFET Amplifier
16
44
14
42
12
40
10
38
3GPP Test Model 1 with 16 DPCH
8
6
36
34
Gain
P1dBm
W-CDMA
4
P1dB (dBm)
Gain (dB)
CR22010 Balanced Broadband Amp
Vds=48V, Idq=500mA/Device; Untuned Prototype
32
2
30
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
Frequency (GHz)
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
Slide 21
75-Watt SiC MESFET Amplifier
2 GHz test fixture for 60 W
MESFET development
50
Power
PAE
48
40
46
30
44
20
42
10
Freq. = 2.0 GHz
40
28
30
32
34
36
Input Power (dBm)
38
0
40
• 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!
Slide 22
PAE (% )
Output Power (dBm )
50
Broadband SiC MESFET Amplifier
200 MHz to 2200 MHz
100 190 ohms
Drain Bias
In
Out
0.6
0.6
1.1
2.4
Gate Bias
All capacitor values in pF
Slide 23
0.5
Ultra Broadband Amplifiers
• Broadband for Multi-Frequency and Multi-Mode
Slide 24
GaN HEMTs for Power Amplifiers
Enabling Feature
Performance Advantage
High Power/Unit Width- Higher Smaller die size per Watt of output power
Watts/pF(10 x GaAs)
Ease of matching, HIGHER BANDWIDTH
High Voltage Operation
(3-5 x GaAs, 1.5-2 x LDMOS)
Eliminate / reduce step down
Capable of 10-50 Volt operation
High Efficiency
(> 60%)
Power saving, reduced cooling
High Cutoff Frequencies
High gain, high efficiency operation
(GaAs like, 15 GHz-mm ftLg )
Superior Thermal Conductivity Higher junction temperature, smaller pitch+
higher device density, ease of packaging
For GaN on SiC
Slide 25
GaN HEMT with 12 W/mm
42
Pout
Gain
40
12.1 W /mm
Output Power (dBm)
38
36
34
32
3.5 GHz
V ds = 90V
V gs = -2.4V
10 ms Pulses, 0.1% Duty Cycle
30
28
26
10
12
14
16
18
20
22
24
26
Gain (dBm)
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
28
Input Power (dBm)
• Peak pulsed power density of 12 W/mm on a 0.5-mm HEMT
• CW power from same device of 9 W/mm
Slide 26
-1.1 dB compression
Pout = 12.5 W
PAE = 46 %
50
f = 4 GHz
45
40
35
Pout
800
Vds = 30 V
Gain
700
PAE
Id
600
30
500
25
20
400
15
10
5
0
300
200
10
15
20
25
Pin (dBm)
30
 P1dB of 12 W, 46% PAE at 4 GHz at 30 Volts
Slide 27
35
Id (mA)
Pout (dBm), Gain(dB), PAE(%)
Packaged GaN HEMTs
Characterization of AlGaN/GaN HEMTs
Using Fixed Load at Varying Voltage Supply
Power sweep
Pout W=300mm
70
Vdd (V): 10, 15, 20, 25, 30, 35, 40
8
50
7
40
6
30
5
20
4
10
3
Drain efficiency at 3dB gain compression
vs. supply voltage
60
0
Increasing Vds
2
-1 0
1
-2 0
0
-3 0
70
60
50
DE(%)
9
Pout (W/mm)
f=8GHz
PAE (%)
PAE;
10
40
30
20
10
0
-5
0
5
10
15
20
25
5
15
25
Vdd (V)
Pin (dB)
Slide 28
35
45
Cellular Base-Station Application
Slide 29
10-Watt Broadband GaN HEMT Amplifier
44
24
P1dB (dBm)
42
22
40
20
38
18
36
16
34
14
32
12
30
10
1.5 1.6 1.7 1.8 1.9
2
2.1 2.2 2.3 2.4
Frequency (GHz)
• 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
Slide 30
Small Signal Gain (dB)
P1dB (dbm)
SS Gain (dB)
GaN HEMTs for Infrastructure:
2 GHz, CW Power from a 24-mm HEMT
Record Power exceeding 100 Watts
50
Peak Power= 108 W CW
Power Density = 4.5 W/mm
35
30
48
25
20
46
15
10
44
32
f = 2 GHz, Vds = 52 V
34
36
38
40
42
Input Power (dBm)
• 103 W at 2.6 dB gain compression
• Peak Drain Efficiency of 54 %
Slide 31
5
0
44
Gain(dB)
Output Power (dBm)
40
High Temperature Operation
• Demonstrated that at a TJ of 180OC
(case temperature of 120OC) SiC
MESFET has a mission life of > 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
Slide 32
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
Slide 33
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
Slide 34
Wide Bandgap enables
Tower-Top Power Amplifiers
Efficiency
= 50% x
85% x 30%
= 12%
If you’re
really
lucky!
Antenna with
Power amplifiers
@ 48 volts
½ X watts
Antenna
½ X watts
3dB Loss
in Cable
Efficiency
= 30%
Power
Amplifiers @
28 volts
(X watts)
Conventional
New Approach
Slide 35
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
Slide 36
Next Steps?
•
•
•
•
•
•
Productization
Customer “Education”
Reliability Facts
Acceptable Dollars/Watt
Introduction of RFICs
Nothing we haven’t been
through with other technologies
– WATCH THIS SPACE!
Slide 37