Depletion width

Fig. 9. Schottky barrier band diagrams (a) a rare situation where the metal work function is less than the semiconductor electron work affinity resulting in an ohmic contact (b) normal Schottky barrier with barrier height When the depletion width Wis <10 nm, an ohmic contact forms.

As k g increases, the depletion width at the drain junction grows and can accommodate more charge. Thus, less charge is needed in the inversion layer to balance the gate charge. Because the surface potential at the drain edge of the channel is k g, when U g — < Up inversion can no longer be... 

The charge earner depletion width, W, at the recti Tying contact, which forms a Schottky barrier, can be calculated using the following Eq. (99) (47) ... 

Figure 9-22. Energy diagram ol a metal/ scmiconductor/meta Schottky barrier (0... workfunction, x,. electron affinity, /,... ionization potential, . ..bandgap, W... depletion width).

The decrease in free carriers (holes) after hydrogenation of p-type Si is also evidenced by the decrease in IR absorption at the longer wavelengths, where free-carrier absorption dominates, and by a decrease in the device capacitance of Schottky-barrier diodes, due to the increase in the depletion width (at a given reverse bias) as the effective acceptor concentration decreases. 

The of the cell was measured as a function of the collector current with different collector supply voltages (Figure 6.29). For = 20V, peaks at about 1.5 GHz, whereas for V = 30V, fj. peaks at about 1.3 GHz. This reduction in f. with collector voltage is expected as the electron transit delay in the collector depletion width increases with increasing collector bias. The sudden drop in at high current for V c = 30V is attributed to a hot spot on the device. 

Fig. 4. Charge distributions in the voltage-biased MOS capacitor (a) accumulation of majority carriers near surface (b) depletion of majority carriers from surface (c) inversion, accumulation of minority carriers near surface (26). VG = gate voltage Qq = gate charge xand xd = depletion widths and...

The depletion width can play a role in analyte-induced modulation of the semiconductor PL [4]. As molecules adsorb onto the surface of the semiconductor, the dead-layer thickness can change, resulting in what can be described as a luminescent litmus test When Lewis bases adsorb onto the semiconductor surface, they donate electron density to the solid, which decreases the electric field and thus decreases the dead-layer thickness. The reduction in D causes an enhancement in the PL intensity from the semiconductor. Figures 2a and 2b present typical PL enhancements observed from an etched n-CdSe substrate Relative to a nitrogen reference ambient, adsorption of the Lewis bases ammonia and trimethylamine cause a reversible increase in PL intensity. In contrast, when Lewis acids adsorb onto the surface, they can withdraw additional electron density, causing the electric field to increase and the PL intensity to decrease. Such effects have been observed with gases like sulfur dioxide [5]. 

The quantitative form of the dead-layer model relates PL intensity to dead-layer thickness, which is assumed to approximate the depletion width W [6,7] ... 

These devices showed EL enhancements to ammonia, methylamine, di-methylamine, trimethylamine, and sulfur dioxide that increased in magnitude with concentration until saturation was reached [14]. The LEDs with larger active layers produced the greatest change in EL intensity with exposure to sulfur dioxide and the amines. Intensity changes were attributed principally to surface recombination velocity effects, as the significant forward biases employed should eliminate the depletion width. 

In practice, this limitation is not seriously restrictive. Moderate electrolyte concentrations (t 0.5N) are required by solution IR drop concerns, yielding ds on the order of a few angstroms. Optimal semiconductor doping levels are dictated by developing H with the approximate value of a l, where a is the semiconductor absorption coefficient, and H is the depletion width. As a consequence, a typical order of magnitude for H is 1 micron (104 A). Thus the ratio H/ds... 

In poly crystalline semiconductor samples, the excited-state lifetime of electron-hole pairs is so short that photocurrent collection is efficient only for carriers created within the space charge (depletion) region. Thin-film processes offer an inexpensive way to prepare large solar arrays, but the semiconductors formed by such processes are almost inevitably polycrystalline. It is not wise to use semiconductor films thicker than the depletion width in such devices because the additional thickness contributes only extra grain barrier boundaries for the majority of carriers to surmount on their way to the back contact. The additional thickness does not provide any additional photocurrent. 

From the fact that 633-nm light passing under the detector was not observed behind the detector, it was concluded that the beam was effectively absorbed. Operating in the photovoltaic mode, the detector voltage was a linear function of the guided wave power (Fig. 2). The rise time of these detectors was 350 nsec. From assumed values for thfe mobility, junction voltage, and depletion width, the dominant limiting factor for the speed of these detectors was the transit time. 

Fig. 9.5. The depletion potential profile of a-Si H with platinum and chromium contacts measured by transient photoconductivity. The inset plots the dependence of the charge collection, Q., on applied bias which shows the shrinking of the depletion width in forward bias (Street 1983).

Fig. 9.9. Calculated doping dependence of the depletion width and depletion field at different bias voltages.

Transient photoconductivity measurements of the depletion width, as described in Section 9.1.3, show that there is an electron accumulation layer at the interface with SijN4 (Street et al. 1985b). In contrast, an oxide interface (either a native or deposited oxide) has a depletion layer (Aker, Peng, Cai and Fritzsche 1983). The band bending causes similar changes in the conductance of the films as is described for adsorbed molecules in Section 9.2.2. 

Doping multilayers are formed by alternating n-type and p-type doping. This results in a smooth modulation of the bands, as illustrated in Fig. 9.31, because electrons transfer from the n-type to the p-type layer. The maximum amplitude of the modulation is equal to the shift of the Fermi energy between the n-type and the p-type material. There is incomplete transfer when the layer width is less than the depletion width, so that the modulation amplitude is smaller. For this... 

Depletion width the depth of the depletion region in a semiconductor... 

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