AlGaN HEMT

7/29/2019 AlGaN HEMT

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U.S. Department of Defense

UCS

UCSBUCS

UCSB

AlGaN/GaN HEMTs and HBTs

Umesh K. Mishra


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UCS

UCSBUCS

UCSB

PART I

AlGaN/GaN HEMTs


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UCS

UCSBUCS

UCSB

1500

260

5000

1300

TmaxJFM

Ratio

BFOM

Ratio

EgMaterial

9.5

9.7

13.1

11.4

700 C8024.63.4GaN

600 C603.12.9SiC

300 C3.59.61.4GaAs

300 C1.01.01.1Si

BFOM = Baligas figure of merit for power transistor performance [K**Ec3]JFM = Johnsons figure of merit for power transistor performance

(Breakdown, electron velocity product) [Eb*Vbr/2]

Materials Properties Comparison


4/42U.S. Department of Defense

UCSUCSBUCSUCSB

NeedNeed Enabling FeatureEnabling Feature Performance AdvantagePerformance Advantage

High Linearity

High Frequency

TechnologyLeverage

High TemperatureOperation

HEMT Topology

High Electron Velocity

Wide Bandgap

Direct Bandgap:Enabler for Lighting

High Power/Unit Width Wide Bandgap, High Field Compact, Ease of Matching

High Efficiency High Operating Voltage Power Saving, ReducedCooling

Optimum Band Allocation

Bandwidth, -Wave/mm-Wave

Rugged, Reliable,Reduced Cooling

Driving Force for Technology:Low Cost

High Voltage Operation High Breakdown Field Eliminate/Reduce Step Down

Low Noise High gain, high velocity High dynamic range receivers

ThermalManagement

SiC Substrate High power devices withreduced cooling needs

NeedNeed Enabling FeatureEnabling Feature Performance AdvantagePerformance Advantage

High Linearity

High Frequency

TechnologyLeverage

High TemperatureOperation

HEMT Topology

High Electron Velocity

Wide Bandgap

Direct Bandgap:Enabler for Lighting

High Power/Unit Width Wide Bandgap, High Field Compact, Ease of Matching

High Efficiency High Operating Voltage Power Saving, ReducedCooling

Optimum Band Allocation

Bandwidth, -Wave/mm-Wave

Rugged, Reliable,Reduced Cooling

Driving Force for Technology:Low Cost

High Voltage Operation High Breakdown Field Eliminate/Reduce Step Down

Low Noise High gain, high velocity High dynamic range receivers

ThermalManagement

SiC Substrate High power devices withreduced cooling needs

Advantages of WBG Devices

CS

CS



UCSUCSBUCSUCSBDC Device Level Issues (HEMTs)

If it aint good @ DC it aint goin to be good @ RF

SG

D

Maximize I Maximize nS,

Maximize nS Maximize PSP, PPE Maximize Al mole fraction

without strain relaxation

Mazimize Minimize effective gatelength

Minimize Lg and gatelength extension

Maximize Minimize dislocations Smooth interface

- - - - - - - - - - - - -AlGaN PSP + PPE+ + + + + + +

Lg

GaN

Vmax

Imax

V

- - - - - - - - - - - - - - - - - - - -

max max max18

S

P V I

I V n

=

=

UCS

UCSBUCS

UCSB



UCSUCSBUCSUCSBIssues With Mazimizing Al Mole Fraction in AlxGa1-xN

Eg

Lattice Constant

GaN

AlN

InN

AlxGa1-xN

G

Pseudomorphic Relaxed with Misfit Morphology MediatedBy Dislocaton

DISLOCATIONS LEAD TO PREMATURE RELAXATION OF AlGaN AND A POTENTIALRELIABILITY PROBLEM BECAUSE OF THE METALLIZED PITS

xAl = 0.2 xAl = 0.4 xAl = 0.6

250 nm 250 nm 250 nm

xHC= 0.3

relaxed

GaN

2.5 3 3.5

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSBIssues With Increasing Mobility

