Name: intrinsic and doped semiconductors
Uploaded: 2024-07-08T13:20:08.000Z
Duration: 1 min 20 s
Channel: JoVE

A pure silicon wafer used in integrated circuits is an intrinsic semiconductor that lacks impurities and exhibits low electrical conductivity.
At zero kelvin, its conduction band is empty. The probability of electron occupancy at different energy levels is given by the Fermi level. Here, it lies in the middle of the band gap.
When the temperature increases, several electrons move to the conduction band, resulting in an equal concentration of electrons and holes in their respective bands.
The conductivity of intrinsic silicon can be tailored by introducing impurities through doping, which converts it into an extrinsic semiconductor.
Silicon has four valence electrons. When doped with a pentavalent impurity, it replaces a silicon atom and donates an excess electron, which acts as the majority carrier. Such semiconductors are called N-type semiconductors. Here, the Fermi level is shifted near the conduction band.
Conversely, trivalent impurities create vacant sites called holes, forming the majority charge carriers. These semiconductors are called p-type semiconductors. Here, the Fermi level is shifted towards the valence band.

types of semiconductors

Types of Semiconductors

Intrinsic semiconductors are highly pure materials with no impurities. At absolute zero, these semiconductors behave as perfect insulators because all the valence electrons are bound, and the conduction band is empty, disallowing electrical conduction. The Fermi level is a concept used to describe the probability of occupancy of energy levels by electrons at thermal equilibrium. In intrinsic semiconductors, the Fermi level is positioned at the midpoint of the energy gap at absolute zero. When the temperature of the semiconductor increases, thermal energy excites some electrons from the valence band to the conduction band, creating electron-hole pairs (EHPs). The creation of EHPs enables conduction because electrons can move freely in the conduction band and holes can act like positive charge carriers in the valence band.
The intrinsic carrier concentration, denoted as ni, is the number of free electrons or holes in a pure semiconductor at thermal equilibrium. It is a temperature-dependent value and can be expressed by the formula:
<img alt="Intrinsic carrier concentration formula: \( n_i = BT^{3/2} e^{-E_g/(2kT)} \); semiconductor physics." src="/files/ftp_upload/15153/15153_Equation_1.svg" style="text-align: center; height: 24px;" data-altupdatedat="1762173717000" data-alt="Equation 1" />
Where B is a material constant, T is the temperature, Eg is the band gap energy, and k is the Boltzmann constant.
At any temperature above absolute zero, EHPs are generated at a rate gi, and they recombine at a rate ri. For the semiconductor to maintain thermal equilibrium, these rates must be equal. The recombination rate is proportional to the product of the electron (n0) and hole (p0) concentrations, described by:
<img alt="Chemical reaction rate equation, formula illustrating dependence on reactants, mathematical expression." src="/files/ftp_upload/15153/15153_Equation_2.svg" style="text-align: center; height: 24px;" data-altupdatedat="1762173717000" data-alt="Equation 2" />
where &#945;r is the recombination coefficient.
Intrinsic semiconductors can be altered to become extrinsic semiconductors by doping, which introduces impurities to change the material's electrical properties. Doping intrinsic semiconductors with pentavalent atoms creates N-type materials by adding free electrons. Conversely, trivalent dopants yield P-type materials with prevalent holes, shifting the Fermi level towards the valence band, thus modifying the semiconductor's conductive properties.

A pure silicon wafer used in integrated circuits is an intrinsic semiconductor that lacks impurities and exhibits low electrical conductivity.
At zero ...

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Electrical Engineering

Unit 1

Basics of Semiconductors

KEY TERMS AND CONCEPTS

intrinsic and doped semiconductors

Intrinsic semiconductors are very pure materials with no impurities. At absolute zero, they act as perfect insulators because the valence electrons are bound and the conduction band is empty. That means electrical conduction cannot occur under those conditions.

The Fermi level helps describe the chance that an energy level is occupied by an electron at thermal equilibrium. In an intrinsic semiconductor, the Fermi level sits near the middle of the band gap at absolute zero. As temperature rises, thermal energy can excite some electrons from the valence band into the conduction band.

This process creates electron-hole pairs, or EHPs. Electrons in the conduction band can move freely, and holes in the valence band behave like positive charge carriers. The intrinsic carrier concentration, ni, is the number of free electrons or holes in a pure semiconductor at thermal equilibrium, and it changes with temperature.

At any temperature above absolute zero, EHPs are generated at a rate gi and recombine at a rate ri. To stay in thermal equilibrium, these rates must be equal. The recombination rate depends on the product of the electron concentration n0 and the hole concentration p0, with the recombination coefficient αr setting the proportionality.

Intrinsic semiconductors can be changed into extrinsic semiconductors by doping, which adds impurities to alter electrical properties. Doping with pentavalent atoms creates N-type material with extra free electrons. Doping with trivalent atoms creates P-type material with more holes and shifts the Fermi level toward the valence band, which changes how the semiconductor conducts electricity.

