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

Der Kontakt zwischen einem Metall und einem Halbleiter bildet einen Übergang mit entweder Schottky- oder Ohm-Verhalten.
Übersteigt die Arbeitsfunktion des Metalls die des Halbleiters, bildet sich ein Schottky-Übergang. Vor dem Kontakt werden die Energiebanddiagramme auf der Vakuumebene ausgerichtet, wobei sich die Fermi-Niveaus an verschiedenen Positionen befinden.
Beim Kontakt richten sich die Fermi-Niveaus aus, wenn Elektronen vom Halbleiter auf das Metall übertragen werden und ein Gleichgewicht erreichen.
Der Elektronenverlust im Halbleiter verringert sein Potential, während ihre Zugabe zum Metall das Potential erhöht und an der Verbindungsstelle eine Schottky-Barriere bildet. Die Barrierenhöhe entspricht der Differenz zwischen der Arbeitsfunktion des Metalls und der Elektronenaffinität des Halbleiters.
Die Sperrschichtpotentialdifferenz widersteht dem Elektronentransfer vom Halbleiter auf das Metall und wird durch die Differenz ihrer Arbeitsfunktionen bestimmt.
Ist die Arbeitsfunktion des Metalls kleiner als die des Halbleiters, bildet sich ein ohmscher Übergang. Hier richten sich die Fermi-Niveaus aus, wenn Elektronen vom Metall auf den Halbleiter übertragen werden.
Diese Ausrichtung erhöht die Elektronenenergien des Halbleiters relativ zum Metall und ermöglicht den Elektronenfluss vom Halbleiter zum Metall. Der ohmsche Übergang leitet also Strom in beide Richtungen.

metall-halbleiter-Übergänge

Metal-Semiconductor Junctions

Der Kontakt von Metall und Halbleiter kann zur Bildung eines Übergangs mit Schottky- oder Ohm'schem Verhalten führen.

Schottky-Barrieren

Schottky-Barrieren entstehen, wenn ein Metall mit einer Austrittsarbeit (Φ_m) einen Halbleiter mit einer anderen Austrittsarbeit (Φ_s) berührt. Zunächst werden Elektronen übertragen, bis sich die Fermi-Niveaus des Metalls und des Halbleiters im Gleichgewicht befinden. Wenn beispielsweise Φ_m > Φ_s ist, ist das Fermi-Niveau des Halbleiters höher als das des Metalls vor dem Kontakt. Das elektrostatische Potential des Halbleiters muss erhöht werden, um die Fermi-Niveaus anzugleichen, was zu einem Verarmungsbereich führt, in dem positive Ladungen von nicht kompensierten Donorionen die negative Ladung des Metalls ausgleichen. Die Verarmungsbreite im Halbleiter kann ähnlich wie bei p-n-Übergängen berechnet werden.

Das Gleichgewichtskontaktpotential (V_o) verhindert eine weitere Elektronendiffusion aus dem Leitungsband des Halbleiters in das Metall. Dieses Potenzial ist die Differenz der Austrittsarbeitspotenziale (Φ_m - Φ_s). Die Potenzialbarrierehöhe (Φ_B) für die Elektroneninjektion vom Metall in das Halbleiterleitungsband ergibt sich aus Φ_m - χ, wobei χ die Elektronenaffinität ist.

Ohmsche Kontakte

In vielen Anwendungen, wie z. B. integrierten Schaltkreisen, ist es entscheidend, ohmsche Metall-Halbleiter-Kontakte mit einer linearen I-V-Kennlinie in beiden Vorspannungsrichtungen zu haben. Ohmsche Kontakte werden gebildet, wenn die im Halbleiter induzierte Ladung zur Ausrichtung der Fermi-Niveaus durch Mehrheitsträger bereitgestellt wird. Beispielsweise werden in einem n-Typ-Halbleiter, bei dem Φ_m < Φ_s, Elektronen vom Metall zum Halbleiter übertragen, um die Fermi-Niveaus auszurichten, wodurch die Elektronenenergien des Halbleiters erhöht werden. Dies führt zu einer kleinen Barriere für den Elektronenfluss, die leicht durch eine kleine Spannung überwunden werden kann. In ähnlicher Weise wird bei p-Typ-Halbleitern, bei denen Φ_m > Φ_s, der Lochfluss über die Verbindung erleichtert, wodurch ein minimaler Widerstand und keine Signalgleichrichtung gewährleistet werden.

Metall-Halbleiter-Übergänge

Der Kontakt zwischen einem Metall und einem Halbleiter bildet einen Übergang mit entweder Schottky- oder Ohm-Verhalten.
Übersteigt die Arbeitsfunktion des Metalls die des ...

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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

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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...

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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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electrical engineering

Schottky and Ohmic Metal Contacts

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