Metal-semiconductor contact

When a metal and a semiconductor, or just two metals, are brought in a contact, potential difference develops at the contact, which depends on the difference between the work functions of contacting materials.

Assume that the work function of a metal is higher than that of an electronic semiconductor (F_m>F_n). With the ideal contact established between the metal and the semiconductor, electrons initially flow from the semiconductor that has a lower work function into the metal with a higher work function. This provides increase in a negative charge on the metal surface that impedes the further transfer of electrons from the semiconductor adjacent layer into the metal. In the equilibrium state, the certain potential difference develops between the metal and the semiconductor which balances out the diffusion flow of electrons from the semiconductor into the metal and the drift flow of electrons in the reverse direction. The diffusion flow arises from the difference in the work functions, and the drift flow appears due to the electric field of the potential difference across the contact.

The state that established at a semiconductor-metal contact differs from the state at a metal-metal contact. The density of free electrons in an n-semiconductor is less than free electron density in a metal surface layer.

So, all electrons in the near-surface layer at the semiconductor are insufficient to ensure the necessary density of surface charge. As a consequence, the electrons will be drawn up from the near-contact semiconductor layer; leaving behind non-compensated positive ions of the donor impurity. The near-contact layer that becomes depleted of majority carriers indicates increased resistance. This depletion layer is also called a blocking or barrierlayer.

At a semiconductor-metal contact a high potential barrier (Shottky) is formed. The height of this barrier will alter considerably with polarity of applied voltage. Such contact has rectifying property and is used in Shottki’s diodes.

If the work function for a metal is lower than the work function for an n-type semiconductor (F_m<F_s) electrons of the metal brought in contact with a semiconductor will flow into a semiconductor and make its near-contact layer rich in majority carriers, thereby the established into a semiconductor layer has high conductivity, which is termed an enriched layer.

Let's consider a contact formed between a metal and a p-type semiconductor, as long as F_s <F_m. At the moment when a metal and a semiconductor come in contact, a certain number of electrons in a semiconductor will move to a metal. Then the electric field that appears in the process will push electrons away into the semiconductor and attract holes, which are majority carriers in the p-semiconductor. In consequence, an enriched layer forms in the near-contact region of the semiconductor.

Setting F_s >F_m, we will have a barrier layer as a result.

On applying to the contact an external potential difference with polarity opposite in sign to the polarity of the contact potential difference, the potential barrier of the contact will come down, so that the flow of electrons from the semiconductor to the metal will rise in intensity. The current flowing through the contact is called the forward current.

If the strength of external electric field applied to a metal-semiconductor contact that has a barrier region is coincident in direction with the strength of the electric field produced by the contact potential difference, then the potential barrier at the contact will grow, and therefore the flow of electrons from the semiconductor to the metal will come down. The current that now passes through the contact is called the reverse or back current.