Metal to Semiconductor Contacts: Injection or Extraction for Either Direction of Current Flow

A study of metal to semiconductor contacts has been carried out using infrared radiation to measure the current versus added carrier density ($J-\Delta p$) characteristics. It is found that the nature of the semiconductor surface rather than the metal is the major factor in controlling the characteristics of the metal to semiconductor contact. Two unusual classes of results are described; viz., where injection into the semiconductor bulk is observed regardless of the direction of current flow and where extraction is observed regardless of the direction current flow. The model proposed to explain these unusual results utilizes an insulating film between the metal and the semiconductor. Two competing effects, voltage controlled injection and extraction, which can explain these results, are discussed. Extraction for either direction of current flow is explained by the occurrence of the field-effect at the metal to semiconductor contact. Injection for either direction of current flow is explained by a change, due to the applied voltage, in the communication of the current carriers in metal with the valence and conduction bands of the semiconductor, resulting in a change of the current composition entering the semiconductor. Relaxation effects, which are attributed to the action of slow surface states on the charge induced in the surface barrier, have also been observed in the $J-\Delta p$ characteristics. The predominance of injection after relaxation is thus attributed to a reduction of the magnitude of the field effect through the shielding effect of the charge in the surface states.