Week 1: The science behind traditional semiconductors
Rohan V -
Hi all, it’s Rohan. Today, I’ll take a deep-dive into some of the foundational science behind traditional semiconductors, showing why they have worked in the past and why the world is currently trying to shift away from them.
Semiconductors have served as the backbone of the electronics industry since its inception. Electricity relies on the flow of electrons to transmit information, and semiconductors offer the ideal balance of significant ability to conduct electrons at controllable levels.
Traditional semiconductors are “metalloids” – substances that have a mix of metallic and non-metallic properties. Some examples include silicon, germanium, and arsenic. Unlike metals, which are very potent conductors of electricity, semiconductors have more moderate conductance. This difference in properties comes from how electrons are shared within the respective groups. In metals, electrons are “delocalized” and can freely move around. Consequently, in the presence of an electric field, electrons in a metal can easily move towards the positive pole. Metalloids, however, have a crystalline structure. Electrons are locked in place around their parent atom, with limited movement. Why, then, are semiconductors able to transmit charge?
The answer is, they cannot – at least, not without modifications made to them. A pure silicon crystal cannot conduct electricity because its electrons are fixed in place. To solve this problem, physicist John Woodyard pioneered the process of doping: introducing impurities into metalloid-based semiconductors to change how electrons move within them. There are two main types of doping: n-doping and p-doping, which can be explained using the classic example of silicon. Silicon, situated in the 14th group of the periodic table, has four valence (outer) electrons. N-doping involves replacing a silicon atom in the crystal with a phosphorus atom, which has five valence electrons. This impurity adds an excess electron to the crystal which is able to move in response to an electric field. P–doping, on the other hand, involves replacing a silicon atom with an atom which has three valence electrons (most frequently, boron) instead of silicon’s four. The missing electron leaves a ‘hole’ in the crystal which other electrons can use to orient themselves in an electric field.
While doping enables semiconductors to conduct electricity, their rates of charge transport are nowhere close to those of metals. This phenomenon is beneficial in electronics because it ensures semiconductors use less power than metals and are easier to control. Control of current flow allows the production of transistors, ‘on-off switches’ that form the basis of modern processing logic.
Despite the benefits associated with traditional semiconductors, the raw materials needed to make them are finite, they have high manufacturing costs, and they consume significant amounts of energy in the production process. These effects have led to a push to switch to carbon-based (organic) semiconductors, which I will discuss in detail in my next post.
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