As the applied electric field, E, across the GaAs material is increased, the charge carriers, that is electrons in this case, gain energy from the applied field. At the same time, through collisions (that is, optical phonon scattering), with the lattice, the electrons also lose a small portion of this energy. So long as the resultant balance is positive, the energy and drift velocity of the charge carriers increases with an increase in the applied field. However, at some point, the energy gained from the field becomes equal to the energy lost as the result of collisions. This result in the drift velocity approaching a limiting value referred to as the saturation velocity, v_sat

Since gallium arsenide is a multi-valley semiconductor, when the energy of lower valley electrons rises sufficiently, that is at electric fields greater than approximately 3500V/cm, electrons become hot. There is a region in the electron velocity-field characteristics where some of the ‘hot’ electrons populate an upper conduction band that is characterized by larger electron effective mass. The resultant effect is a reduction in the number of high mobility electrons and hence the drift velocity.

In this region the drift velocity is no longer proportional to the electric field, but instead passes through a maximum of about 2*10⁷ cm/sec with increasing field, and decreases to an electric field independent saturation value of about 1.4*10⁷ cm/sec.

The velocity-field characteristics illustrating the three regions of interest are shown in the figure below.

Electron Velocity vs Electric field

For convenience of comparison, characteristics for silicon are also illustrated. From the figure it can readily be noted that in low electric field regions, silicon has a much lower mobility than gallium arsenide. This increases monotonically until the drift velocity saturates at a value of about 1*10⁷ cm/sec.

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One of the important characteristics that is attributed to GaAs is its superior electron mobility brought about as the result of its energy band structure as shown in the figure below. Gallium arsenide is a direct gap material with valence band maximum and conduction band minimum coinciding in k space at the Brillouin zone centers. Valleys in the band structure that are narrow and sharply curved correspond to electrons with low effective mass state, while valleys that are wide with gentle curvature are characterized by larger effective masses. The curvature of the energy versus electron momentum profile determines the effective mass of electrons travelling through the crystal. The minimum point of gallium arsenide’s conduction band is near the zero point of crystal-lattice momentum, as opposed to silicon, where conduction band minimum occurs at high momentum. Now, mobility, µ, depends upon

• Concentration of impurity, N
• Temperature, T
• And is also inversely related to the electron effective mass, m.

Energy Band Strucure of GaAs

For GaAs, the effective mass of these electrons is 0.067 times the mass of free electron (that is, 0.067m_e, where m_e is the free electron rest mass). This means electrons travel faster in gallium arsenide than in silicon as the result of their superior electron mobility brought about by the shapes of their conduction bands. Electrons in the higher valleys have high mass and strong inter-valley scattering and therefore exhibit very low mobility, which is very similar to conduction electrons in silicon. Furthermore, gallium arsenide is a direct-gap semiconductor. Its conduction band minimum occurs at the same wave vector as the valence band maximum , which means little momentum change is necessary for the transition of an electron from the conduction band, to the valence band. Since the probability of photon emission with energy nearly equal to the band gap is somewhat high, GaAs makes an excellent light-emitting diode. Silicon on the other hand, is an indirect-gap semiconductor since the minimum associated with its conduction band is separated in momentum from the valence band minimum. Therefore it cannot be a light-emitting device.

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The whole concept of crystal orientation becomes important during

• The etching of the crystal
• Ion-implantation
• Passivation

This introduces an orientation dependency that influences the properties of GaAs field effect transistor. For example, during implantation, when a high energy ion enters a single crystal lattice at a critical angle to the major axis of the GaAs crystal, the ion is steered down the open directions of the lattice. This steering is called axial channelling. This implies that of a random equivalent direction is not used during ion implantation, the depth distribution will be greater than those predicted by range statistics which are used to establish penetration depth.

The channelling effect is not as dramatic in the <100> direction when compared with <110> direction. Many of the current GaAs wafers employ the <100> direction. It should be noted that the profile difference between the aligned <100> direction implant and any other direction of implant has a significant influence upon the threshold voltages of the fabricated devices.

