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Starting from the top: A chip, LED, and transistor are all crafted from semiconductor materials.Semiconductors have made a profound impact on our world. These tiny components are essential in microprocessor chips and transistors, making them central to everything from computers to devices that use radio waves.
Currently, the majority of semiconductor chips and transistors are made from silicon. You've likely heard terms like 'Silicon Valley' and the 'silicon economy,' which highlights silicon's crucial role in powering all electronic devices.
A diode is the most basic semiconductor component, making it a perfect starting point to understand the functioning of semiconductors. In this article, we’ll explore what a semiconductor is, how doping works, and how a diode is formed using semiconductor materials. But first, let’s dive into the properties of silicon.
Silicon is an abundant element, found for instance in sand and quartz. On the periodic table, silicon is located next to aluminum, beneath carbon, and above germanium.
Silicon is positioned next to aluminum and just below carbon in the periodic table.Carbon, silicon, and germanium (which, like silicon, is also a semiconductor) share a unique characteristic in their electron arrangement — each possesses four electrons in its outer orbital. This enables them to form orderly crystals. The four electrons create strong covalent bonds with four adjacent atoms, forming a lattice. In carbon, we recognize this crystalline structure as diamond, while in silicon, it appears as a silvery, metallic-looking material.
Within a silicon lattice, each silicon atom forms perfect bonds with four neighboring atoms, resulting in no free electrons to carry electric current. This property makes silicon a non-conductive material rather than a conductor.Metals are typically good conductors of electricity because they contain 'free electrons' that move freely between atoms, allowing electricity to flow. While silicon crystals may appear metallic, they are not actually metals. All the outer electrons in silicon crystals are part of perfect covalent bonds, preventing them from moving. As a result, pure silicon behaves more like an insulator with minimal electric current flow through it.
Through a process known as doping, you can alter all of this.
Doping Silicon
Silicon's behavior can be changed into that of a conductor by doping it, where a small amount of an impurity is added to the silicon crystal.
There are two main categories of impurities:
- N-type - N-type doping involves introducing small amounts of phosphorus or arsenic into silicon. These elements have five outer electrons, which don't quite fit into the silicon lattice. As a result, the fifth electron becomes free to move around, allowing electricity to flow through the silicon. Even a tiny amount of impurity creates enough free electrons for conduction. N-type silicon is a good conductor, with electrons carrying a negative charge, hence the 'N' in N-type.
- P-type - P-type doping involves the introduction of boron or gallium, which each have only three outer electrons. These elements create 'holes' in the silicon lattice, where an electron is missing. The absence of an electron creates a positive charge, hence the 'P' in P-type. These holes can conduct electricity by accepting electrons from neighboring atoms, allowing the hole to shift across the lattice. P-type silicon also serves as a good conductor.
Even a tiny amount of either N-type or P-type doping transforms a silicon crystal from being a good insulator to a reasonably decent conductor, which is why it's called a 'semiconductor.'
On their own, N-type and P-type silicon aren't particularly remarkable; however, when combined, they exhibit fascinating behavior at the junction, as seen in a diode.
A diode is the most basic form of semiconductor device. It permits current to flow in one direction but blocks it in the opposite. It's similar to turnstiles at sports stadiums or subway stations that only allow passage in one direction. In essence, a diode acts as a one-way gate for electrons.
When N-type and P-type silicon are placed together, as shown in this diagram, it results in an intriguing effect that grants a diode its distinctive characteristics.
Although N-type silicon is conductive on its own, and P-type silicon is also conductive by itself, the combination illustrated in the diagram doesn't conduct electricity. The negative electrons in the N-type silicon are drawn to the positive terminal of the battery, while the positive holes in the P-type silicon are attracted to the battery's negative terminal. Since the electrons and holes are moving in opposite directions, no current flows through the junction.
If you reverse the battery, the diode allows electricity to flow perfectly. The free electrons in the N-type silicon are pushed away by the battery's negative terminal, while the holes in the P-type silicon are pushed away by the positive terminal. At the junction between the N-type and P-type materials, electrons and holes meet. The electrons fill the holes, causing them to disappear, and new holes and electrons form in their place. This process enables current to flow through the junction.
In the upcoming section, we'll explore the applications of diodes and transistors.
Diodes and Transistors
A diode is a component that prevents current from flowing in one direction while allowing it to flow in the other. Diodes are used in various applications, such as in devices that run on batteries. These devices often include a diode that prevents damage if the batteries are inserted backward. The diode blocks current from flowing out of the battery when reversed, thus safeguarding the device's delicate electronics.
When a diode is reverse-biased, it would ideally block all current. In reality, a diode allows a small amount of current, about 10 microamps, to pass through — though this is still not perfect. If enough reverse voltage (V) is applied, the junction will break down, allowing current to pass. However, the breakdown voltage is typically much higher than what the circuit will experience, making it insignificant.
When forward-biased, a small voltage is required to activate the diode. For silicon, this voltage is approximately 0.7 volts. This voltage initiates the process of combining holes and electrons at the junction.
A significant technology closely tied to diodes is the transistor. There are many similarities between transistors and diodes.
Transistors
A transistor is formed by stacking three layers, in contrast to the two layers found in a diode. These layers can be arranged as either an NPN or PNP configuration. Transistors function as both switches and amplifiers.
A transistor may appear as two diodes placed back-to-back. It might seem that no current could flow through the transistor, since back-to-back diodes would block current in both directions. This is true. However, when a small current is introduced to the center layer of the transistor, a much larger current can flow through the entire device. This is what gives the transistor its switching function, allowing a small current to control a larger one.
A silicon chip is a small piece of silicon capable of housing thousands of transistors. These transistors function as switches, allowing for the creation of Boolean gates, which in turn can be used to build microprocessor chips.
The evolution from silicon to doped silicon, then to transistors, and eventually to chips is what has made microprocessors and other electronic devices so affordable and widespread in today's world. The underlying principles are surprisingly simple, but the true marvel lies in the continuous enhancement of these concepts, enabling the production of millions of transistors on a single chip at a low cost.
For further details on semiconductors, diodes, chips, and more, explore the links provided below.
