Name: schottky and ohmic metal contacts
Uploaded: 2024-07-08T13:20:09.000Z
Duration: 1 min 24 s
Channel: JoVE

Il contatto tra un metallo e un semiconduttore forma una giunzione con comportamento Schottky o Ohmic.
Se la funzione di lavoro del metallo supera quella del semiconduttore, si forma una giunzione Schottky. Prima del contatto, i diagrammi delle bande di energia sono allineati a livello del vuoto, con i livelli di Fermi in posizioni diverse.
Al contatto, i livelli di Fermi si allineano mentre gli elettroni si trasferiscono dal semiconduttore al metallo, raggiungendo l'equilibrio.
La perdita di elettroni nel semiconduttore ne diminuisce il potenziale, mentre la loro aggiunta al metallo aumenta il potenziale, formando una barriera Schottky alla giunzione. L'altezza della barriera è uguale alla differenza tra la funzione di lavoro del metallo e l'affinità elettronica del semiconduttore.
La differenza di potenziale di giunzione resiste al trasferimento di elettroni dal semiconduttore al metallo ed è determinata dalla differenza nelle loro funzioni di lavoro.
Se la funzione di lavoro del metallo è inferiore a quella del semiconduttore, si forma una giunzione ohmica. Qui, i livelli di Fermi si allineano quando gli elettroni si trasferiscono dal metallo al semiconduttore.
Questo allineamento aumenta le energie degli elettroni del semiconduttore rispetto al metallo, consentendo il flusso di elettroni dal semiconduttore al metallo. Quindi, la giunzione ohmica conduce la corrente in entrambe le direzioni.

giunzioni metallo-semiconduttore

Metal-Semiconductor Junctions

Il contatto tra un metallo e un semiconduttore può portare alla formazione di una giunzione con comportamento Schottky o Ohmico.

Barriere Schottky

Le barriere Schottky si formano quando un metallo con una funzione di lavoro (Φm) entra in contatto con un semiconduttore con una funzione di lavoro diversa (Φs). Inizialmente, gli elettroni si trasferiscono finché i livelli di Fermi del metallo e del semiconduttore non si allineano all'equilibrio. Ad esempio, se Φm > Φs, il livello di Fermi del semiconduttore è superiore a quello del metallo prima del contatto. Il potenziale elettrostatico del semiconduttore deve essere aumentato per allineare i livelli di Fermi, risultando in una regione di svuotamento in cui le cariche positive provenienti da ioni donatori non compensati, bilanciano la carica negativa sul metallo. L'ampiezza di svuotamento nel semiconduttore può essere calcolata in modo simile a quella delle giunzioni p-n.

Il potenziale di contatto di equilibrio (Vo) impedisce un'ulteriore diffusione degli elettroni dalla banda di conduzione del semiconduttore nel metallo. Questo potenziale è la differenza nei potenziali della funzione lavoro (Φm - Φs). L'altezza potenziale della barriera (ΦB) per l'iniezione di elettroni dal metallo nella banda di conduzione del semiconduttore, è data da Φm - χ, dove χ è l'affinità elettronica.

Contatti ohmici

In molte applicazioni, come i circuiti integrati, è fondamentale disporre di contatti metallo-semiconduttore ohmici con una caratteristica I-V lineare in entrambe le direzioni di polarizzazione. I contatti ohmici si formano quando la carica indotta nel semiconduttore per allineare i livelli di Fermi, è fornita dai portatori maggioritari. Ad esempio, in un semiconduttore di tipo n dove Φm < Φs, gli elettroni si trasferiscono dal metallo al semiconduttore per allineare i livelli di Fermi, aumentando le energie degli elettroni del semiconduttore. Ciò si traduce in una piccola barriera al flusso di elettroni, facilmente superabile da una piccola tensione. Allo stesso modo, per i semiconduttori di tipo p dove Φm > Φs, il flusso delle lacune attraverso la giunzione è facilitato, garantendo una resistenza minima e nessuna rettifica del segnale.

Giunzioni Metallo-Semiconduttore

Il contatto tra un metallo e un semiconduttore forma una giunzione con comportamento Schottky o Ohmic.
Se la funzione di lavoro del metallo supera quella del ...

Education

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JoVE Core

Electrical Engineering

Ingegneria elettrica

Unit 1

Unità 1

Basics of Semiconductors

Fondamenti dei semiconduttori

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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 higher than the metal's before contact. The semiconductor potential then rises, and a depletion region forms where positive donor ions balance the negative charge on the metal.

The depletion width in the semiconductor can be calculated in a way similar to a p-n junction. The equilibrium contact potential Vo stops more electrons from diffusing from the semiconductor conduction band into the metal. This potential equals the work function difference, Φm - Φs. The barrier height ΦB for electron injection from the metal into the semiconductor conduction band is Φm - χ, where χ is the electron affinity.

Ohmic contacts are needed in many devices, including integrated circuits, because they give a linear I-V characteristic in both bias directions. They form when the charge needed to align the Fermi levels comes from majority carriers. In an n-type semiconductor with Φm less than Φs, electrons move from the metal to the semiconductor, and only a small barrier remains. In a p-type semiconductor with Φm greater than Φs, hole flow is favored, so the contact has low resistance and does not rectify the signal.

Schottky and Ohmic Metal Contacts

energy bands in solids

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p-n junction

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metal-semiconductor junctions

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

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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 metal-semiconductor contacts form Schottky barriers or Ohmic contacts, and why work function differences control current flow and resistance.

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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

Schottky and Ohmic Metal Contacts

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