Intrinsic and Doped Semiconductors

energy bands in solids

Valence and Conduction Bands in Silicon

Valence and conduction bands explain how electrons behave in solids. In isolated atoms, electrons occupy discrete energy levels that the Bohr model describes well. The hydrogen atom, for example, has specific electron energies labeled by the quantum number n, and higher n values give levels that are less negative and closer together. When atoms move close together in a solid, those discrete levels start to split. This happens because electron orbitals from neighboring atoms overlap. The Pauli...

semiconductors

Energy Bands in Metals and Insulators

Materials conduct electricity in different ways because their energy bands are arranged differently. Metals, semiconductors, and insulators can be compared with energy band diagrams. These diagrams show how easily electrons can move when an electric field is applied. Metals such as copper, zinc, and lead have low resistivity. Their conduction band is not fully occupied, or it overlaps with the valence band, so a band gap is essentially absent. Electrons in the highest valence energy levels can...

Intrinsic semiconductors are very pure materials with no impurities. At absolute zero, they act as perfect insulators because the valence electrons are bound and the conduction band is empty. That means electrical conduction cannot occur under those conditions. The Fermi level helps describe the chance that an energy level is occupied by an electron at thermal equilibrium. In an intrinsic semiconductor, the Fermi level sits near the middle of the band gap at absolute zero. As temperature...

carrier generation and recombination

Semiconductor Charge Carriers: Creation and Loss

Carrier generation and recombination control how free charge carriers change inside a semiconductor. Carrier generation creates electron-hole pairs, or EHPs, when electrons move from the valence band into the conduction band and leave holes behind. Carrier recombination does the opposite and lowers the number of free carriers. In direct-bandgap semiconductors such as gallium arsenide, generation happens efficiently because energy absorption can move a valence electron straight to the...

carrier transport

Drift and Diffusion in Semiconductors

Drift and diffusion are the two main ways electric current forms in semiconductors. These charge-carrier motions help determine how semiconductor devices work and how well they perform. The carriers involved are electrons and holes. Drift current begins when an external electric field acts on charge carriers. Electrons and holes move between collisions with lattice atoms, and this motion creates drift velocity. For electrons, drift velocity depends on electron mobility and the electric field...

p-n junction

P-N Junction Depletion Region

A p-n junction forms when p-type and n-type semiconductor materials are joined together. At the boundary, holes from the p-side and electrons from the n-side diffuse into the opposite side because of the concentration gradient. This movement leaves a region with no free charge carriers near the junction, called the depletion region. The charge density in the depletion region can be described on the n-side and p-side using the donor and acceptor doping concentrations, ND and NA. The fixed...

biasing of p-n junction

P-N Junction Diode Under Bias

P-N junction diodes change behavior under equilibrium, forward bias, and reverse bias. In equilibrium, no external voltage is applied across the junction. Carriers diffuse at the boundary and leave behind charged dopants. Acceptors remain on the p-side, and donors remain on the n-side. These fixed charges form a depletion region and an electric field that blocks further diffusion. The band diagram at equilibrium shows the Fermi levels on both sides aligned. That alignment means the junction is...

metal-semiconductor junctions

Schottky and Ohmic Metal Contacts

Metal contacts with semiconductors can form either Schottky or Ohmic junctions. The result depends on the work function of the metal, the work function of the semiconductor, and how electrons move when the two materials touch. At equilibrium, the Fermi levels align, so charge shifts until both sides match in energy. Schottky barriers form when a metal with work function Φm contacts a semiconductor with a different work function Φs. If Φm is greater than Φs, the semiconductor Fermi level starts...

biasing of metal-semiconductor junctions

How Bias Changes Metal-Semiconductor Current

Biasing metal-semiconductor junctions means applying a voltage across the junction. The metal connects to the voltage source, and the semiconductor is grounded. This setup helps control the direction and size of current in devices such as diodes, transistors, and photovoltaic cells. In a Schottky junction with an n-type semiconductor, a positive voltage on the metal lowers its Fermi level. A Fermi level is the energy level that helps describe how electrons are distributed. Lowering this level...

fermi level

Fermi Level in Semiconductors

The Fermi level is a key energy level in semiconductors. It sits between the lower-energy valence band and the higher-energy conduction band. The Fermi-Dirac function, which follows an S-shaped curve, shows the chance that an energy state is filled by an electron at a given temperature. At absolute zero, electrons occupy every energy state up to the Fermi level. States above it stay empty. As temperature rises, electrons gain energy and can move into vacant states above the Fermi level. In an...

fermi level dynamics

Vacuum Level and Schottky Barriers

Vacuum level and Schottky barriers help explain how electrons move between materials. The vacuum level is the energy needed for an electron to escape a material surface. In a semiconductor, it usually sits above the conduction band and serves as a reference for comparing electron energy. Electron affinity is the energy gap between the conduction band minimum and the vacuum level. It shows how easily a semiconductor can accept extra electrons. The work function is the least energy needed to...

key terms and concepts

Learn how pure semiconductors conduct through electron-hole pairs, then see how doping creates N-type and P-type materials by adding specific impurities.

Intrinsic and Doped Semiconductors | JoVE K12

JoVE Core Electrical Engineering offers easy-to-understand animated videos that cover fundamental concepts of electric circuits along with circuit analysis techniques. It incorporates first-order and second-order circuits using operational amplifiers and circuit transformation concepts. The inclusion of practical design examples and real-world applications through scientist-in-action videos adds significant educational value, making complex concepts accessible.

electrical engineering

Intrinsic and Doped Semiconductors

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