Semiconductors saturation current

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... 

The equilibrium hole concentration can be increased by raising the temperature of the semiconductor. Uhlir (7) found that the temperature variation of the saturation current density across the barrier between anodic n-type germanium and 10% potassium hydroxide solution is quite like that of a p-n junction. About a tenfold increase in the saturation current density is obtained for each 30° C rise in temperature as shown in Fig. 2. 

SiC p-n diodes clearly illustrate the advantages of wide bandgap semiconductors in general and SiC in particular. The elementary theory of p-n junctions yields the following expression for the reverse saturation current density, JR, for a p+-n junction [1] ... 

Some care must be exercised when using the reverse saturation current obtained from the semilogarithmic current voltage plot and equation 12 to determine the metal-semiconductor barrier height c()g. Card and Rhoderick have shown that if the interfacial oxide is sufficiently thick so that the electron tunnelling transmission coefficient is no longer unity then the reverse saturation current is reduced to a value equal to the product of the reverse saturation current when no interfacial layer is present and the transmission coefficient of the interfacial oxide> that is... 

The power characteristics of a photocell can be constructed from the individual current voltage curves of the photoelectrode and the counter electrode. This is shown in Fig. IV.4, again for an n-type semiconductor. In this figure the influence of the counter electrode is indicated. If the redox reaction there is slow, i.e. if it needs large overvoltages, the power characteristics are very much worsened. A similar effect has the photoelectrode if the photocurrent does not increase very steeply below the open circuit photovoltage and does not reach the saturation current very soon. 

The organic photovoltaic device can be approximated by the equivalent circuit shown in Fig. 8.4 [2], which is similar to the circuit used to represent inorganic photovoltaics based on p-n junctions. The circuit consists of a photocurrent source (Iph) in parallel with a diode, which produces an opposing dark current. The optimum value of the diode s ideality factor n is one, but it is often larger depending on the recombination mechanism. The saturation current Iq is the current in the dark at reverse bias. The series resistance (Rs), which has to be minimized, results from the finite conductivity of the semiconducting material, the contact resistance between the semiconductors and the electrodes, as well as the resistance associated with electrodes and interconnections. The shunt resistance (Rp), which... 

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