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

Un wafer di silicio puro utilizzato nei circuiti integrati è un semiconduttore intrinseco che manca di impurità e presenta una bassa conduttività elettrica.
A zero kelvin, la sua banda di conduzione è vuota. La probabilità di occupazione di elettroni a diversi livelli di energia è data dal livello di Fermi. Qui, si trova nel mezzo della banda proibita.
Quando la temperatura aumenta, diversi elettroni si spostano verso la banda di conduzione, determinando un'uguale concentrazione di elettroni e lacune nelle rispettive bande.
La conduttività del silicio intrinseco può essere adattata introducendo impurità attraverso il drogaggio, che lo converte in un semiconduttore estrinseco.
Il silicio ha quattro elettroni di valenza. Quando drogato con un'impurità pentavalente, sostituisce un atomo di silicio e dona un elettrone in eccesso, che funge da vettore di maggioranza. Tali semiconduttori sono chiamati semiconduttori di tipo N. Qui, il livello di Fermi viene spostato vicino alla banda di conduzione.
Al contrario, le impurità trivalenti creano siti vuoti chiamati buchi, formando la maggior parte dei portatori di carica. Questi semiconduttori sono chiamati semiconduttori di tipo p. Qui, il livello di Fermi è spostato verso la banda di valenza.

nozioni di base sui semiconduttori

Types of Semiconductors

I semiconduttori intrinseci sono materiali altamente puri, senza impurità. Allo zero assoluto questi semiconduttori si comportano come isolanti perfetti, perché tutti gli elettroni di valenza sono legati e la banda di conduzione è vuota, impedendo la conduzione elettrica. Il livello di Fermi è un concetto utilizzato per descrivere la probabilità all'equilibrio termico di occupazione dei livelli energetici da parte degli elettroni. Nei semiconduttori intrinseci, il livello di Fermi è posizionato nel punto medio del gap energetico allo zero assoluto. Quando la temperatura del semiconduttore aumenta, l'energia termica eccita alcuni elettroni dalla banda di valenza alla banda di conduzione, creando coppie elettrone-lacuna (EHP). La creazione di EHP consente la conduzione perché gli elettroni possono muoversi liberamente nella banda di conduzione e le lacune possono agire come portatori di carica positiva nella banda di valenza.


La concentrazione intrinseca del portatore, indicata come ni, è il numero di elettroni o lacune liberi in un semiconduttore puro all'equilibrio termico. È un valore dipendente dalla temperatura e può essere espresso con la formula:


<img alt="Equation 1" src="/files/ftp_upload/15153/15153_Equation_1.svg" style="height: 24px;">

Dove B è la costante del materiale, T è la temperatura, Eg è l'energia della banda proibita e k è la costante di Boltzmann.

A qualsiasi temperatura superiore allo zero assoluto, gli EHP vengono generati a una velocità gi e si ricombinano a una velocità ri. Affinché il semiconduttore mantenga l'equilibrio termico, queste velocità devono essere uguali. La velocità di ricombinazione è proporzionale al prodotto delle concentrazioni di elettroni (n0) e lacune (p0), descritto da:

<img alt="Equation 2" src="/files/ftp_upload/15153/15153_Equation_2.svg" style="height: 24px;">

dove αr è il coefficiente di ricombinazione.

I semiconduttori intrinseci possono essere modificati per diventare semiconduttori estrinseci mediante drogaggio, che introduce impurità per modificare le proprietà elettriche del materiale. Il drogaggio di semiconduttori intrinseci con atomi pentavalenti, crea materiali di tipo N aggiungendo elettroni liberi. Al contrario, i droganti trivalenti, producono materiali di tipo P con lacune prevalenti, spostando il livello di Fermi verso la banda di valenza, modificando così le proprietà conduttive del semiconduttore.

Nozioni di base sui semiconduttori

Un wafer di silicio puro utilizzato nei circuiti integrati è un semiconduttore intrinseco che manca di impurità e presenta una bassa conduttività elettrica.
A zero ...

Education

Didattica

JoVE Core

Electrical Engineering

Ingegneria elettrica

Unit 1

Unità 1

Basics of Semiconductors

Fondamenti dei semiconduttori

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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...

types of 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...

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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