Examples of High-speed Harmonic Load Pull

Investigations of High-Efficiency GaN Power

Transistors

Mauro Marchetti1, T. Maier2, V. Carrubba2, S. Maroldt2, M. Mußer2, R. Quay2

1Anteverta-mw B. V., Feldmannweg 17, 2628 CT, Delft, The Netherlands2Fraunhofer Institute for Applied Solid-State Physics (IAF), Tullastr. 72, 79108

Freiburg, Germany

Outline

• Introduction

• Technical overview of available load-pull techniques

• MT2000 mixed-signal active harmonic load-pull system

• Harmonic load-pull examples on AlGaN/GaN power

transistors:

– S band measurement examples

– X band measurement examples

• Conclusions

Load pull

1) Present impedance to DUT

2) Measure parameters (Pout, Eff, etc.)

3) Repeat 1+24) Determine ideal

impedance for application

Airline

Probe

VSWR α Gamma α 1/Ω10:1 VSWR = Γ=0.82 = 5Ω20:1 VSWR = Γ=0.9 = 2.5Ω

Γ = a/b

Mechanical TunerGamma comes from probe (slug) inserted into airline

Airline

X YProbe

Y

X

Passive tuning

Multi-carriage tuners for harmonic load-pull

Active Tuner Gamma comes from signal generator and amplifier

Γ=1 or Γ>1

Multiple generators are used for harmonic tuning

Active tuning

b2

a2 A fA

Amplifier

Active Load

DUT

ΓLoad

b2

a2 A fA

Amplifier

2f0 LoadDUT

ΓLoad

A fA

Amplifier

f0 Load

A fA

Amplifier

3f0 Load

Load Tuning Techniques: Summary

Passive Systems Active Systems

Simplicity & high-power handling capabilities

× Constrained by losses (Γ1)

Fast (3-5 times faster than Passive Systems)

Modulation is possible with mixed signal active

× Traditionally narrowband (multiple signal generators)

× High power amplifier

b2

a2 A fA

Amplifier

Active Load

DUT

ΓLoad

b2

a2 A fA

Amplifier

Active Load

DUT

ΓLoad

TUNER

Active loads and mechanical tuners

can be combined: Hybrid active

load pull

All the advantages of active load pull

Improve power handling/reduce

required power for active load

× Increased system complexity/cost

Traditional load pull (SG+PM)

Traditional load pull (SG+PM)

Measure scalar power parameter from power meter or spectrum analyzer

Measurements are de-embedded to DUT reference plane from power calibration data

Relies on pre-calibration repeatability and accuracy of tuner for presenting impedances

No information about large signal input impedance of DUT, do not know delivered input power

Signal Type: CW, pulsed-CW, two-tone, modulatedMeasured Parameters: Pin_avail, Pout, Transducer Gain, Efficiency, IMD, ACPR, EVM

Traditional load pull (SG+PM)

2 2 2 22 2 21 1

12 2

out loadP b a b

2 2 2 2, 1 1 11 1

12 2

in del inP a b a

2 2

2

2 2, 1

1

1

loadout

p

in del in

bPG

P a

,out in del

DC

P PPAE

P

Vector-receiver load pull (VNA)

Vector-receiver load pull (VNA)

Main Advantages:

• Measure vector a- and b-waves instead of scalar parameters Pin delivered, ΓIN and

therefore true Power Gain and PAE, AMPM

• Measurements are made at calibrated DUT reference plane no de-embedding\

• Most accurate measurement does not rely on pre-calibration repeatability or

accuracy of tuner

• Allows Active Load pull

Signal Type: CW, pulsed-CW, two-toneMeasured Parameters: Pin_avail, Pin_del, Pout, Transducer Gain, Power Gain, PAE, IMD, AM/PM, Time domain voltage and current, Behavioral modeling

Vector-receiver load pull

Mixed-signal active load-pullMixed Signal Active Load Pull (MT2000) combines traditional analog and microwave techniques with low-frequency signal acquisition (A/D converters) and generation (Wideband AWG).

