GALLIUM ARSENIDE TECHNOLOGY1

PRESENTED BY: NIKHILA GEORGE ROLL NO: 11 VLSI & ES

CONTENTS2

Introduction

GaAs crystal structure Doping Process Channeling Effect

Energy Band Structure Electron velocity field behaviour

INTRODUCTION3

GaAs technology is an alternative to Si technology

for faster devices. Discovered in 1926. It is a compound of elements gallium and arsenic. Group III/V compound. It does not displace Si, but is used in conjunction with Si for high speed requirements.

INTRODUCTION4

Advantages of GaAs over Si:

for same lithographical processes.Higher electron mobility. Higher saturated electron velocity. Less noise than Si devices, especially at high frequencies, hence lower parasitics. Direct band gap semiconductors , emit light. Less power dissipation.

GaAs crystal structure5

Both Gallium (Ga) and arsenic (As) are toxic materials.

Gallium: By-product in both the zinc and aluminium

production processes. Arsenic: Produced from ores such as As2S3 or As2S4. The oxidation reaction of the ores is first entailed to produce As2O3. Later through the reduction with carbon, arsenic is produced.

GaAs crystal structure6

GaAs crystal structure7

Ga has a positively charged nucleus of +31, while

arsenic has +33. Ga has 3 valence electrons while As has 5. GaAs is a compound SC . 3 valence electrons of Ga combines with 5 valence electrons of As to form GaAs. GaAs forms chemical bonds through sharing of valence

electrons.

Arrangement of atoms in GaAs substrate8

GaAs crystal structure9

Gallium and arsenic atoms are placed in alternate

positions in their exact crystallographic locations. Gallium arsenide is a binary semiconductor Special care is required during the processing to avoid high temperatures that could result in dissociation of the surface. This is one of the basic difficulties in the growth of GaAs bulk material.

Doping Process10

Dopants of two types:

N type P type N type:Group IV elements like Si can act as donors at Ga sites and as acceptors at As sites. Ga are larger than As atoms, hence Si atoms occupy Ga sites.

N type material11

Doping Process12

P type:Be

or Mg (group II) elements can be used. Be is the lightest dopant, hence deep implantation of dopant atoms can be accomplished with relatively less lattice damage.

Channeling Effect13

The concept of crystal orientation becomes important

during: Etching of the crystal. Ion implantation Passivation The orientation influences the properties of GaAs FETs. Axial Channeling: When a high energy ion enters a single crystal lattice at a critical angle to the major axis of GaAs crystal, the ion is sheered down the open directions of the lattice.

Channeling Effect14

If a random equivalent direction is not used during

ion implantation, the depth distribution will be greater than those predicted by range statistics. Channeling effect is less in direction crystal. Other directions of implant has a significant influence on threshold voltages of fabricated device.

Energy band Structure15

The important characteristics of GaAs is its superior

electron mobility. This is a result of its energy band structure. Curvature of energy vs momentum graph determines the effective mass of electron travelling through the crystal. In band gap structure, narrow and sharply curved valleys corresponds to electrons with low effective masses. Valleys with wide and gentle curvature corresponds to larger effective masses.

Energy band structure of Si and GaAs16

Energy band structure17

Mobility depends on:Concentration of impurity. Temperature Electron effective mass (inversely proportional)

For GaAs, effective mass is 0.067 times mass of free

electron. Electron travels faster in GaAs. e- in higher valley have high mass and strong inter valley scattering , therefore exhibits low mobility.

Energy band structure18

GaAs is a direct band gap SC.

CB minimum and VB maximum at same value of k. Little momentum change for transition from CB to

VB. Probability of photon emission with energy equal to the band gap is high, GaAs makes an excellent light emitting diode.

Electron velocity field behaviour 19

As applied electric field E across GaAs is increased,

electrons gain energy from applied field. Some portion of this energy is lost due to collision. As long as gain is more than loss, energy and drift velocity increases with an increase in applied field. At any point when gain equals loss, drift velocity is called saturation velocity.

Electron velocity field behavior 20

At electric field greater than 3.5kV/cm, energy of

lower valley electrons rises sufficiently, ie, they become hot. Some of the hot electrons populate the upper conduction band with larger electron effective mass. This reduces no. of high mobility e- and drift velocity. In this region drift velocity passes through a max of 2*107 cm/sec with increasing field and decreases to a saturation value of 1.4*107cm/sec.

Electron velocity field behavior21

Electron velocity field behavior22

Si has lower mobility at lower electric fields.

Electron velocity of Si increases monotonically until

the drift velocity saturates at a value of about 1*107 cm/sec. Si attains saturation velocity at a higher electric field. Due to higher electron mobility , GaAs finds application in ultra speed devices.

References23

Basic VLSI Design, Douglas A Pucknell, Kamran

Eshraghian.

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