Ohmic Contact

An ohmic contact is a non-rectifying junction—a region in a semiconductor device that has been prepared so that the current–voltage (I-V) curve of the region is linear and symmetric. Usually it is a metal–semiconductor junction between a metal lead and semiconductor material. If the I-V characteristic is non-linear and asymmetric, the contact is not ohmic, but is a blocking or Schottky barrier contact, which creates a rectifying junction, known as a Schottky diode. In the Schottky barrier model, rectification is dependent on the difference between the work function of the metal and the electron affinity of the semiconductor. However, in practice, metal–semiconductor interfaces do not exactly follow the Schottky model, because the presence of extrinsic surface states at the interface (i.e., oxides and defects) can make the behavior of the junction almost independent of the difference of work function and electron affinity. In semiconductor device fabrication (integrated circuit packaging), to make an ohmic contact, the contact region is doped, creating an extrinsic semiconductor to ensure the type of contact wanted (usually an n+ doped contact for an n-type silicon wafer and aluminum wires).

Typical ohmic contacts on the semiconductors are sputtered or evaporated metal pads that are patterned using photolithography. Low-resistance, stable contacts are critical for the performance and reliability of integrated circuits and their preparation and characterization are major efforts in circuit fabrication. As a rule, ohmic contacts on semiconductors form when the semiconductor is highly doped. A high doping would narrow the depletion region at the interface and allow electrons to flow in both directions easily at any potential by tunneling through the barrier.