High Power GaN Based Power Amplifiers for Flexible RF ... · The Baliga high-frequency figure of merit A figure of merit is derived for power semiconductor devices operating in high-frequency

High Power GaN Based Power Amplifiers for Flexible RF

Communications

Compact Microwave Power

NMI Workshop: Devices, Sensors and Systems for Flexible RF Communications

Ian Davis: Diamond Microwave

Diamond Microwave

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Compact High Power RF Amplifiers based on Gallium Nitride (GaN) semiconductor technology for pulsed operation from 2GHz to 18GHz with Psat up to 1kW

04 Nov 2015 NMI Workshop: Devices, Sensors and Systems for Flexible RF Communications

Devices, Sensors and Systems for Flexible RF Communications

1. Flexible RF Communications – A digital agenda for Europe

2. Enablers: GaN

3. GaN: Practical Considerations

4. Enablers: Class S Amplifiers

5. Class S: Practical Considerations

6. Summary

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Content


Digital Agenda for Europe

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Image reproduced by courtesy of European Commission

The European Commission’s Digital Agenda for Europe


CAPACITY LATENCY RELIABILITY ENERGY EFFICIENCY

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Executive Summary

“Several challenges still need to be addressed …. energy efficiency (Target 90% less consumption for the same service compared to 2010 levels)”

“A new 5G system concept needs to drastically reduce the energy consumption per Mbit.”

(Figure of merit Mbits/Joule)




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The Roadmap published in the NetWorld2020 whitepaper identifies enablers to meet its challenging energy efficiency targets for wireless subsystems… “New transceiver designs based on GaN semiconductor technology to implement Class S amplifier topologies by 2020”


Enablers: Gallium Nitride Power


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Enablers: GaN

The Baliga high-frequency figure of merit A figure of merit is derived for power semiconductor devices operating in high-frequency circuits. Using this figure of merit, it is predicted that the power losses incurred in the power device will increase as the square root of the operating frequency and approximately in proportion to the output power. By relating the device power dissipation to the intrinsic material parameters, it is shown that the power loss can be reduced by using semiconductors with larger mobility and critical electric field for breakdown. Baliga, B.Jayant, "Power semiconductor device figure of merit for high-frequency applications," in Electron Device Letters, IEEE , vol.10, no.10, pp.455-457, Oct. 1989


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Enablers: GaN

After Diamond (which isn’t available) the second semi-conductor which offers high frequency switching capability is GaN.

Property Si GaAs 6H-SiC 4H-SiC GaN Diamon

d

Bandgap,Eg (eV) 1.12 1.43 3.03 3.26 3.45 5.45

Dielectric Constant, εr 11.9 13.1 9.66 10.1 9 5.5

Electric breakdown field, Ec (kV/cm) 300 400 2500 2200 2000 10000

Electron mobility, µn (cm2/V.s) 1500 8500 500 1000 1250 2200

Hole mobility, µp (cm2/V.s) 600 400 101 115 850 850

Thermal conductivity, λ (W/cmK) 1.5 0.46 4.9 4.9 1.3 22

Saturated electron drift velocity, vsat (x107 cm/s) 1 1 2 2 2.2 2.7

Johnson's Figure of Merit JFM - 1 1.8 277.8 215.1 215.1 81000

Baliga's Figure of Merit BFM - 1 14.8 125.3 223.1 186.7 25106

Baliga's HF Figure of Merit BHFFM - 1 16 13 34 100 1080


• Properties of GaN High Electron Mobility Transistors (HEMT)

- High breakdown field: 2 MV/cm

- High Vsat @ 2.5 x 107 cm/s - Thermal conductivity: 3 x GaAs - Large channel charge: > 1x1013 cm-2

- Good electron mobility: >1200 cm2/V-s - Power density 5 x GaAs: up to 12 W/mm

Standard AlGaN/GaN HEMT structure

Enablers: GaN

04 Nov 2015 NMI Workshop: Devices, Sensors and Systems for Flexible RF Communications Page 10

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Enablers: GaN

While GaAs has a basic power density of about 1.5 W/mm, GaN has a power density ranging from 5 to 12 W/mm. Higher W/mm results in less combining loss, hence increased Power Added Efficiency

GaAs GaN


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GaN I-V Characteristic

A GaN-based high-electron-mobility transistor (HEMT) is considered as an excellent candidate for future power devices due to its high breakdown voltage, high saturation drain current and low on-resistance (Ron).


GaN– Practical Considerations

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GaN I-V Characteristic

However, current collapse, i.e., dispersion of drain current or increased dynamic Ron, is regarded as one of the most critical issues to be solved for actual power-switching applications.


