ppt hemt(high electron mobility transistor)
TRANSCRIPT
HEMT
(High Electron Mobility Transistor) A REVOLUTION FOR HIGH SPEED DEVICES
•SCHOOL OF MATERIAL SCIENCE AND NANOTECHNOLOGY
OUTLINE•BRIEF HISTORY•LIMITATIONS OF CONVENTIONAL TRANSISTORS•HEMT INTRODUCTION•HETEROJUNCTION REQUIREMENT•BAND STRUCTURE AND BAND BENDING •HOW THEY WORK??•SWITCHING IN HEMT•APPLICATIONS •CONCLUSIONS•REFERENCES
BRIEF HISTORY• 1960 GaN small crystals was made.• 1980 Takashi Mimura, Fujitsu laboratories
designed the features of the first HEMT.• 1985 HEMT was announced the lowest noise
device.• 1994 the first AlGaN/GaN HEMT was
demonstrated .• After 2010 it is widely used for high speed
devices.
LIMITATIONS OF CONVENTIONAL TRANSISTORS
•Short channel effects •Gate leakage current•Gate power dissipation•Ionizing Impurities•Lattice and impurities scattering•Less mobility•Less transconductance
HEMT INTRODUCTIONHIGH ELECTRON MOBILITY TRANSISTORS (HEMT)•High speed device•Referred to as heterojunction field effect transistors.•Two layer of different semiconductor with different band gap energies.
2-D ELECTRON GAS
LESS ELECRON COLLISIO
N
LESS NOISE+HIGH
MOBILITY
HETERO JUNCTION REQUIREMENT•Two disimmilar semiconductor having different band energies.•Lattice match between them. Alx Ga(1-x)As/GaAs have excellent match.
= 1.798 eV= 1.424 eVEg1
EC1
EF1EV1
EC2
EF2
EV2
Eg2
p-type GaAs
Heavily doped
n-type Al0.3Ga0.7As
Moderately doped
BAND STRUCTURE AND BAND BENDING
Notch formation in hetero junction
Eg1
EC1
Evac
qfm1
EF1EV1
qc1
Evac
qfm2qc2
EC2
EF2
EV2
Eg2
Electron affinities qc1 for GaAs and qc2 AlxGa1-xAs
Eg1
EC1
Evac
qfm1
EF1EV1
qc1
Results so far: EC1 and EC2 band-bending:
J
AlGaASaGaAs
Evac
qfm2qc2
EC2EF2
EV2
Eg2
Eg1
EC1
Evac
qfm1
EF1EV1
qc1
Keeping the electron affinities correct resulted in a triangular quantum well in EC (for this heterojunction combination): J
In this region: a triangular quantum well has developed in the conduct
DEC
GaAs
AlGaAs
HOW DO THEY WORK?We have got electron which have got a high mobility, put a contact between two ends. If we apply a vltage between them the electron gets tansported.
Electrons locked up here can be used up by tranporting them.
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HEMT STRUCTURE
SWITCHING IN HEMT
APPLICATIONSOriginally for high speed applications•High power/ high temperature microwave applications•Power amplifiers•Oscillators•Cell Phones•Radar•Most MMIC’s radio frequency •Comaptible for nano devices
CONCLUSIONS• Fantastic mobility. 2,50,000cm2/Vsec at 77K and 20,00,000cm2/Vsec at 4K. APROX 200-300 times more than conentional transistors.
•Its two main features are low noise and high frequency capability.
•A heterojunction is two layers different semiconductors with different band gap energies.
•The 2-D electron gas is essential to the low noise feature.
•AlGaAs and GaAs are the most common materials for heterojunction.