Semiconductors and Energy Bands

Semiconductors

• A semiconductor's ability to conduct electrical current falls between that of conductors and insulators.
• Pure (intrinsic) semiconductors are neither good conductors nor good insulators.
• Common single-element semiconductors include silicon, germanium, and carbon.
• Compound semiconductors like gallium arsenide are also used.
• Single-element semiconductors are characterized by atoms with four valence electrons.

Energy Bands

• The valence shell of an atom forms a band of energy levels where valence electrons are confined.
• When an electron gains enough energy, it can leave the valence shell and become a free electron in the conduction band.
• An energy gap separates the valence band from the conduction band.
• The energy gap represents the amount of energy a valence electron needs to jump to the conduction band.
• Once in the conduction band, electrons can move freely throughout the material.
• Insulators have a wide energy gap, meaning valence electrons rarely jump to the conduction band unless under extremely high voltages (breakdown conditions).
• Semiconductors have a narrower energy gap, allowing some valence electrons to become free electrons in the conduction band.
• Conductors have overlapping energy bands, resulting in a large number of free electrons.

Silicon vs Copper

• Silicon is a semiconductor, while Copper is a conductor due to primary reasons relating to atomic structure.
• Silicon's valence shell contains four electrons, whereas Copper's valence shell contains one electron.
• A valence electron in copper experiences an attractive force of +1, while in silicon, it experiences +4.
• There is four times more force holding a valence electron to a silicon atom compared to a copper atom.
• Copper's valence electron is in the fourth shell and is farther from the nucleus than silicon's valence electron in the third shell.
• Electrons farthest from the nucleus have more energy.
• Valence electron in copper has less force holding it to the atom and more energy than silicon's valence electron.
• Valence electrons in copper require less additional energy to escape and become free electrons than in silicon.
• Copper has a large number of free electrons at room temperature.

Semiconductor Characteristics

• Silicon, germanium, and carbon are shown with their atomic structures.
• Silicon is the most widely used material in diodes, transistors, integrated circuits, and other semiconductor devices.
• Silicon, germanium, and carbon each have four valence electrons.
• Valence electrons in germanium are in the fourth shell, while silicon's are in the third shell, closer to the nucleus
• Germanium valence electrons require less additional energy to escape the atom.
• Silicon is preferred over germanium because germanium is more unstable at high temperatures.
• Carbon's valence band is closer to the nucleus, giving it insulator-like properties that behave differently when doped with impurities.

Covalent Bonds in Silicon

• Silicon atoms position themselves with four adjacent silicon atoms to form a silicon crystal.
• Silicon atom shares an electron with each neighbor, creating eight valence electrons per atom and chemical stability.
• Shared valence electrons form covalent bonds that hold the atoms together.
• Each shared electron is equally attracted by two adjacent atoms.
• Covalent bonding occurs in an intrinsic (pure) silicon crystal with no impurities.
• Germanium's covalent bonding is similar to silicon's due to having four valence electrons.

Conduction in Semiconductors

• Electrons in an atom exist within prescribed energy bands.
• Each shell around the nucleus corresponds to an energy band and is separated from adjacent shells by energy gaps.
• Silicon crystal with unexcited silicon atoms is shown at 0 Kelvin.

Conduction Electrons and Holes

• Intrinsic silicon crystal at room temperature has enough heat energy for some valence electrons to jump from the valence band to the conduction band, becoming free (conduction) electrons.
• A vacancy that is left in the valence band is called a hole.
• For every electron raised to the conduction band, one hole is left in the valence band, creating an electron-hole pair.
• Recombination is when a conduction-band electron loses energy and falls back into a hole.
• An intrinsic silicon piece at room temperature has a number of unattached, drifting conduction-band (free) electrons.
• An equal number of holes exists in the valence band, created when these electrons jump into the conduction band.

Electron and Hole Current

• When a voltage is applied across intrinsic silicon, the generated free electrons in the conduction band move toward the positive end, this movement of free electrons is called electron current.
• Current occurs at the valence level where holes exist.
• Valence electrons are still attached to their atoms and aren't free to move randomly, but can move with little energy between a nearby hole.
• Movement of a valence electron leaves another hole where it came from effectively moves the hole from one place to another.

Doping in Semiconductors

• Semi-conductive materials do not conduct current well and are of limited value in their intrinsic state.
• The limited number of free electrons in the conduction band and holes in the valence band, are the reason for their limited value.
• An increase the number of free electrons or holes is needed to modify Intrinsic silicon or germanium, which increase conductivity and make it useful in electronic devices.
• Doping, or adding impurities to the intrinsic material.
• N-type and P-type are the two types of extrinsic semi-conductive materials (key building blocks for electronic devices).
• The conductivity of silicon and germanium can be increased by controlled addition of impurities (doping) to the intrinsic material.
• N-type and P-type, both categories of impurities, increase the number of current carriers (electrons or holes).

N-Type Semiconductor

• Pentavalent impurity atoms are added to intrinsic silicon to increase conduction-band electrons.
• The atoms with five valence electrons are arsenic (As), phosphorus (P), bismuth (Bi) and antimony (Sb).
• Four of arsenic’s valence electrons form covalent bonds with silicon atoms, leaving one extra electron.
• The extra electron becomes a conduction electron because it is unattached to any atom.
• The pentavalent atom is called a donor atom, because it gives up an electron.
• The number of conduction electrons can be controlled by the number of impurity atoms added to the silicon.
• A conduction electron created by this doping process does not leave a hole in the valence band.

Majority and Minority Carriers

• Silicon or germanium doped with pentavalent atoms is an N-type semiconductor because most of the current carriers are electrons.
• Electrons are called the majority carriers in N-type material.
• Holes in an N-type material are called minority carriers and are created when electron-hole pairs are thermally generated.
• Such holes are not produced by the addition of the pentavalent impurity atoms.

P-Type Semiconductor

• Trivalent impurity atoms are added to increase the number of holes in intrinsic silicon.
• Atoms with three valence electrons are boron (B), indium (In) and gallium (Ga).
• Trivalent atom (boron) forms covalent bonds with four silicon atoms.
• A hole results when each trivalent atom is added and all three of the boron atom's valence electrons are used in the covalent bonds (four electrons are required).
• A trivalent atom can take an electron, so it is referred to as an acceptor atom.
• The number of holes can be controlled by the number of trivalent impurity atoms added to the silicon.
• A hole created by this doping process is not accompanied by a conduction (free) electron

Majority and Minority Carriers

• Silicon or germanium doped with trivalent atoms is called a P-type semiconductor, because most of the current carriers are holes.
• Holes can be thought of as positive charges because the absence of an electron leaves a net positive charge on the atom and are the majority carriers in P-type material.
• Free Electrons, are created when electron-hole pairs are thermally generated, and are the minority current carriers in P-type material.
• The free electrons are not produced by the addition of the trivalent impurity atoms.