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GaN HEMTs for RF Applications

March 5, 2024

^*E-mail: nadim@eee.buet.ac.bd

Acknowledgments

FA9550-22-1-0367

(K. Goretta)

M.A. Jabbar

Fermi-Level Pinning Effect in Gate Region

A Case Study of Multimetal Gated AlGaN/GaN HEMT�for High RF Linearity

Toiyob Hossain^1,2, Bejoy Sikder¹, Md. Tasnim Azad¹, Qingyun Xie³, Mengyang Yuan³, Eiji Yagyu⁴, Koon Hoo Teo⁵,Tomás Palacios³ and Nadim Chowdhury^1,*

¹Dept. of EEE, Bangladesh University of Engineering and Technology, Dhaka-1205, Bangladesh

²Neural Semiconductor Limited, Dhaka-1230, Bangladesh

³Massachusetts Institute of Technology, Cambridge, MA 02139, U.S.A.

⁴Mitsubishi Electric Corporation, Amagasaki 661-8661, Japan

⁵Mitsubishi Electric Research Laboratories, Cambridge, MA 02139, U.S.A.

8^th IEEE Electron Devices Technology and Manufacturing Conference

March 3–6, 2024 Bangalore, India

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Motivation for high RF linearity
Multimetal Gated AlGaN/GaN HEMT
Fermi-level pinning (FLP) effect in gate region
Robustness of MMG HEMT to FLP
Summary

Fermi-Level Pinning Effect in Gate Region

A Case Study of Multimetal Gated AlGaN/GaN HEMT for High RF Linearity

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Efficiency issue in 5G Telecommunication

High data rate

(peak 10/20 Gbps)

RF Front

End Module

Power amplifiers plays a dominant role in linearity and efficiency of the transmitter

Vision of 5G telecommunication

Efficiency

~10%

4G

~ 35-45%

Superior performance

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Linearity-Efficiency Trade off in GaN PAs

[1] Joglekar et al. IEEE IEDM (2017)

[1]

However, Efficiency remains a concern for GaN Power Amplifiers.
To understand it, we need to look into this graph. It shows output power and efficiency vs input power.
GaN PA's maximum efficiency point of operation is often highly non-linear.
So to meet the linearity of signal, we have to back of from the maximum efficiency point to a linear region.

This power back-off eventually leads to the drop in efficiency of the Power amplifiers.

So, we need to make the devices more linear right? So that we do not need to back off much.
In that regard, what is source of non-linearity in power amplifiers? When we input a signal we will be having harmonic outputs along with the fundamentals.
In recent reports, A breakdown of intermodulation contribution shows, a large portion of GaN HEMT non-linearity emerges from transconductance and its derivatives, especially the second order transconductance called gm3.

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Recent Works on Linearity of GaN

Epitaxial Modification

PolFET

OSU [1]

Graded channel GaN HEMT

HRL/ UND [2]

AlGaN/GaN/- Graded-AlGaN:Si-doped/GaN

Xidian U.[3]

Coupled Double Channel

HKUST [4]

Device Architecture

Variable Fin Width

MIT [5]

Selective Charge Implantation

Xidian U.[6]

Transitional Recessed

Gate

Xidian U. [7]

[1] Sohel et al. IEEE EDL (2019)

[2] Moon et al. IEEE IMS (2020)

[3] Yu et al. IEEE EDL (2023)

[4] Song et al. IEEE EDL (2021)

[5] Joglekar et al. 2017 IEEE IEDM (2017)

[6] Zhang et al. IEEE EDL (2022)

[7] Wu, et al. IEEE EDL (2019)

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Variable Fin Width

MIT [1]

Implanting region

Non -Implanting region

Selective Area Charge Implantation

Xidian U. [2]

[1] Joglekar et al. 2017 IEEE IEDM (2017) [2] Zhang et al. IEEE EDL (2022) [3] Azad et al. IEEE T-ED (2023).

