Semiconductor Principles

Semiconductor Principles

Semiconductor technology has a huge impact on our society. You can find semiconductors in microprocessor chips and at the heart of transistors. Any product that uses computers or radio waves also relies on semiconductors.

Currently, most semiconductor chips and transistors are made of silicon. You may have heard of "Silicon Valley" and the "Silicon Economy" because silicon is at the heart of all electronic devices.

Clockwise from top: Chips, LEDs, and transistors are all made of semiconductor materials.

The diode is probably the simplest semiconductor device, so it's a good place to start if you want to understand how semiconductors work. In this article, you'll learn what semiconductors are, how they work, and the process of using them to make diodes. Let's start by learning about the element silicon.

Silicon is a very common element - for example, it's the main component of sand and quartz. If you look up silicon on the periodic table, you'll find it next to aluminum, below carbon, and above germanium.

Silicon is located next to aluminum and below carbon in the periodic table.

Carbon, silicon, and germanium (germanium, like silicon, is a semiconductor) have a unique property in their electronic structures—they all have four electrons in their outermost orbits, which allows them to form very good crystals. The four electrons form perfect covalent bonds with four neighboring atoms, creating a crystal lattice. We all know carbon in its crystalline configuration as diamond, and silicon in its crystalline configuration as a silvery, metallic-looking substance.

­ In the silicon crystal lattice, all silicon atoms form perfect bonds with four neighboring atoms, so there are no free electrons available to conduct electric current. So the silicon crystal is an insulator rather than a conductor.

Metals are usually good conductors of electricity because they generally have "free electrons" that can move easily between atoms, and the flow of electrons creates an electric current. Although silicon crystals look like metals, they are not actually metals. In silicon crystals, all of the outer electrons are in perfect covalent bonds, so the electrons can't move around. Pure silicon crystals are almost insulators - only a small amount of current can flow through them.

But silicon can be doped - by mixing small amounts of impurities into the silicon crystal, this property can be changed, turning it into a conductor.

Two types of impurities can be introduced:

• N-type - N-type doping is the addition of small amounts of phosphorus or arsenic to silicon. Phosphorus and arsenic both have five electrons in their outer shells, so they don't stay in the right place when they enter the silicon lattice. The fifth electron has nothing to bind to, so it can move around freely, and only a small amount of impurities can create enough free electrons to allow electricity to flow through the silicon. N-type silicon is a good conductor. Electrons have a negative charge, so it is called N-type silicon.
• P-Type - For P-type doping, boron or gallium is used as the dopant. Both boron and gallium have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where the silicon electrons do not form bonds. Because of the lack of an electron, it has a positive charge, hence the name P-type silicon. Holes can conduct electricity, and holes easily attract electrons from neighboring atoms, allowing the holes to move between atoms. P-type silicon is a good conductor.
  

A small amount of N-type or P-type dopants can transform a silicon crystal from a good insulator into a conductor that can conduct electricity (but not very well) - hence the name "semiconductor".

There's nothing magical about N-type and P-type silicon on their own, but when you put them together, the combination has some interesting behaviors.

A diode is probably the simplest semiconductor device, and it only allows current to flow in one direction. You may have seen the turnstiles at the entrance of a stadium or subway station, where people can only pass through in one direction. A diode is like a one-way turnstile for electrons.

If you put N-type silicon and P-type silicon together (as shown in the figure), an interesting phenomenon will occur, which is a unique characteristic of diodes.

Although N-type and P-type silicon are conductors in their own right, they do not conduct any current when combined as shown. The negative electrons in the N-type silicon are attracted to the positive terminal of the battery, and the positively charged holes in the P-type silicon are attracted to the negative terminal of the battery, and no current flows through the junction because the holes and electrons are moving in the wrong direction.

If you flip the battery over, the diode will conduct current just fine. The free electrons in the N-type silicon are repelled by the negative electrode of the battery, and the holes in the P-type silicon are repelled by the positive electrode. The holes and electrons meet at the junction of the N-type silicon and the P-type silicon, and the electrons fill the holes. These holes and free electrons disappear, and new holes and new free electrons come out to take their place, which will form a current at the junction.

A diode is a device that blocks the flow of electrical current in one direction but allows it to flow in the other direction. Diodes can be used in a variety of ways. For example, devices that use batteries often include a diode to protect the device if the battery is plugged in the wrong way. If the battery is plugged in the wrong way, the diode blocks the flow of current from the battery - thus protecting sensitive electronic components in the device.

The performance of semiconductor diodes is not perfect, as shown in the following figure:

When connected in reverse, an ideal diode should block all current. In reality, a diode lets about 10 mA through—not a lot, but still not perfect. Also, if enough reverse voltage (V) is applied, the junction will break down and allow current to flow. Usually, the breakdown voltage is much greater than the normal voltage, so this isn't a problem.

When connected in the forward direction, only a small voltage is required to turn the diode on. For silicon, this voltage is about 0.7 volts, which is necessary to start the hole-electron combination process at the junction.

Unlike diodes, which use two layers, transistors contain three layers. A sandwich structure can be created that is either NPN or PNP, and the transistor can be used as a switch or an amplifier.

A transistor looks like two diodes placed back to back. You might think that no current can flow through the transistor because the diodes placed back to back block current in both directions, and this is true. However, if a small current is applied to the middle layer of the sandwich, a larger current will flow through the entire sandwich. This makes the transistor behave like a switch, where a small current can turn a large current on or off.

A silicon chip is a piece of silicon that can hold thousands of transistors. By using transistors as switches, logic gate circuits can be created, and through logic gates, microprocessor chips can be created.

The natural progression from silicon to doped silicon to transistors to chips is why microprocessors and other electronic devices are so cheap and ubiquitous today. The basic principles are so simple, and the miracle is that these principles have been explored in depth until today, tens of millions of transistors can be integrated on a chip at a very low price.