Deep depletion

Fig. 6. Band model for the charge mode detector biased to deep depletion. The charge, integrates in the potential well defined by the insulator and...

Fig. 5-44. Space charge layers of n-type semiconductor electrodes (c) an inversion layer, (d) a deep depletion layer.

Fig. 6-48. Differential capacity of a space charge layer of an n-type semiconductor electrode as a function of electrode potential solid cunre = electronic equilibrium established in the semiconductor electrode dashed curve = electronic equilibrium prevented to be established in the semiconductor electrode AL = accumulation layer DL = depletion layer IL = inversion layer, DDL - deep depletion layer.

The thickness of depletion and deep depletion layers may be approximated by the effective Debye length, Lo, ff, given in Eqn. 5-70 Ld, is inversely proportional to the square root of the impiuity concentration, In ordinary semiconductors... 

The energy levels in the solution are kept constant, and the applied voltage shifts the bands in the oxide and the silicon. The Gaussian curves in Figure 4b represent the ferrocyanide/ferricyanide redox couple with an excess of ferrocyanide. E° is the standard redox potential of iron cyanide. With this, one can construct (a) to represent conditions with an accumulation layers, (b) with flatbands, where for illustration, we assume no charge in interface states, and (c) with an inversion or deep depletion layer (high anodic... 

A fifth type of space charge layer, the deep depletion layer, may be formed under non-equilibrium conditions at the semiconductor surface when a high voltage is applied such that an inversion layer should form, but either (a) minority carriers are not available to accumulate at the surface in the time allotted or (b) the minority carriers are consumed in an electrochemical reaction as soon as they reach the surface. Such a space charge layer is unlikely to form within semiconductor electrodes at open circuit and is included here solely for completeness. 

Although the reverse current of an ideal Schottky barrier is J, in practice there are other current soitfces. Imperfect contacts have a leakage current which generally increases exponentially with bias. Even with an ideal contact, there is a thermal generation current caused by the excitation of electrons and holes from bulk gap states to the band edges. This mechanism determines the Fermi energy position under deep depletion conditions. The current density is the product of the density of states and the excitation rate and is approximately. 

FIGURE 1.8. Variation of the space charge capacity with a band bending of V, on an n-type semiconductor with an accumulation layer or an inversion layer. Mobile carriers are at the surface in the inversion so the capacity is high. If minority carriers do not accumulate at the surface at a large band bending, a deep depletion curve results. After Morrison. ... 

Figure 5.9. Spectra of 0.1 M Na2S04 in a 1 cm quartz cuvette in water obtained with a Chromex Raman 2000 spectrometer and EEV 15-11 deep depletion CCD (—90°C), 50 mm slit, 600 line/mm grating, 50 mW of 785 nm light at sample, 20 integrations of 40 sec each were averaged. Lower spectrum is difference between the two upper spectra.

Deep depletion refers to controlled doping of the silicon to enhance long wavelength response, usually applied to front-illuminated CCDs. A Q vs. A, curve for a deep depletion CCD is compared to a conventional front-illuminated response in Figure 8.29. Although deep depletion CCDs cost approximately the same as the conventional front-illuminated CCDs, they have higher dark current and generally must be cooled to lower temperatures. 

Figure 8.29. Quantum efficiency curves for conventional, deep depletion, and UV-enhanced front-illuminated CCDs. (Adapted from Horiba/ISA product literature.)...

Figure 8.32. Temperature dependence of dark spectrum for an EEV 15-11 deep depletion CCD. Integration time was 60 sec in all cases. Positive spikes are due to cosmics negative spike at 1450 cm is due to a column in this CCD with weak response.

In contrast to r measurements, in which the decay of excess carriers is monitored the generation lifetime is determined from the reverse-biased pn junction leakage current or from the pulsed MOS capacitor (22.) latter and the more popular of the two, an MOS-C is pulsed into deep depletion and the capacitance is monitored as a function of time. An appropriate analysis of the C-t response yields t. ... 

CCD (direct detector mode) 20 10x10 17x26 512x512 770x1152 40 40 -250x250 -385 x 576 30 pm silicon deep depletion... 

The CCD is a semi-conductor detector. It can be used as an X-ray detector (for reviews see Allinson (1982), Milch et al (1982) and Allin-son (1989)) either by direct illumination of the X-rays onto the silicon or by conversion of the X-rays in a phosphor to visible light which is then incident onto the silicon. The previous section dealt with phosphor coupled systems based on television cameras. The CCD can replace the television camera. This is the basis of the system being developed by Strauss et al (1987) and Westbrook (1988) see figure 5.25. An alternative is to use direct illumination onto a so-called deep depletion CCD. CCD... 

For small values of applied bias or for metal-oxide-semiconductor (MOS) structures, the Fermi energy may be expected to extend through the barrier region as shown in Fig. 1. However, for laiger reverse bias applied to a Schottky barrier this picture is modified and leads to the formation of a quasi-Fermi level near midgap in the deep-depletion region as shown in Fig. 2. 

Fig. 2. Space-charge region for deep depletion. In the deep-depletion region the charge density is approximately constant as determined by the position of the quasi-Fermi energy. Parameters are defined in the text.

When sufficient reverse bias is applied to the Schottky diode, the potential barrier f/(x) depletes the region near the interface of free carriers to the point that the carrier concentration arises predominantly either from the leakage over the barrier from the metal contact or from the intrinsic thermalization of electrons and holes from the valence and conduction bands. This leads to the formation of a quasi-Fermi level in the deep-depletion region. For the... 

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