CONTENTS INTRODUCTION HISTORY DEVICE STRUCTURE FABRICATION WORKING PRINCIPLE TYPES OF HEMTS GROWTH CHARACTERSTICS ADVANTAGES APPLICATIONS CONCLUSION

INTRODUCTION

High Electron Mobility Transistors (HEMTs) have

emerged as a promising candidate for microwave (f > 1

GHz) power amplification with applications ranging from

satellite links to wireless communications, from highways

to electronic warfare.

HEMTs were primarily based on AlGaAs/GaAs,

AlGaAs/InGaAs, AlInAs/InGaAs and related epitaxial

films grown on GaAs or InP substrates.

The HEMT is also known as MODFET (Modulation-

doped FET), TEGFET (Two-dimensional Electron Gas

FET), SDHT (Selectively Doped Heterostructure

Transistor) or simply, HFET (Heterojunction FET).

The unique feature of the HEMT is channel formation

from carriers accumulated along a grossly asymmetric

heterojunction i.e. a junction between a heavily doped

high band gap and a lightly doped low band gap region.

HISTORY The physics of carrier transport parallel to a heterojunction

was first considered in 1969.

First demonstrated in AlGaAs/GaAs hetero junctions in

1979, and applied to demonstrate a HEMT in 1980. Electron

mobility enhancement at AlGaN/GaN heterojunction was

first reported in 1991.

Enhancement was attributed to the 2D nature of the

electrons, based on observations of mobility increase

with lowering of the temperature.

DEVICE STRUCTURE

We have fabricated AlGaN/GaN HFETs with the

source-to-drain spacing from 2 µm to 7 µm, the gate

length from 0.25 µm to 5 µm and the total gate width

from 50 µm to 150 µm (2×25 µm to 2×75 µm).

This reduces the parasitic source inductance and

improves thermal dissipation.

fig. of HEMT device

FABRICATION The Substrate:- Sapphire (Al2O3) and SiC are the most

popular substrate materials used currently. Sapphire

substrates are cheaper for GaN growth than SiC.

The Contacts:- The shape of the gate contact is

crucial to device performance. T- or Y shaped

cross-sections are employed.

Growth of the semiconductor epitaxial and

insulator layers.

Photolithography for ohmic contact openings.

Ohmic contact metallization.

Rapid Thermal Annealing (RTA) of ohmic

contacts.

Photolithography for device isolation level.

Reactive ion etching or ion implantation for

device isolation.

WORKING PRINCIPLE Charge transfer takes place across the interface to equalize

the Fermi energy on both sides. Electrons from the donor

impurities of the highly doped n-type Ga1-xAlxAs are

transferred to the conduction band of the nearly intrinsic p-

type GaAs.

Positively charged donor ions are therefore left near the

interface on the n-type side and negatively charged acceptor

ions are left near the interface on the p-type side

TYPES OF HEMTS Lattice-matched HEMTs: same lattice constant

Non-lattice matched or pseudomorphic

HEMT (pHEMT): slightly different lattice

constants

metamorphic HEMT (mHEMT): a buffer layer

is grown between materials with different lattice

constant.

GaN HEMT technology development

SUBSTRATES-Material selection: SiC, Si, Sapphire-SiC material quality-Diameter expansion 2’’ to 3’’

EPITAXY-GaN HEMT epi wafer growth-Advanced materials

DEVICES-Device processing-High voltage passives-Device modeling-Reliability & robustness evaluation

MMIC-Design-Fabrication-Test

INTEGRATION-Thermal management-Assembly-Packaging-System impact

2005

2009

CHARACTERSTICS DRAIN CURRENT

CHARACTERSTICS:

The measured DC

characteristics of a

power HEMT are

shown in Fig.

Negative resistance

These devices show negative resistance for high currents and voltages.

ADVANTAGES

Used in Analog circuits as amplifiers

Excellent high frequency characteristics

(several GHz)

Low noise

High power-gain

High efficiency

APPLICATIONS

Thermal Optimization of GaN HEMT

Transistor Power Amplifiers Using New Self-

heating Large-signal Model.

High-Power, High-Efficiency GaN HEMT

Power Amplifiers for 4G Applications.

GaN-Based High Electron-Mobility Transistors

for Microwave and RF Control Applications.

CONCLUSION

High power density

Very good high frequency

characteristics

Low on-resistance

High temperature stability (wide

bandgap materials)

THANKS…and Queries.. ?