GaN– Practical Considerations

Enablers: Class S Amplifiers


• Today’s RF power amplifiers offer only limited efficiency (as low as 20%) for modulation formats with large peak-to-average power ratio (PAPR) and high linearity requirements.

• One method to overcome this problem is to use linearization techniques like enhanced digital pre-distortion that increase efficiency, but the fundamental problem remains that the power amplifier operates most of the time in back-off mode with the average power.

• Theoretically, Class-S Amplifiers can offer up to 100 percent efficiency, which is a key enabler for very compact transceivers. The design also offers multiband capabilities, which, together with metamaterial based RF duplex filters, and a frequency agile radio, could enable a universal single board transmitter.

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• A class S power amplifier is a non-linear switching mode amplifier similar in operation to the class D amplifier.

• As the digital signal of this switching amplifier is always either fully “ON” or “OFF” (theoretically zero power dissipation), efficiencies reaching 100% are possible.

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Sigma Delta

modulator driver

GaN Switching Amplifier

Band Pass Filter


The class S amplifier converts analogue input signals into digital square wave pulses by a delta-sigma modulator, and amplifies them to increases the output power before finally being demodulated by a band pass filter.


Reaching the target of a universal high efficiency switch mode RF GaN-Based power amplifier that can be reconfigured to operate at different frequency bands with high efficiency requires solving a number of essential challenges, including but not limited to:

• Wideband Amplifiers: The specific challenges here are to maintain a good rectangular shape of the signal. Considering the harmonics of rectangular signals, for a 1GHz transmit frequency a bandwidth in the order of 20 GHz is needed (Estimate).

• Speed of the switching amplifier: The switching amplifier typically must switch 4 times faster than the RF carrier frequency. To transmit a 2GHz signal requires a sample rate of 8Gs/s

• Current Collapse in GaN HEMT devices: Increased dynamic on-resistance, is regarded as one of the most critical issues to be solved for actual power-switching applications.

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Class S – Practical Considerations


Wideband GaN HEMT:

GaN has high electron mobility, meaning it has the potential to amplify signals well into the upper-gigahertz ranges.

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Devices available which operate from DC to 18GHz

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Transistors’ parasitic elements limit the maximum switching conditions, reduce power capability, and decrease maximum operation frequency.

Minimizing all parasitic components is particularly critical during the design of high-efficiency Switch-Mode Power Amplifiers (SMPA)


Speed of the switching amplifier:

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Overcoming Parasitic Elements at the Amplifier Module level:

1. Minimise all path lengths - Locate Drain Current supply close to the RF circuits

2. Avoid thermal degradation (run cool!)

3. Reduce pulse transient effects (overshoot) by compensating for reactive components in the matching networks.


Page 22 04 Nov 2015 NMI Workshop: Devices, Sensors and Systems for Flexible RF Communications


Impact of increased dynamic on-resistance (Ron) on a Class S amplifier

Studies indicate that the current collapse is suppressed by good passivation film, changes to the device structure and optimised bias conditions.

I

Effect of increased Ron

Distortion of the signal when demodulated by a band pass filter

Current Collapse in GaN HEMT devices

Summary


CLASS-S amplifiers implemented using GaN HEMT semiconductors promise potential benefits to RF Communication applications including:

• First approximations have shown a significant improvement in efficiency - 50 to 60 percent as compared to 30 percent with pre-distorted Class-AB amplifiers and to 35 to 40 percent with Doherty architectures.

• The increased efficiency leads to significant savings in cooling, which reduces CAPEX.

• By reducing the power consumption of RF power amplifiers directly, operators also can realize significant savings in electricity, extending battery life or reducing cost.


Summary

CLASS-S amplifiers implemented using GaN HEMT semiconductors promise potential benefits to RF Communication applications including:

• First approximations have shown a significant improvement in efficiency - 50 to 60 percent as compared to 30 percent with pre-distorted Class-AB amplifiers and to 35 to 40 percent with Doherty architectures.

• The increased efficiency leads to significant savings in cooling, which reduces CAPEX.

• By reducing the power consumption of RF power amplifiers directly, operators also can realize significant savings in electricity, extending battery life or reducing cost.

BUT … Viable Class S amplifiers for RF Comm applications have yet to be demonstrated. Very significant issues are yet to be overcome for implementation in 2020 to be realised. Literature search identified 1GHz as ‘best of class’.


Summary

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High Power GaN Based Power Amplifiers for Flexible RF ... · The Baliga high-frequency figure of merit A figure of merit is derived for power semiconductor devices operating in high-frequency