GaN HEMT Power Switch 의 특성 향상 방안

2004- 21648

최영환

Contents Introduction GaN HEMT 의 구조 및 특성 Power Switch Design

Breakdown voltage On-resistance Switching speed Substrate

Conclusion

Introduction GaN 특성

Wide band gap (3.4 eV) Very low intrinsic carrier concentration High breakdown voltage (VB EG

5)

High electron mobility 2D e- Gas (1300 cm2/Vs ~ 2000 cm2/Vs) Bulk GaN (900 cm2/Vs)

High saturation velocity 약 1.5x107 cm/s

Introduction

Material Bandgap (eV)Electron mobility (cm2/Vs)

Critical electric

field (V/m)

Thermal conductivity (W/m

K)

Tmax

(℃)

Si 1.1 1300 300,000 130 300

GaAs 1.4 5000 400,000 55 300

SiC 2.9 260 2,300,000 110 600

GaN 3.4 900 3,000,000 700 700

Properties of Competing Material on Power Electronics

Introduction

• Breakdown voltage 와 specific on-resistance 비교

• 타 물질보다 우수한 특성

GaN HEMT 의 구조

Substrate : sapphire or SiC

Nucleation layer

GaN

AlGaN

Source DrainGate

2DEG

• AlGaN/GaN interface• Large conduction band offset• Spontaneous polarization• Piezoelectric polarization

2D e- Gas (2DEG) 형성

• The lack of GaN substrate• Heteroepitaxy 필요

• Lattice mismatch• Nucleation layer 생성

Power Switch Design (1) High breakdown voltage

Field crowding 방지 Circular design

Electric field 완화 Field plate

Impact ionization 억제 Low gate leakage current

Circular design

• Field crowding

• 소자의 blocking 특성 저하 • Circular device design

• Electric field 의 고른 분산

Blocking 능력 저하 방지

Field plate

• Gate edge 의 electric field 완화• Electric field 를 보다 균일하게 분산

Breakdown voltage 증가

Double field plate

• Drain 에 field plate 추가• Drain edge 의 electric field 감소

향상된 breakdown voltage

Low gate leakage current

• Gate 절연막

• Gate leakage current 감소

• Impact ionization 억제

Breakdown voltage 증가

Power Switch Design (2) Low on-resistance

2DEG carrier density AlGaN layer 의 doping AlGaN layer 에 Al 함량 증가

Contact resistance Source/drain ohmic contact area Contact resistivity

RON-VB trade-off

• AlGaN layer doping 농도 증가 • Ohmic contact 면적 감소

• 낮은 contact resistivity

Breakdown voltage 감소

On-resistance 향상On-resistance 향상

Power Switch Design (3)

Switching speed Surface trap

Dielectric material Parasitic capacitance

Substrate

Surface trap

• Turn-off 상황• Gate 에서 electron 이 surface trap 으로 inject

• Channel 이 vertical depletion

• Electric field peak 감소

Breakdown voltage 증가

Dispersion

• I-V 특성 차이• Deep trap 에 의해서 발생• Slow emission process

• Dielectric material• SiN (shallow trap 형성 )• SiO2 (deep trap 형성 )

Switching speed 감소

Dielectric material 선택

• 이중 gate 절연막

• SiO2 와 Si3N4 의 장점을 결합

• Dispersion 감소• Shallow trap 형성

Switching speed 향상

Power Switch Design (4) Substrate

Conductive High breakdown voltage Large switching loss

Insulating Small switching loss Low breakdown voltage

SiC vs Sapphire Cost Thermal conductivity

SiC vs Sapphire (1)

• Sapphire substrate

• Breakdown voltage 감소

• Undoped AlGaN layer

• Gate leakage 감소

SiC vs Sapphire (2)

• Drain-source capacitance

• Switching speed 결정• Parasitic 성분이 dominant

• Insulating Substrate

• Parasitic capacitance 감소

Switching speed 향상

Power efficiency 향상

Switching speed 감소

Conclusion GaN HEMT

High power, high efficiency device GaN HEMT Design

Insulating substrate High breakdown voltage RONA 와 VB 의 trade-off 관계 개선 Shallow trap 형성

Future Work Epi 기술 개발 Cost 절감

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a, Tsuneo Ogura, and Hiromichi Ohashi, “High Breakdown Voltage AlGaN–GaN Power-HEMT Design and High Current Density Switching Behavior”, IEEE Trans. Electron Device, VOL. 50, NO 12, 2003

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