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.

eeee

e

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.