0

5

10

15

20

25

30

0

1 104

2 104

3 104

4 104

5 104

6 104

7 104

10 100 1000

NS,

1012c

m-2

,cm2

/Vs

1000/T, 1/K

x = 0.09

High Mobility AlGaN/GaN StructuresGrown by RF Plasma Assisted MBE

xAl

0.0 0.2 0.4 0.6

ns

[1

013cm-2]

0.60.81.0

1.21.41.61.8

300

300

300

300

[cm2/Vs]

0

1000

1100

1200

1300

1400

1500

INCREASING Al MOLE FRACTION DECREASES MOBILITY

RMS [nm]

0.1 0.2 0.3 0.4 0.5 0.6

300

300

300

300

[cm

2/Vs]

600

800

1000

1200

1400

Al0.3

Ga0.7

NMobility v. Al Fraction Plot

Relaxed

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSBMinimizing Gate Length Extension

ELECTRONS IN SURFACE STATES AND/ORBUFFER TRAPS DEPLETE THE CHANNELCAUSING GATE LENGTH EXTENSION

SEVERE CONSEQUENCE: DISPERSIONBETWEEN SMALL SIGNAL AND LARGESIGNAL BEHAVIOR BECAUSE OF THE LARGETRAP TIME CONSTANTS

Vds

(V)

Id(5mA

)

DC

AC

Load line

Dispersion

WHY DO THESE TRAPS ARISE?

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

GaAsGaAs High PowerAmplifier Module

Equivalent

High Power

GaNGaN AmplifierModule

I

N

O

U

T

I

N

O

U

T

10-x power density ( > 10 W/mm)

10-x reduction in power-combining

Improved efficiency (> 60 %)

Improved reliability

Compact size

Superior Performance at reduced costGaAsGaAs High PowerAmplifier Module

Equivalent

High Power

GaNGaN AmplifierModule

I

N

O

U

T

I

N

O

U

T

I

N

O

U

T

I

N

O

U

T

10-x power density ( > 10 W/mm)

10-x reduction in power-combining

Improved efficiency (> 60 %)

Improved reliability

Compact size

Superior Performance at reduced cost

Example of Advantage of WBG Devices


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UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

AlGaN

2DEG

SOURCE DRAINGATE

GaN

Nucleation

LayerGaN, AlGaN or AlN

Substrate: Typically Sapphire or SiC

SiN Passivation

Schematic of Device Structure

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSBBall and Stick Diagram of the GaN Crystal

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSBPolarization World

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

AlxGa1-xN

GaN

+ + + + + + + + + + +

+ + + + + + + + + + +

,Q AlGaN

,Q AlGaN

,Q GaN

,Q GaN

+ + + + + + + + + + +

( ) ( )( ) ,,x GaNAlGaNP Q Q = +

includes the contribution of spontaneous and piezo-electric contributionsQ

How does the electron gas form in AlGaN/GaN structures? - A

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

InxGa1-xN

+ + + + + + + + + + +

+ + + + + + + + + + +

,Q InGaN

,Q InGaN+

,Q GaN

,Q GaN

( ) ( )( ) ,,x InGaNGaNP Q Q = +

includes the contribution of spontaneous and piezo-electric contributionsQ

How does the electron gas form in AlGaN/GaN structures? - B

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

GaNAlGaN

sp2

( )Q cm

sn2( )Q cm

vE

,Eg GaN

l , Maximum Dipole MomentA GaN g GaN vV E E+ =!

How does the electron gas form in AlGaN/GaN structures? - C

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

EFGaNAlGaN

EDD

DD+ 2( )Q cm

Q sn

How does the electron gas form in AlGaN/GaN structures? - D

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

Vds

(V)

Id(5

mA)

DC

AC

Load line

Dispersion

Dispersion in AlGaN/Ga HEMTs

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

Lg

AlGaN

GATE

Electrons in

Surface States

GaN

Source Drain

Depletion of 2DEG

caused by occupied

surface states

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSBPerformance of Passivated AlGaN/GaN HEMT on Sapphire

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30

0

20

40

60

80

100

120

140

160

Pout

Gain

PAEId

Pin

(dB)

P

out

(dB),Ga

in(dB),PAE(%)

Id(mA)