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TAKE A LOOK : GALLIUM ARSENIDE (GaAs) CRYSTAL STRUCTURE

GaAs – A Compound Semiconductor

Gallium arsenide is a compound semiconductor which may be defined as a semiconductor made of a compound of two elements (as opposed to silicon, which is a single element semiconductor).

The figure below shows the arrangement of atoms in a gallium arsenide substrate material. Note the alternate positioning of gallium and arsenic atoms in their exact crystallographic locations. Since 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 being one of the basic difficulties in the growth of GaAs bulk material.

GaAs Atom Arrangement

GaAs Doping Process

Much as it is with silicon, it is necessary to introduce impurities into the semi insulating Ga As material in order to facilitate the creation of switching devices Selection of the impurity and its concentration density determine the behaviour of the switching clement. According to the dopant used, both n-type and p-type material can be realized.

• n-type material

Group IV elements such as silicon can act as either donors (that is, on Ga sites) or acceptors (that is, on As sites). Since arsenic is smaller than gallium and silicon (the covalent radius for Ga is 1.26 A and for As is 1.18 A), group IV impurities tend to occupy gallium sites. Thus, silicon is used as the dopant for the formation of n-type material as shown in the figure below.

The shrinkage of atomic radii across a given row of the periodic table can best be explained by noting that in any given period, electrons are added to s and p orbitals, which are not able to shield each other effectively from the increasing positive nuclear charge. Thus an increase in the positive charge of the nucleus results in an increase in the effective nuclear charge, thereby decreasing the effective, atomic radius. This is why, for example, an As atom is smaller than a Ga atom.

n-type material

• p-type material

Beryllium (Be) or magnesium (group II) can be used for the formation of p-type material. Since Be is the lightest p-type dopant for GaAs, deep implantation of the dopant atoms can be accompiished with relatively less lattice damage. Nevertheless, Mg is also finding its way as a suitable dopant in a number of processes.

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Gallium (Ga), a toxic material, is produced as a by-product in both the zinc and aluminium production processes. Similarly, arsenic (As), which is also very toxic, is produced from ores such as AS₂S₃ or AS₂S₄. The process entails firstly oxidation of the ores to form AS₂O₃ and subsequently, through reduction with carbon, arsenic is produced.

In order to better appreciate the structure and the properties of gallium arsenide crystal, it is appropriate to focus some attention on the characteristics of the individual atoms themselves. The figure below shows Bohr’s model of the atomic structures for gallium and arsenic. Similar representation for silicon is also illustrated for comparison.

Bohr's Model For Si, Ga, and As

Gallium possesses a positively charged nucleus of +31, while the arsenic atom’s nucleus has a positive charge of+33. In each case, the total positive charge of the nucleus is equalized by the total effective negative charge of the electrons.

Electrons, travelling within their respective orbits, possess energy since they are a definite mass in motion (that is, rest mass of electron is 9.108*10^-23gm). This means each electron in its relationship with its parent nucleus exhibits an energy value and functions at a distinct energy level. This energy level is dictated by the electron’s momentum and its physical proximity to the nucleus. The closer the electron is to the nucleus, the greater is the holding influence of the nucleus on the electron and the greater is the energy required for the electron to break loose and become free.

Outer orbit electrons are said to be stronger than inner orbit electrons because of their ability to break loose from the parent atom, and as a result they are referred to as ‘valence electrons’. The outer orbit in which valence electrons exist is called the ‘valence band’. It is the electrons from this band that are being considered in much of the discussions in the section to follow.

Crystal chemical bonds result through sharing of valence electrons. In materials such as Si, Ga and As, the outer-shell valence configuration can be represented by

Si→3s² 3p²

Ga→4s²4p¹

As→4s² 4p³

Here the core is not shown and the superscripts denote the number of electrons in the subshells (that is, s and p orbitals).

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TAKE A LOOK : GALLIUM ARSENIDE (GaAs) DOPING PROCESS