Positive hole, recombination and reciprocity failure
This is something from material science of semiconductor materials, and not a common term in photography (unless you study emulsion chemistry). I'll try to make gross simplification.
Silver halide crystals are like silicone crystals in a sense that they are indirect gap semiconductors. Such materials have covalent bonds between molecules and there are some electrons that are tightly shared between adjacent molecules. Those electrons are usually locked in and they stay there. You can think of it as a highway in heavy traffic jam, where cars aren't flowing. (Cars are electrons that are locked in.)
When one photon is absorbed by the crystal, this excites the chemical bonds between molecules, and knock one electron out. This makes a void of electron in the covalent bond from which the electron was ejected. This is hole. You can think of one car is given extra energy from the photon to jump up and escape from the traffic.
In order to make a developable latent image center, one crystal must absorb several photons, and collect several electrons to one site (sensitivity center) where electrons reduce a few silver halide molecules metallic silver in a tiny cluster (speck). This is where the developer can act on to develop exposed crystals.
Given the constant crystal size and shape, in order to increase the sensitivity of the crystal and also reduce reciprocity failure, it is essential to have a mechanism to concentrate generated electrons to one place (ther fewer the better!) so that the electrons are most efficiently and most quickly used to make a developable latent image center. (One LI center is sufficient to develop the entire grain.) It is best to make latent image center very quickly because intermediate forms (latent subimages) are unstable and they may break if electrons don't gather at the location fast enough. A LOT of progress in this area was made by Fuji, Kodak and AGFA researchers from mid 1980s and on. Most exciting findings are from Kodak people in 1980s and Fuji people in 1990s. You can think of this as having fewer recycle bin (for photoelectrons) would fill them up faster than having more recycle bins. Full recycle bins are the latent image centers.
One problem with holes is that, they attract electrons and electrons want to jump back in to fill the void. This is called recombination. Once this happens, the electron has lost its power to make latent image center. (Cars get in to any space in traffic jams.)
Thus, formation of a latent image center is at a competition with recombination. So another factor in increasing sensitivity is to minimize recombination by somehow stabilizing the void, or filling the void of electrons with electrons that come from elsewhere other than those generated by light.
There are some compounds that take up the hole and release second electron. This can double the number of useful electrons to make latent image center, and it can increase the photographic speed of the same crystal. Pioneer of this research was AGFA people and French scientists, but Kodak is also active in this approach using more practical materials.
Anyway, Foma is not very close to most of the above stuff in terms of technology.
Roger Hicks said:
When an electron (negative charge) is knocked out by a photon, it leaves a 'positive hole'. If the 'positive hole' is refilled by another electron (or the same one, insofar as that means anything), the development site weakens or disappears. 'Trapping' the 'positive hole' increases sensitivity and decreases reciprocity failure.
