E7 Semiconductor Physics

Topic

Semiconductors conduct electricity at a level somewhere between a good conductor and an insulator, owing to a relatively narrow forbidden energy gap between electrons bound to atoms and electrons free to move through the material. At absolute zero, a pure semiconductor behaves as a perfect insulator; at room temperature, thermal agitation is sufficient to excite some electrons into the conduction band, simultaneously generating a free electron and a "hole"—the absence of that electron, which behaves like a mobile positive charge.

The more free electrons are thermally generated, the higher this intrinsic carrier concentration rises; this process is highly sensitive to both temperature and the width of the forbidden gap itself: a material with a wider gap has far fewer free carriers at a given temperature than one with a narrower gap.

Doping is the technique that makes semiconductors useful: it involves deliberately introducing impurity atoms that either contribute additional free electrons (n-type doping) or accept electrons, thereby creating additional holes (p-type doping), which allows for precise control over the material's conductivity. Within a semiconductor, current flows via two mechanisms that can coexist: drift current, driven by an external electric field, and diffusion current, driven simply by the tendency of carriers to move from areas of higher concentration to areas of lower concentration—much like perfume diffusing through a room. The same underlying statistical physics governs both the ease with which carriers move under the influence of a field—their mobility—and the ease with which they diffuse due to concentration gradients; thus, while the two phenomena appear distinct, they are not independent of one another. That is why this module—which opens with a band gap separating bound electrons from free electrons—concludes by showing that doping a semiconductor does not create a new material; rather, it deliberately tilts—in one direction or the other—the very balance between electrons and holes that temperature itself spontaneously establishes.

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