Accumulation mode mobility

As indicated above, there is a relationship between particle concentration, equilibrium factor and the amount of highly mobile radioactive particles. Removal of the accumulation mode particles may decrease the decay product exposure, but increase the dose because of the high effectiveness of the "unattached activity in dose deposition. Thus, air cleaning may not succeed in lower risk unless both factors are taken into account. Jonassen explores electrostatic filtration in this context. Finally, design considerations are presented for a possible alternative control system using activated carbon in an alternating bed system. 

As discussed earlier in Section 5.2, introducing thin n-type buried layers under the gate oxide is expected to improve channel mobility. The accumulation mode DMOS structure, referred to as ACCUFET, has been patented by Baliga in 1996 [35]. Experimental results on this structure implemented in 6H-SiC have been published in work [36]. Similar devices implemented in 4H-SiC and referred to as Accu-DMOSFET have been reported later by Singh et al. [37]. 

Although the SEMS represents a marked advance in the state of the art for measurement of aerosol size distribution, an important gap remains in current measurement technology, namely, the ability to make rapid, high-resolution measurements of the accumulation-mode aerosols on-line. The limitation of the DMA or SEMS for measurement of particles larger than 0.2 xm in diameter is the multiple charging that allows particles of two or more different sizes to contribute a given mobility fraction. Regardless of... 

In a-Si H, it is difficult to operate CCDs in the same mode as conventional SCCDs. This is because the ideal inversion at the p-type a-Si interface has not been observed as yet (Sugiura and Matsumura, 1982), and as for H-type a-Si H, whose interface can be inverted, the drift mobility of the signal charge (i.e., hole) is very low. However, the band gap of a-Si H is about 1.7 eV, which is much wider than that of a single-crystal silicon. This indicates that the carrier concentration in undoped a-Si H under thermal equilibrium condition is so low that the accumulation mode can store transient signals sufficiently long for CCD operation, as described later. 

The usual TFT structure is shown in Fig. 10.7 and comprises the a-Si H channel, a gate dielectric, and source, drain, and gate contacts. N-channel accumulation mode operation using an undoped a-Si H channel is the only structure widely used. Depletion mode devices are prevented by the high defect density of doped material, which makes it difficult to deplete the channel. The much lower mobility of holes compared to electrons gives p channel devices a lower current by about a factor 100, which is undesirable. 

Illustrated in Figure 24.4 is the output characteristic of a pentacene OFET with Au drain-source electrodes and a 200 nm Si02 dielectric [32]. The OFET exhibits unipolar p-type behaviour with a hole mobility = 0.165 cmWs, a threshold of = -4.5 V as well as an On/Off ratio of >10. These parameters have been derived from the respective transfer characteristics. The absence of an s-shaped feature in the linear range of the characteristic indicates ohmic contacts between the Au electrodes and the pentacene active layer. This is attributed to the good matching of the ionisation potential of the organic semiconductor and the Au work frmction. However, employing a Ca drain-soirrce metallisation, with an otherwise identical OFET device structure, the transistor did not exhibit any current in the electron accumulation mode. This is unexpected, since the metal work frmction is well matched to the electron affinity of pentacene. 

Contemporary OFETs are based on undoped organic semiconductors, and mobile charges in these devices must be injected from the metallic contacts. These devices can potentially operate in the electron- and the ftote-accumulation modes, depending on the polarity of the gate voltage (the so-called ambipolar operation). Often, however, the injection barrier at the contact or the held-effect threshold for either n- or... 

Electric mobility—can measure nuclei and accumulation modes can resolve Og of 1.4 without correction and 1.25o with correction 0.006 1 15 1 pg/m Diffusion charging and electric mobility measurement... 

Field-effect mobility for the accumulation mode operation. 

Figure 21 shows the transistor characteristics of a typical printed device fabricated with regioregular PHT [89]. All the transistors are p-channel devices and can operate both in accumulation-mode and depletion-mode. The field-effect mobility was found to be between 0.01 and 0.03cm s . This is one of the... 

In Eq. (6) L is the channel length while Mi,2 re either the mobility of electrons (1 —> n) and holes (2 —> p), respectively, or vice versa that of holes (1 p) and electrons (2 —> n). The former holds for OFETs driven in electron accumulation mode ( electron accumulation mode we define as Vg > OV following the... 

Fig. 6 Simulated transfer characteristic in electron accumulation mode at Vj = lOOV for different ratios of the electron and hole mobility /t = lO m V s , IV = 8mm, L = 100(tm, Vth,n = Vth,p = 0 V, C = 7.2 X 10 m. With permission adapted from [43], copyright 2008...

The decompositions of hydroperoxides (reactions 4 and 5) that occur as a uni-or bimolecular process are the most important reactions leading to the oxidative degradation (reactions 4 and 5). The bimolecular reaction (reaction 5) takes place some time after the unimolecular initiation (reaction 4) provided that a sufficiently high concentration of hydroperoxides accumulates. In the case of oxidation in a condensed system of a solid polymer with restricted diffusional mobility of respective segments, where hydroperoxides are spread around the initial initiation site, the predominating mode of initiation of free radical oxidation is bimolecular decomposition of hydroperoxides. 

Lithium(I) ions are small but strongly hydrated and could interfere with Mg(II) biochemistry. However, the favored mode of action is interference with Ca(II) metabolism via inhibition of enzymes in the inositol phosphate pathways (470-472). Inositol phosphates are responsible for mobilizing Ca(II) inside cells in response to external stimnlii. Lithium also stimulates glutamate release presumably via activation of the AT-methyl-D-asparate receptor and leads to Ca(II) entry (473). The increased influx of intracellular Ca(II) may activate phospholipase C and stimulate accumulation of inositol 1,4,5-triphosphate (473). 

Silica gel is a polar material. The presence of silanol groups is responsible for the acidic catalytic effect of this material (the pK of Si OH is comparable to that of phenol). The mode of action of silica gel is based on adsorption (Fig. 3.9), a phenomenon that leads to the accumulation of a compound at the interface between the stationary and mobile phases. In the simplest case, a monolayer is formed (known as a Langmuir isotherm) but there is also some attraction and interaction between molecules that are already adsorbed and those still in solution. This contributes to the asymmetry of the elution profile. Although it demonstrates good resolution and a high adsorption capacity, bare silica gel is seldom used for analytical purposes. For most applications, it must be deactivated by partial rehydration (in 3-8% water). 

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