Main Advantages:

High Gammas (Γ=1 or Γ>1) Fast (100 times faster than traditional systems) Full control of f0, 2f0 and 3f0 source and load impedancesUp to 240 MHz Modulation Bandwidth control (complete control of reflection coefficient over the bandwidth) Turn-Key System (no need for external equipment: NVNA, Synthesizer, PM)

Input section

To DC

Bias

Tee

I V

On-wafer configuration

To DC

I V

Output section

LO LO

PA @ 2f0

PA @ f0

Bias

Tee

PA @ 2f0

PA @ f0

DUT

a1 b1 b2 a2

PXIe system

PA @ 3f0 PA @ 3f0

f02f03f0

LOLO

f0 2f0 3f0

AWG AWG AWG AWG AWG AWGADC ADC ADC ADCTrigger and Clock

High Speed Capability

MT2000/MT1000 approach:

• Multiple powers and terminations can be measured at once, by using multiple time segments with different amplitude and phase information

• Full synchronization between measurement andsignal generation is used to track the info embedded ineach wave segment

• Load pull sweeps iterations are performed to obtain thedesired Γ for each segment.

Mixed-signal load-pull, thanks to signal processing at low frequency, is able to set multiple impedance points at the same power level resulting in improvements in terms of speed (up to100 times faster).

Measurements Examples: High Speed

10 mm GaN device peak output power ~ 10 W

• Measurement conditions

• Swept available power:

• 15 to 27 dBm (1 dB step)

• Swept load fundamental :

• 64 load impedances

768 measurement

points in30 sec!

Optimum fundamental load

69.2 % PAE @35.3 dBm

Modulated Signal Capability

TUNER Adaptor

Cable

Probe

phase delay

MT2000 approach:

• A non-linear device excited with a modulated signal will emanate waves (b-waves) at the harmonic and baseband frequencies

• MT2000 divides the BW in multiple tones (thousands) and sets by software iterations the desired impedance versus each tone.

The electric delay phenomena in conventional passive and active setups limits the use of modulated signals. An uncontrolled Gamma shift will be presented to the DUT reference plane.

This phenomena is eliminated completely in MT2000, where a full control of the impedance versus bandwidth is performed on the DUT reference plane.

Passive Trad LP Passive VR LP Hybrid-Active LP Mixed Signal LP

VSWR/Γ atDUT

Relatively low VSWR (reduced further by losses of cables, fixtures…)

Relatively low VSWR -reduced from trad LP by addition of coupler

Γ=1 or Γ>1 possible Γ=1 or Γ>1 possible

HarmonicImpedance Control

Available (requires one tuner per freq or large multi-freqtuner)

Available (requires one tuner per freq or large multi-freq tuner)

Available (requires one signal source per harmonic freq)

Integrated with 4-loop system

Tuning Speed Relatively slow tuning speed due to mechanical movements (1-5 seconds)

Relatively slow tuning speed due to mechanical movements (1-5 seconds)

Relatively slow tuning on mechanical paths, faster tuning speeds on active paths (0.3-1 seconds)

Up to 1000 impedance/power states per minute

Measurement Speed

Relatively slow measurements due to instruments (1-15 seconds)

Faster measurement speeds (

Passive Trad. LP Passive VR LP Hybrid-Active LP Mixed Signal LP

Signal Types CW, Pulsed-CW, Modulated

CW, Pulsed-CW CW, Pulsed-CW CW, Pulsed-CW, Modulated

WidebandImpedance Control

Uncontrolledwideband

Uncontrolledwideband

N/A Up to 240 MHz instantaneous impedance control

On-Wafer Integration

Requires appropriate probe station and/oradvanced integration

Requires appropriate probe station and/oradvanced integration

Requires appropriate probe station and/oradvanced integration

Easy, no vibration

Oscillations Risk due to wideband reflection

Risk due to wideband reflection

Risk due to wideband reflection

Low probability

Time-domain No Yes (NVNA dependent)

Yes (NVNA dependent) Yes (direct support)

Production Testing

Fixed impedance only

Fixed impedance only

Fixed impedance or basic load pull

Advanced load pull and power sweep

UpgradableCapabilities

Requires new instruments (if available)

Requires new instruments (if available)

Requires new instruments (if available)

Software/hardware upgradable

Comparison of Solutions

Data Comparison

GaN 10 W, 2.5GHz CW

Case study: IAF AlGaN/GaN Technology

• Fraunhofer IAF AlGaN/GaNHEMT grown on a 3-/4- inch semi-insulating SiC substrate

