Semiconductors Understanding the difference between n- and p-type semiconductors

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Semiconductors can be differentiated as intrinsic and extrinsic as per the matter of purity concerned. P-type and N-type semiconductors both come under extrinsic semiconductors. So, what's the difference?

Semiconductors have a monumental impact on our world. They are found at the heart of any electrical device that is computerized or uses radio waves. They are often made of silicon, hence the name Silicon Valley where many of today’s biggest tech companies can be found – silicon is at the core of virtually any electronic device.

Silicon is used so widely in semiconductors because it is an abundant element – it can be found in sand and quartz, for example – which has an ideal electronic structure. With four electrons in its outer orbital, silicon can form nice crystal structures and the four electrons can form perfect covalent bonds with four neighboring atoms to create a lattice.

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In carbon, another element with four electrons in its outer orbital, this crystalline structure is known as a diamond. In silicon, this crystalline structure is a silvery, metallic-looking substance. Although they look metallic, silicon crystals are not, in fact, metals; a silicon crystal is a near insulator and only a small amount of electricity will flow through it.

By doping silicon, however, all this can be changed, and this is when p- and n-type semiconductors are formed.

Understanding p- and n-type semiconductors

In semiconductors like silicon, doping is a process that intentionally introduces impurities into an intrinsic semiconductor. It involves a chemical reaction that allows impurities to form ionic bonds with silicon atoms in its crystal.

The purpose of doping is to modulate its electrical, optical and structural properties. When a semiconductor has undergone doping, it is then referred to as an extrinsic semiconductor. In contrast, a semiconductor in a pure undoped form is an intrinsic semiconductor.

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In silicon doping, there are two types of impurities: n-type and p-type.

In n-type doping, arsenic or phosphorus is added in small quantities to the silicon. Both of these elements have five electrons in their outer orbitals and so they are not out of place when they get into the silicon crystalline structure. Since the fifth electron has nothing to bond to, it is free to move around, allowing an electric current to flow through the silicon.

In p-type doping, boron or gallium is used as the dopant. These elements each have three electrons in their outer orbitals. When they are mixed into the silicon lattice, they form ‘holes’ in the valence band of silicon atoms. This means the electrons in the valence band become mobile, and the holes move in the opposite direction to the movement of the electrons. Because the dopant is fixed in the crystal lattice, only the positive charges can move. Due to the positive holes, these semiconductors are known as “p-type” (or “p-conductive” or “p-doped”).

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So, what’s the difference?

In n-type silicon, the electrons have a negative charge, hence the name n-type. In p-type silicon, the effect of a positive charge is created in the absence of an electron, hence the name p-type.

The material difference between n- and p-type doping is the direction in which the electrons flow through the deposited layers of the semiconductor. Both n- and p-type silicon are good (but not great!) conductors of electricity.

Putting them together

N- and p-type silicon are nothing amazing alone. When you put them together, however, interesting behavior is exhibited at the junction between the two.

A diode is the simplest possible example of a semiconductor device that uses both n- and p-type silicon. It allows an electrical current to flow in one single direction. Imagine a turnstile at a football stadium – a diode is a one-way turnstile gate for electrons.

Everything comes down to the p-n junction. N-type silicon has extra electrons and there are atoms on the p-side that need electrons, so electrons migrate across the junction. (Alternatively: the p-side has extra holes, and there are atoms on the n-side that need holes, so the holes migrate across the junction.) These electrons and holes – carriers of electric charge – near the junction combine and cancel one another out, leaving a neutral ‘depletion’ zone where no electric charge flows.

However, atoms at either side of the depletion zone want to acquire electrons/get rid of holes to become neutral, but since there are no free charge carriers at the depletion zone, they cannot do that. They pull on the charge carriers that crossed the junction but because the depletion zone doesn’t have any charge carriers to give up, nothing moves across.

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By applying an electric field to the p-n junction (e.g. by using up a battery) you can either turn the diode’s junction into an insulator or a conductor.

If you connect the negative (-ve) end of the battery to the p-side and the positive (+ve) end to the n-side (‘reverse bias’), free charge carriers are pulled aside, and the depletion zone widens. This turns the junction into an insulator and further inhibits electrical current flow.

However, if you connect the -ve end of the battery to the n-side and the +ve end to the p-side (‘forward bias’), charge carriers are pushed into the middle, knocking out the depletion zone and turning the p-n junction into a conductor. This is because holes from the p-side are repelled by the +ve end of the battery and electrons in the n-side are repelled by the -ve end of the battery. Atoms at the junction can now hand off charge carriers to one another, allowing current to flow freely.

This is a very basic example of how the most elementary type of semiconductor device, the diode, works. Put a few billion of these together back-to-back and you’ve got a computer chip!

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