Contact charging metal-semiconductor

Eg and Eh are denoted as transport levels. To carry a stationary current, they must be populated and depopulated. At each of the contacts, the contact barriers which exist there, that is the differences of the energies of the transport levels of the two contacting materials (metal/semiconductor or semiconductor 1/semiconductor 2), must be overcome in order for the contact to allow the passage of charges. And, on the other hand, the energy gap 1 ... 

T. Ioannides, and X.E. Verykios, Charge transfer in metal catalysts supported on Doped Ti02 A Theoretical approach based on metal-semiconductor contact theory, J. Catal. 161,560-569 (1996). 

The photovoltage is esentially determined by the ratio of the photo- and saturation current. Since io oomrs as a pre-exponential factor in Eq. 1 it determines also the dark current. Actually this is the main reason that it limits the photovoltage via Eq. 2, The value of io depends on the mechanism of charge transfer at the interface under forward bias and is normally different for a pn-junction and a metal-semiconductor contact. In the first case electrons are injected into the p-region and holes into the n-region. These minority carriers recombine somewhere in the bulk as illustrated in Fig. 1 c. In such a minority carrier device the forward current is essentially determined... 

As with metal-semiconductor contacts, the electric field in the space charge, which is given by the diffusion voltage Fd and the width of the space-charge region W, separates optically generated electron-hole pairs. W is given by 41> ... 

In bulk heterojunction solar cells, the metal/semiconductor interface is even more complex. Now the metal comes into contact with two semiconductors, one p-type (typically the polymer) and one n-type (typically the fullerene) semiconductor. A classical electrical characterization technique for studying the occurrence of charged states in the bulk or at the interface of a solar cell is admittance spectroscopy. If a solar cell is considered as a capacitor with capacitance C, the complex admittance Y is given by... 

Schottky barrier — Energy barrier formed at metal-semiconductor junctions. When a metal is intimately brought into contact with a semiconductor, charge... 

Consider a plane-parallel condenser of capacitance C whose plates are a p-type semiconductor (e.g., a CP) and a metal, and polarize the latter negatively. Excess positive charges (i.e., holes) appear at the surface of the semiconductor, and since its conductivity is low, they are in fact distributed over a certain thickness within the material. These excess holes, or at least part of them, should take part in the conduction. Applying a voltage to an external electrode not in contact with the semiconductor modulates its conductivity. This is the principle of the field effect, and clearly this control of the current through a gate electrode opens the possibility of transistor action without requiring the existence of p-n junctions. 

The simplest case to consider is the situation that describes the contact between an n-type semiconductor and a metal. As discussed above, the factor that controls the charge transfer process is the electrochemical potential, or Fermi level, E-g, of each phase. The initial difference in the electrochemical potentials of the two phases indicates that, after contact, charge must flow between the phases in order to reach equilibrium. We will consider the case where the electrochemical potential of an isolated n-type semiconductor (Af.sc) is more negative than the electrochemical potential of an isolated metal phase... 

The condition of thermodynamic equilibrium in a metal-semiconductor contact states that the electrochemical potential should be uniform throughout the system. If, before contact, the metal and the semiconductor have different electrochemical potential, then upon contact, charge will flow to the material with the smaller potential, until the potentials are equalized. When the two materials have no net charge, Jlf = Om and Ji = where M and S denote the metal and the semiconductor, respectively. If Om > the electron flux will be toward the metal. At equilibrium, the common electrochemical potential will be... 

A mathematical model of metal-semiconductor contacts has been employed to estimate the quantity of charge transferred through the interface, based on parameter values that pertain to the M/Ti02 system [88]. The direction of electron flux in a metal-semiconductor contact depends on the relative values of the work function of the two materials. The work function of the semiconductor is a function of the kind (valence) and concentration of the dopant and of temperature. Doping of... 

A silicon electrode with a small amount of deposited noble metals such as Pt does not behave as a buried junction and the photoelectrode properties are still determined by contact of the electrolyte, not the metal. The photovoltaic response of the Pt-deposited silicon electrode is due to the incomplete coverage of the surface by Pt so that the interface energetics remains dominated by the contact of the semiconductor surface with the electrolyte. The mechanism of the hydrogen reduction processes involves first photoexcitation of electron-hole pairs followed by charging of the small Pt islands with excited electrons. The charged Pt islands react with H2O at a high rate. 

---