Current reverse saturation

Lp Pi 50 pm. and the reverse saturation current would be 17 x 10 = 17 pA for a square centimeter of junction area. Typical reverse saturation currents are about one thousand times greater as a result of generation—recombination currents in the depletion region (9). As the reverse voltage bias increases, the field increases in the depletion region until avalanche breakdown occurs, resulting in the characteristic shown in Figure 7. 

The MOSEET has three regions of operation. The cutoff region occurs for V g < Up. In this region, the drain-to-source current is the reverse saturation current of the back-to-back source and drain junctions. This leakage current is small but nonzero and allows charge to leak off capacitors which are isolated by cutoff MOSFETs. Because this is how bits are stored in dynamic memory (DRAM) ceUs, DRAMs must be regularly refreshed to retain their memory. 

Is = reverse saturated current (temperature dependent), amperes E = diode biasing voltage (+E for forward bias —E for reverse bias), volts... 

The reciprocal derivatives of the j/V curve under illumination at open circuit and short circuit represent the series and parallel resistances Rs and Rp, respectively, of the diode device. Of course, for non-linear effects in the diode, these quantities are not constant but depend on voltage V, current density j (illumination level), reverse saturation current density jtev, and temperature T. 

Rectifier current-voltage (IV) plot. The reverse current before Zener breakdown is /rs, the (negative) reverse saturation current. The equation should be the Ebers-Moll equation, Eq. (9.6.1), / = /rs [exp (eV/kBT) -1], which does not work very well in the Zener breakdown region. 

In Eq. (9.6.1) the current I is zero at V = 0 I grows very rapidly in the forward direction at V < 0, but before Zener33 breakdown, I is the small, negative, voltage-independent negative reverse saturation current Jrs = — Irs. ... 

The total current in the emitter IE, not too far from the emitter-to-base junction, consists of two contributions the electron current (drift current) 1 e, which proceeds under forward bias toward the emitter-to-base junction, and the much smaller hole current (a diffusion current), which originates in the base and proceeds in the opposite direction, but decays exponentially, as the distance from the junction increases. The total current in the collector, close to the base-to-collector junction, consists of electrons l c (a large fraction of 1 e) that have somehow evaded capture within the base and proceed against reverse bias in the collector region. The rest of the electron current in the collector is what in pn diodes is called reverse saturation current frs, and here it is called collector current with zero emitter current Iqo = hs- l c is the "useful" electron flow in the transistor. 

The presence of the reverse saturation current IIS = ICo (this is usually a small effect). 

The total collector current Ic depends on the reverse saturation current IIS (as explained for pn junctions above also called Ico, or collector current at zero emitter current) and on the emitter voltage VE by the Ebers-Moll equation [already introduced in Eq. (9.6.1)] [14] ... 

In 1983 the application of doped tin oxide films to silicon solar cells has been reported [5]. lida and coworkers realized a setup with the following characteristics 7sc = 14 mAcm", Kic = 800 mV, efficiency = 7.5 % and fill factor = 0.67. Vishwakarma et al. [166, 167] prepared arsenic-doped tin oxide films for silicon solar cells and investigated the diode properties of Sn02 As/Si02/n-Si and Sn02 As/n-Si cells. The barrier height 0 was 0.78-0.89 eV and 0.68-0.69 eV, respectively, and the reverse saturation current density 7u was 2-45 pAcm and 0.07-9.2 pAcm", respectively, with diode quality factors of 2.2-2.9 and 1.7-1.9. The optimized results for solar cell applications are given below... 

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] ... 

The temperature effect can also be understood from the increase of n with temperature, which increases the reverse saturation current... 

Jp and Jq are forward bias and reverse saturation current densities... 

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 effect of the presence of a thin interfacial oxide on the dark current voltage characteristics of an SBSC has been examined in sections 2.2 and for the min MIS cell in 2.5. The exact role of the interfacial oxide is not fully understood at present, although it is known that both minority and majority carrier flow over the barrier decrease with increasing oxide thickness. If the oxide is sufficiently thick so that its transmission coefficient is no longer unity, the reverse saturation current density obtained from the forward vs V, characteristics is reduced causing an increase in the effective barrier height < ) This effect is... 

Some calculation of the efficiency of such tandem cell systems was first conducted by Jackson in 1955. A more complete calculation was recently (1979) published by Gokcen and Loferski, and we shall focus on the results of that paper. Their calculation is applicable to solar cells in which the reverse saturation current... 

Expressions for the reverse saturation current I are calculated from the ambipolar diffusion equation which governs the transport of the thermally generated minority carriers in the homogeneous, quasi-neutral regions of the cell. For the one dimensional case, this equation has the form... 

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