Pout = 10.3 W/mmPAE= 41.6%

f=8GHz,Tuned for Power

0

5

10

15

20

25

30

35

40

45

0 5 10 15 20 25 30

0

20

40

60

80

100

120

140

160

Pout

Gain

PAEId

Pin

(dB)

P

out

(dB),Ga

in(dB),PAE(%)

Id(mA)

Pout = 10.3 W/mmPAE= 42%

f=8GHz,Tuned for Power

Performance of AlGaN/GaN HEMT on SiC (CLC)

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

0

1

2

3

4

5

6

7

8

9

10

-5 0 5 10 15 20 25

-30

-20

-10

0

10

20

30

40

50

60

70

Pin (dB)

Pout(

W/mm)

f=8GHz

PAE(%)

Increasing Vds

PAE; Pout

0

1

2

3

4

5

6

7

8

9

10

-5 0 5 10 15 20 25

-30

-20

-10

0

10

20

30

40

50

60

70

Pin (dB)

Pout(

W/mm)

f=8GHz

PAE(%)

Increasing Vds

PAE; Pout

Drain Bias Dependence of Rf Power (CLC)

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSBFlip-chip AlGaN/GaN HEMT for Thermal Management

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

Vds (V/divsion)

Id(A/d

ivsion)

Vg start: +2V, Step: -2 V

I-V Curves from 8mm-wide HEMT

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSBLow Flip-chip Wide Bandwidth Amplifier

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

0

5

10

15

20

25

30

35

40

18 22 26 30 34 38 42

10

15

20

25

30

35

40

45

50

GainDEPAEPout

Pin (dBm)

Gai

n(dB),DE(%),PA

E(%)

Pout(dBm)

0

5

10

15

20

25

30

35

40

18 22 26 30 34 38 42

10

15

20

25

30

35

40

45

50

GainDEPAEPout

Pin (dBm)

Gai

n(dB),DE(%),PA

E(%)

Pout(dBm)

Pulse Power Performance of mm-flipped Device

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

IncludesIncludes ALL LEADINGALL LEADING players in the fieldplayers in the fieldCREE =CREE = Cree LightingCree Lighting++ CreeCree--DurhamDurham

1996 1998 2000 2002

Total power

Year

Cree

Totalpower(W)

HRL

CREE

Cree

UCSBCornell

0

10

20

30

40

50

60

Early

Player

s

Sapphire

SiC

0

1

2

3

4

5

6

7

8

9

10

1996 1998 2000 2002Year

Powerde

nsity(W/mm)

CREE

Cree

Cree

Cornell

UCSB

HRL

Early

Play

ers

Sapphire

SiC

11 Power densityCornell

UCSB

NEC

IncludesIncludes ALL LEADINGALL LEADING players in the fieldplayers in the fieldCREE =CREE = Cree LightingCree Lighting++ CreeCree--DurhamDurham

1996 1998 2000 2002

Total power

Year

Cree

Totalpower(W)

HRL

CREE

Cree

UCSBCornell

0

10

20

30

40

50

60

Early

Player

s

Sapphire

SiC

0

1

2

3

4

5

6

7

8

9

10

1996 1998 2000 2002Year

Powerde

nsity(W/mm)

CREE

Cree

Cree

Cornell

UCSB

HRL

Early

Play

ers

Sapphire

SiC

11 Power densityCornell

UCSB

NEC

Power Performance vs. Year

Cree108W,

CW Cree

UCS

UCSBUCS

UCSB


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UCSUCSBUCSUCSB

Part II

High Voltage Operation (> 330 V) ofAlGaN/GaN HBTs

UCS

UCSBUCS

UCSBBipolar transistor key issues


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UCSUCSBUCSUCSBBipolar transistor key issues

Injection

!1

n! 1[I=I0 exp (qv/nkt)]

Output ConductanceOutput Conductance

IC/VCE!!!! 0

(WB/VCE!!!! 0)

CollectionCollection

Cbc!!!! 0

v!!!! vsat [2 x 107 cm/s] (Kolnik et. al.)

Vbr!!!! Ecrit WC [Ecrit ~2 MV/cm] (Bhapkar and Shur.)