Multimetal Gated HEMT

BUET [3]

Parallel Connected HEMT

Recent Works on Linearity of GaN

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Motivation for high RF linearity
Multimetal Gated AlGaN/GaN HEMT
Fermi-level pinning (FLP) effect in gate region
Robustness of MMG HEMT to FLP
Summary

Fermi-Level Pinning Effect in Gate Region

A Case Study of Multimetal Gated AlGaN/GaN HEMT for High RF Linearity

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Multimetal Gated HEMT

[1] Azad et al. IEEE T-ED (2023).

Multimetal Gated HEMT

BUET [1]

Device to Circuit

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g_m3 minimization in MMG HEMT

We have talked about that gm3 is one of the main factor in GaN HEMT linearity. Having a low gm3 will work in the benefit of the device. In this slide we will be discussing, how MMG HEMT achieves linearity through gm3 suppression.

We can see in this curve, Different metals have different work-functions, and this will lead to different threshold voltages as we will be having different Schottky barrier height of the metal/AlGaN contact.

when these metals are placed along the width of the devices, we get modulated threshold voltage and spatial positions of gm3 in the Vgs axis. The superposition of gm3 profiles of different metals correspond to minimization scope. In the minimization scope, metal widths can be adjusted to get a low gm3 peak value.
In the MMG HEMT paper, we reported a 3 times reduction of gm3 peak by using this technique.

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Large Signal Results of MMG HEMT

Fermi Level Pinning needs to be considered

Along with the device level improvement of the gm3, the MMG HEMT showed significant improvement in Gain, OIP3/Pdc etc.
In this curve we can see that the Gain of MMG HEMT keeps linear for larger portion of output power. We compared the devices in 4 quiescent current bias points. At 70 mA/mm, which is the deep class AB operation, MMG HEMT shows 6.2 dB improvement in OIP3/Pdc
One crucial thing in MMG HEMT is the shottky barrier height variation due to metal work-function variation. This barrier height is affected by Fermi Level Pinning. Fermi level Pinning is the behavior at metal-semiconductor interface that defies Schottky-mott model and creates unexpected height of the Schottky barrier.
It can be represented by the Schottky slope parameter S, named as Fermi level Pinning factor.
In our previous work, Fermi level pinning was not considered. We present this work with the inclusion of FLP as a practical consideration for MMG HEMT.

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Motivation for high RF linearity
Multimetal Gated AlGaN/GaN HEMT
Fermi-level pinning (FLP) effect in gate region
Robustness of MMG HEMT to FLP
Summary

Fermi-Level Pinning Effect in Gate Region

A Case Study of Multimetal Gated AlGaN/GaN HEMT for High RF Linearity

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Fermi Level Pinning in Gate Region

FLP reduces the range of V_t

MMG HEMT works on the principle of V_t modulation.

S = 1.00

S = 0.70

S = 0.43

Ta

Ti

Cr

W

Ag

Ru

Ni

Au

Ir

Pt

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g_m3 Suppression in the Presence of FLP

The optimized results benefit from the consideration of FLP

4 metals

6 metals

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Motivation for high RF linearity
Multimetal Gated AlGaN/GaN HEMT
Fermi-level pinning (FLP) effect in gate region
Robustness of MMG HEMT to FLP
Summary

Fermi-Level Pinning Effect in Gate Region

A Case Study of Multimetal Gated AlGaN/GaN HEMT for High RF Linearity

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Results and Discussion

10.7 dB improvement of OIP3/P_DC

S = 0.43

S = 0.70

S = 1.00

Conv.

Dotted: MMG(S=0.43)

Solid: Conv.

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Results at Output Power Back-off (OBO)

IMD3 13.9 dB lower at 6 dB OBO

A higher PAE at high OBO is desired

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Results at High Power

1

2

3

4

S = 1

S = 0.70

S = 0.43

Delayed gain compression

Better Linearity

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Summary

Fermi-Level Pinning Effect in Gate Region

A Case Study of Multimetal Gated AlGaN/GaN HEMT for High RF Linearity

Multimetal Gated HEMT provides Higher Linearity

Fermi Level Pinning Effect in Gate Region

FLP reduces the range of V_t, hence important in MMG

Robustness of MMG HEMT to FLP

Finite FLP factors of 0.70 and 0.43 shows device level g_m3 suppression

Corresponding Author

Nadim Chowdhury

nadim@eee.buet.ac.bd