It is, by now, well known that McIntyre's localized carrier-multiplication theory cannot explain the suppression of excess noise factor observed in avalanche photodiodes (APDs) that make use of thin multiplication regions. We demonstrate that a carrier multiplication model that incorporates the effects of dead space, as developed earlier by Hayat et al. provides excellent agreement with the impact-ionization and noise characteristics of thin InP, In/sub 0.52/Al/sub 0.48/As, GaAs, and Al/sub 0.2/Ga/sub 0.8/As APDs, with multiplication regions of different widths. We outline a general technique that facilitates the calculation of ionization coefficients for carriers that have traveled a distance exceeding the dead space (enabled carriers), directly from experimental excess-noise-factor data. These coefficients depend on the electric field in exponential fashion and are independent of multiplication width, as expected on physical grounds. The procedure for obtaining the ionization coefficients is used in conjunction with the dead-space-multiplication theory (DSMT) to predict excess noise factor versus mean-gain curves that are in excellent accord with experimental data for thin III-V APDs, for all multiplication-region widths. SECTION I.

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Asymmetry in packing the peptide amide dipole results in larger positive than negative regions in proteins of all folding motifs. The average side chain potential in 305 proteins is 109 +/- 30 mV (2. 5 +/- 0.7 kcal/mol/e). Because the backbone has zero net charge, the non-zero potential is unexpected. The larger oxygen at the negative and smaller proton at the positive end of the amide dipole yield positive potentials because: 1) at allowed phi and psi angles residues come off the backbone into the positive end of their own amide dipole, avoiding the large oxygen; and 2) amide dipoles with their carbonyl oxygen surface exposed and amine proton buried make the protein interior more positive. Twice as many amides have their oxygens exposed than their amine protons. The distribution of acidic and basic residues shows the importance of the bias toward positive backbone potentials. Thirty percent of the Asp, Glu, Lys, and Arg are buried. Sixty percent of buried residues are acids, only 40% bases. The positive backbone potential stabilizes ionization of 20% of the acids by >3 pH units (-4.1 kcal/mol). Only 6.5% of the bases are equivalently stabilized by negative regions. The backbone stabilizes bound anions such as phosphates and rarely stabilizes bound cations.

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