Subcollector

Base

Collector

Emitter

TransportTransport-- !!!! 1

UCS

UCSBUCS

UCSBHurdles with GaN bipolar transistors


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UCSUCSBUCSUCSBHurdles with GaN bipolar transistors

Lack of low damage etch toreveal base

Leaky E/B junction

Bad base contact

No etch stop High RB

Poor p-GaN base contact

Low p-GaN base

conductivity Deep acceptor (~160 meV)

Subcollector

Base

Collector

Emitter

Surface leakageSurface leakage

due to etch damagedue to etch damage

Dislocation causesDislocation causes

leakageleakage

Hard to control junction placement inHard to control junction placement inMOCVD due to memory effect of pMOCVD due to memory effect of p--dopantdopant MgMg

Low minority carrierLow minority carrier

lifetimelifetime

UCS

UCSBUCS

UCSB Demonstration of dislocation enhanced leakage


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UCSUCSBUCSUCSB g

1 10-15

100 10-15

10 10-12

1 10-9

100 10-9

10 10-6

-20 -15 -10 -5 0 5 10 15 20

LeakageCurrent[A]

Applied Bias [V]

Leakage from Collector to Emitter,

Wing vs Window

LEO used to investigate leakage of devicesLEO used to investigate leakage of deviceswithout dislocations. (Lee McCarthy et al.)without dislocations. (Lee McCarthy et al.)

Dislocated

LEO

AFM scan of wing vs Windowon LEO GaN

Regrowth Mask Regrowth Mask

StandardtemplateLEO GaN

Results: LEO device demonstratedReduction in LeakageStable operation past 20V

Gain unchangedDevices on dislocated material alsofunctional

ExplanationThick substrate sufficiently reduces

dislocations to prevent C/E short in

window regionGain (e) not currently limited by

dislocation density

UCS

UCSBUCS

UCSBStrategy: Thick Collector


32/42


gy

Decent dislocation density

High quality MOCVD templates achievedDislocation density ~ 5e8 cm-2

Low background doping

ND < 1e16 cm-3 (Assuming uniform doping ND and Ecritical = 2 MV/cm,requires 10 m to achieve 1 KV breakdown voltage.)

Doping vs. Depth

(010704GA, 8 m collector)

1.E+14

1.E+15

1.E+16

1.E+17

0 1 2 3

Depth (m)

Doping(cm-3)

UCS

UCSBUCS

UCSBEmitter Regrowth Process Flow


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g

Selectively grow MOCVDemitter on base-collector

structures.1. Pattern regrowth mask

2. Regrow emitter layer byMOCVD

3. Remove mask and contactbase and etch to collector

4. Contact collector, emitter

n+ Subcollector

Sapphire Substrate

n- Collector

p Base

Mask

n+ Subcollector

Sapphire Substrate

n- Collector

p Base

n+

Emitter

n+ Subcollector

Sapphire Substrate

n- Collector

p Base

n+

Emitter

n+ Subcollector

Sapphire Substrate

n- Collector

p Base

n+

Emitter

1. 2.

3. 4.Pd/Au

Al/Au

Al/AuRegrown emitter

UCS

UCSBUCS

UCSB SIMS: Mg by MOCVD


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Severe memory effect observed in non-interrupted MOCVD growthSlow decay tail into GaN:Si regrown on as-grown GaN:Mg layer

Growth directionGrowth direction

1E+15

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

0 0.5 1 1.5 2 2.5 3

Depth (m)

Concentration(cm-3)

Low Mg

background in

clean reactor

Mg

Si

Turnoff of Mg source

Decay rate ~ 140

nm/decade

230 nm

400 nm

1.E+16

1.E+17

1.E+18

1.E+19

1.E+20

1.E+21

1 1.5 2 2.5 3 3.5

Depth (m)

Concentration(cm-3) Mg

Si

Regrown surface

Decay rate

~ 115 nm/decade

~ 30 nm/decade

Memory effect Slow decay tail in regrowth

UCS

UCSBUCS

UCSBSIMS: regrowth with surface treatment


35/42


Regrown in Mg-free reactor and all grown by MOCVD Presence of an excessive amount of Mg on the surface, which

can be removed by acid etch

Occurrence of Mg diffusion, ~ 40 nm/decade sharpness achieved

1E+15

1E+16

1E+17

1E+18

1E+19

1E+20

1E+21

1E+22

1E+23

0 500 1000 1500 2000

Depth (nm)

Mg

concentratio

n(a.u.)

