(a) Explain why no hole is generated by the electron excitation involving a donor impurity atom. (b) Explain why no free electron is generated by the electron excitation involving an acceptor impurity atom.

In the world of semiconductors, the addition of impurities is a calculated and deliberate process. Donor impurity atoms are integral to this process, usually being elements from group V of the periodic table, such as phosphorus or arsenic. These atoms come with an extra valence electron compared to the four of silicon or germanium, which are the standard semiconductor materials.

This extra electron is only loosely bound to the donor atom and is rather easily excited to the conduction band, contributing to electrical conductivity. Interestingly, because this electron is donated without creating an absence (or 'hole') in the valence band, it enhances conductivity without compromising the balance of charge carriers. This precise mechanism is why no hole is generated when an electron is excited from a donor impurity atom—the extra electron simply shifts from the donor to the conduction band. The semiconductor thus becomes an 'n-type' semiconductor, named for the negatively charged free electrons that predominate.

Now let's converse about the flip side: acceptor impurity atoms. These atoms are typically from group III of the periodic table, like boron or gallium, with one less valence electron than the semiconductor matrix atoms. When these atoms are introduced into the semiconductor lattice, they 'accept' an electron from a neighboring atom's valence electron, thereby creating a hole.

Because the addition of an acceptor impurity atom does not introduce extra loose electrons but instead generates holes, it is referred to as p-type doping—the 'p' indicating the holes carry a positive charge. Here, when an electron gets excited and is captured by the acceptor atom, it simply fills one of these holes without creating a free electron in the process. Thus, in the case of an acceptor impurity, no free electron is generated with electron excitation, maintaining the dominance of 'holes' as the majority charge carriers.

The concept of electron excitation is foundational to understanding how semiconductors conduct electricity. At a simple level, excitation refers to an electron gaining enough energy to move from a lower energy state to a higher energy state. For pure semiconductors, this would mean an electron jumping from the valence band to the conduction band, consequently creating a hole in the valence band.

In doped semiconductors, the energy required to excite an electron from a donor or an acceptor impurity is much less than moving it from the valence band to the conduction band. Once an electron from a donor atom becomes excited to the conduction band, it freely moves and contributes to electrical conductivity, again without generating a hole in the valence band, due to the extra electron provided by the donor.

N-type semiconductors are crafted through the introduction of donor impurities that add loosely bound extra electrons to the existing lattice. These additional electrons become the majority charge carriers i.e., there are more free electrons than holes in such a semiconductor.

The beauty of n-type semiconductors lies in their enhanced electrical conductivity; however, they maintain electrical neutrality because the number of protons in the donor atoms' nuclei compensates for the additional negative charge carriers. The existence of these additional free electrons makes n-type materials quite significant in electronics, as they are used in various components such as diodes, transistors, and integrated circuits.

Conversely, p-type semiconductors rise from the strategic placement of acceptor impurity atoms within the crystal lattice. These atoms effectively create 'holes' which act as positive charge carriers. Here, the holes are more abundant than the electrons, tailoring the electrical properties of the semiconductor to favor hole-based conductivity.

P-type materials are essential in the semiconductor industry, pairing with n-type materials to form p-n junctions, the fundamental building blocks of many electronic devices. The interaction between the holes in p-type and the electrons in n-type materials is crucial for the function of devices like solar cells and light-emitting diodes (LEDs).