Semiconductors: Types and Energy Bands

Introduction to Semiconductors

• Semiconductors have conductivity between conductors and insulators.
• Intrinsic semiconductors aren't good conductors or insulators.
• Common single-element semiconductors include silicon, germanium, and carbon.
• Compound semiconductors like gallium arsenide are also used.
• Single-element semiconductors have atoms with four valence electrons.

Energy Bands

• The valence shell represents a band of energy levels where valence electrons are confined.
• An electron can leave the valence shell and become a free electron in the conduction band if it gets enough extra energy.
• The energy gap is the energy difference between the valence and conduction bands.
• Valence electrons must jump the energy gap to reach the conduction band.
• Electrons in the conduction band move freely through the material.
• Insulators have a wide energy gap and require high voltages for electrons to jump to the conduction band.
• Semiconductors have a narrower energy gap, allowing some valence electrons to become free electrons.
• Conductors have overlapping energy bands and a large number of free electrons.

Silicon vs. Copper

• Silicon is a semiconductor due to its atomic properties, while copper is a conductor.
• Silicon has 4 valence electrons while Copper has 1 valence electron.
• Silicon's valence electrons experience four times the attractive force to the nucleus compared to copper.
• Copper's valence electron is in the fourth shell, further from the nucleus than silicon's in the third shell.
• Copper's valence electron has less force holding it to the atom and more energy than silicon's, making it easier to become a free electron.
• Copper has many free electrons at room temperature.

Semiconductor Characteristics

• Silicon is commonly used in diodes, transistors, integrated circuits, and other semiconductor devices.
• The valence electrons, in germanium, are in the fourth shell, which are at higher energy levels than those in silicon (third shell).
• Germanium needs less additional energy for valence electrons to escape, making it less stable at high temperatures; this makes silicon preferable.
• Carbon's valence band is very close to the nucleus, creating insulator-like properties.
• A semiconductor's crystal's ability to accept dopants is a key factor.
• Heavily doped diamond behaves like a large band gap semiconductor or an insulator, requiring high voltages and is rarely used.

Covalent Bonds in Silicon

• Silicon atoms form a crystal structure where each silicon atom shares an electron with four neighbors.
• This creates eight valence electrons per atom, leading to chemical stability.
• Covalent bonds hold the atoms together, with shared electrons attracted by two adjacent atoms.
• An intrinsic crystal is pure silicon without impurities.
• Germanium's covalent bonding is similar to silicon due to having four valence electrons.

Conduction in Semiconductors

• Electrons exist only within prescribed energy bands.
• Energy bands correspond to shells around the nucleus.
• Energy gaps separate adjacent shells where no electrons can exist.
• This only occurs at 0 Kelvin.

Conduction Electrons and Holes

• Intrinsic silicon at room temperature has enough thermal energy for valence electrons to jump to the conduction band and become free electrons.
• Free electrons are also called conduction electrons.
• A hole, or vacancy, is left in the valence band when an electron jumps from the valence to the conduction band.
• An electron-hole pair consists of an electron raised to the conduction band and the resulting hole left in the valence band.
• Recombination occurs when a conduction-band electron falls back into a hole in the valence band.
• Intrinsic silicon at room temperature contains free electrons, unattached to any atom, drifting randomly; there is an equal number of holes in the valence band.

Electron and Hole Current

• Applying voltage to intrinsic silicon causes free electrons in the conduction band to move to the positive end. This is electron current.
• Holes also contribute to current at the valence level.
• Valence electrons can move into nearby holes with little change in energy, effectively moving the hole through the crystal structure.

Doping

• Semiconductors in their intrinsic state have limited conductivity.
• Doping is the process of adding impurities to intrinsic silicon or germanium to increase conductivity.
• N-type and P-type materials are the key building blocks for electronic devices.
• Doping increases the number of current carriers (electrons or holes).
• N-type and P-type are the two impurity categories.

N-Type Semiconductors

• N-type semiconductors are created by adding pentavalent impurity atoms (five valence electrons) such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb) to intrinsic silicon.
• Four of arsenic's valence electrons form covalent bonds with silicon atoms and one extra electron is left.
• Since it's not attached to any atom, the extra electron becomes a conduction electron.
• The pentavalent atoms donating an electron is called a donor atom.
• The number of conduction electrons is precisely controlled by the number of impurity atoms.
• Doping and creating a conduction electron leaves no hole in the valence band.

Majority and Minority Carriers

• N-type semiconductors are created when silicon (or germanium) is doped with pentavalent atoms.
• Electrons are the majority carriers in N-type material.
• There are also a few thermally generated holes, which are not produced by pentavalent impurity atoms.
• Holes in N-type material are called minority carriers.

P-Type Semiconductors

• P-type materials are created by adding trivalent impurity atoms (three valence electrons) like boron (B), indium (In) and gallium (Ga) to intrinsic silicon.
• Each trivalent atom (e.g., boron) forms covalent bonds with four adjacent silicon atoms.
• Boron's three valence electrons results in a hole because four electrons are required.
• Because the trivalent atom can take an electron, it is often referred to as an acceptor atom.
• The number of holes can be accurately controlled by adding the trivalent impurity atoms to the silicon.
• A hole created by this doping process is not accompanied by a conduction (free) electron.
• Holes are considered positive charges in P-type, and the majority carriers are holes.
• Free electrons can also be generated thermally and these free electrons are not produced by the addition of the trivalent impurity atoms.
• Electrons are the minority carriers in P-type material.