C: as grown, ~100 nm/decadeD: etch in HFand HCl

for 5 min, ~ 50 nm/decadeE: etch in BHF (1:20) for

15 sec and HCl for 20 sec,

~40 nm/decade

C

D

E

Regrowth surface

Detection limit

UCS

UCSBUCS

UCSBSelectively regrown n/p diodes


36/42


Mask enhanced growth complicatesthe analysis

Regrowth rate depends on the

mask layout, diode size etc

Bunny ear regrowth profile isoften seen

Only the emitter edge is active

in device operation due to

highly resistive base layer

The junction quality depends on

how the regrowth is initiated,

e.g. Temp, P, flows, presence of

Si and Al etc.

Bunny ear regrowth profile of two differentsquare diodes

UCS

UCSBUCS

UCSB


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UCS

UCSBUCS

UCSBRegrown n/p Diodes Characteristics


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Comparison of various structures regrown on 0.5 m GaN:Mg

1100/300

1100/300

1100/300

1100/300

1140/760

~60xAl~ 5%

30 nm GaN:Si

30 nm AlGaN->GaN:Si

730 nm AlGaN:Si

60 nm GaN->AlGaN

60 nm GaN

010517AA

~40450 nm GaN:Si (1e18)

30 nm GaN

010513GC

~30xAl~ 5%

250 nm AlGaN:Si

(1e18)75 nm GaN->AlGaN:Si

(1e18)

000714AF

~30250 nm Al0.06GaN:Si(1e18)

000714AE

~30400 nm GaN:Si

(4e18 cm-3)

0005

10GF

Growth Parameter

G.R. Temp/Press

(nm/min) (C/Torr)

Layer structureRun#

-20 -15 -10 -5 0 5

10-6

1x10-5

1x10-4

10-3

10-2

10-1

100

101

102

Idensity(A

/cm

2)

V (Volts)

000510GF

000714AE

000714AF

010513GC010517AA

Year 2000

Year 2001

UCS

UCSBUCS

UCSBHBT with 8 mm GaN collector


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Current gain () > 20 Common emitter operation > 300 V Non-passivated Base thickness 1000

Al0.05GaN emitter Vbr ~330 V

UCS

UCSBUCS

UCSB Device structure


40/42


Utilization of uid GaN spacer and grading layer- HBTs with high emitter injection coefficiency

Etch damage and current mask layout limits Vbr

n+ Subcollector

Sapphire Substrate

1000 p Base

n+

Emitter

Sapphire

2 m GaN:Si (1e18 cm-3) subcollector

8 m uid GaN (4e15 cm-3) collector

100 nm GaN:Mg (2e19 cm-3) base

8 nm uid GaN spacer

8 nm GaN->Al0.05

GaN (?3e18 cm-3) grading

4 nm GaN:Si (1e18 cm-3) contact

4 nm Al 0.05 GaN->GaN:Si (1e18 cm-3) grading

105 nm Al 0.05 GaN:Si (1e18 cm-3) emitter

8 m n- collector

Effective collectorthickness ~ 2-3 m

UCS

UCSBUCS

UCSBI-V Characteristics


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reasonable base contacts

Improved B/E diodes Rectifying B/C diodes, Vbr > 300 V

Base-collector diode

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10

-0.04

-0.020.00

0.02

0.04

0.06

0.080.10

0.12

I(mA)

VBE

(V)

Base-emitter diode

-10 -8 -6 -4 -2 0 2 4 6 8 10-50

-40

-30

-20

-10

0

10

20

30

40

50

I(A)

V (Volts)

UCS

UCSBUCS

UCSBSummary


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Conclusion

In selective emitter regrowth, a sharp Mg profile, ~ 40

nm/decade, enables the precise junction placement

Improvement of regrown-emitter/base diodes

Demonstration of high Vbr (> 300 V) with high (DCcommon emitter operation up to 35)

Documents

AlGaN HEMT