• LG= 250 nm

• Harmonic load-pull is used to search for peak PAE to evaluate the technology:

1. 6x200 um device at 2 GHz

2. 8x125 um device at 8.7 GHz

• The following examples are performed with mixed-signal active load-pull

6x200 um f0=2 GHz - fundamental load-pull

• fundamental load impedance (ZL,F0) sweep

• second and third harmonic load (ZL,2F0, ZL,3F0) and the second harmonic source ZS,2F0 at 50 Ohm

• A peak PAE of 67.9 % is found

• Measurement time is below 1 minute

Pout, W

6x200 um f0=2 GHz - harmonic load pull

• ZS,2F0 , ZL,2F0, ZL,3F0 are swept while keeping ZL,F0 tuned at the optimum for PAE

• Final fundamental load-pull ZL,F0 sweep is conducted while keeping ZS,2F0, ZL,2F0 and ZL,3F0 at the optimum

• Peak PAE of 86.7% at Pout = 3.6 W

•DE, power gain GP, and transducer power gain GT of 90%, 14.3 dB and 10 dB

6x200 um f0=2 GHz – Time domain

• Drain voltage and current waveforms at peak PAE (ZS,2F0=91.7+j14.4 Ω, ZL,F0=140+j92 Ω, ZL,2F0=1+j100 Ω and ZL,3F0=0+j0 Ω).

• Calculated at the device intrinsic plane (after de-embedding output capacitance CDS+CGD=0.34 pF)

•Peak voltage reaches 3 times VDDdue to non zero phase of second harmonic load termination (class-F like waveforms)

•Very high efficiency thanks to low Vknee

8x125 um f0=8.7 GHz – fundamental load-pull

•fundamental and second harmonic load impedances (ZL,F0, ZL,2F0) , the second harmonic source impedance (ZS,2F0) have been tuned for peak PAE.

• fundamental load impedance (ZL,F0) sweep

• second harmonic load ZL,2F0 and the source ZS,2F0 at 50 Ohm

• A peak PAE of 59.7% is found at Pout=36.1 dBm

8x125 um f0=8.7 GHz – optimizing harmonics

• A systematic measurement procedure is followed where each harmonic is optimized independently. At each step ZL,F0 is re-tuned:

1. ZL,2F0 sweep while keeping ZL,F0 at peak PAE. Max PAE 61.3%.

2. ZL,F0 re-tuning while keeping ZL,2F0 at peak PAE. Max PAE 63.4%.

3. ZS,2F0 sweep while keeping ZL,F0 and ZL,2F0 at peak PAE. Max PAE 65.3%.

4. Final ZL,F0 re-tuning while keeping ZL,2F0 and ZS,2F0 at peak PAE.

• Each measurement takes between 1 and 6 minutes.

8x125 um f0=8.7 GHz – final ZL,F0 re-tuning

• Final performance shows PAE=66.1 %, drain efficiency of 71.2% , while delivering Pout=35 dBm (3.2 W) and GP=11.5 dB.

• Thanks to the high speed of the mixed-signal active load pull a complete wafer can be scanned in an automated fashion, and each measurement on a DIE only takes a couple of seconds

• User-level API interface allows users to setup custom test patterns from their own testing environment (C/C++, NI Labview, Agilent Vee, etc.).

• Software allows easy integration with Cascade Nucleus for probe station control.

High-speed wafer mapping

High-speed wafer mapping

Pin_del (dBm)

PA

E(%

),

Gp

(dB

),

Po

ut(

dB

m)

Pin

_av

(dB

m)

• IAF custom developed software to perform complete automated wafer mapping

• The whole wafer can be measured in a fully automated approach to establish, e.g. the PAE variation over various wafers (evaluate process variations)

• The distribution of the most important parameters show the devices deliver Pout> 35 dBmGp>12dB with an average PAE of 65% across the whole wafer.

Conclusions

• An overview of the different solution for (harmonic) load pull has been discussed:– Scalar passive harmonic load pull

– Vector receiver passive harmonic load pull

– Hybrid active harmonic load pull

– Mixed signal active harmonic load pull

• A few user’s examples on GaN power transistors have been shown

– 6x200 um GaN HEMT at f0=2 GHz

– 8x125 um GaN HEMT at f0=8.7 GHz