Question

The Group \(3 \mathrm{~A} /\) Group 5 A semiconductors are composed of equal amounts of atoms from Group \(3 \mathrm{~A}\) and Group \(5 \mathrm{~A}-\) for example, InP and GaAs. These types of semiconductors are used in lightemitting diodes and solid-state lasers. What would you add to make a p-type semiconductor from pure GaAs? How would you dope pure GaAs to make an n-type semiconductor?

Short answer

To make a p-type semiconductor from pure GaAs, you would add a trivalent dopant like magnesium (Mg) from Group 2A, which has fewer valence electrons. For creating an n-type semiconductor, you would dope pure GaAs with a pentavalent dopant like sulfur (S) from Group 6A, which has more valence electrons.

Step-by-step solution

Determine valence electrons for GaAs and possible dopants

GaAs is a semiconductor composed of 3A (gallium) and 5A (arsenide) groups. Gallium in Group 3A has 3 valence electrons, and arsenic in Group 5A has 5 valence electrons. To make a p-type semiconductor, we should add a trivalent dopant (fewer valence electrons), such as one from Group 2A. On the other hand, to make an n-type semiconductor, we should add a pentavalent dopant (more valence electrons), such as one from Group 6A.

Selecting dopants for p-type semiconductors

To create a p-type semiconductor, you should select an element with fewer valence electrons (Group 2A) and add it to the GaAs crystal. One example of a Group 2A element is magnesium (Mg). This element will serve as a dopant to create a p-type semiconductor, as magnesium has only two valence electrons.

Selecting dopants for n-type semiconductors

To create an n-type semiconductor, you should select an element with more valence electrons (Group 6A) and add it to the GaAs crystal. One example of a Group 6A element is sulfur (S). This element will serve as a dopant to create an n-type semiconductor, as sulfur has six valence electrons.

Doping GaAs for p-type semiconductor

Add a suitable amount of magnesium (Mg) to the GaAs crystal. This results in the creation of a p-type semiconductor where magnesium atoms will accept the extra valence electrons, causing an increase of electron holes in the material.

Doping GaAs for n-type semiconductor

Add a suitable amount of sulfur (S) to the GaAs crystal. This results in the creation of an n-type semiconductor where sulfur atoms will donate extra valence electrons, increasing the count of negatively charged electrons in the material.

Key concepts

P-Type Semiconductor

A p-type semiconductor is created by introducing impurities into an intrinsic semiconductor material. These impurities have fewer valence electrons compared to the semiconductor, typically one less. In the context of semiconductors like GaAs, which combines elements from Group 3A (three valence electrons) and Group 5A (five valence electrons), a Group 2A element such as magnesium (Mg) would be introduced.

This addition results in a surplus of 'holes', or places where an electron could be, but isn't. These holes can move throughout the lattice structure and carry positive charge, which is why this type of doped semiconductor is referred to as 'p-type', indicating the positive nature of the predominant charge carriers.

N-Type Semiconductor

An n-type semiconductor, on the other hand, is achieved by doping an intrinsic semiconductor with an element that has more valence electrons, usually one more than the semiconductor material. For GaAs, this would mean using a Group 6A element like sulfur (S) which possesses six valence electrons.

When sulfur atoms are added, they contribute extra electrons that are not bound to any particular atom in the crystal lattice. These additional free electrons enhance the electron density and give the material its 'n-type' characteristic, where 'n' refers to the negative charge of the mobile electrons that act as the main carriers of electrical current.

GaAs Semiconductor

Gallium Arsenide (GaAs) is a compound semiconductor, a fusion of elements from Group 3A and Group 5A of the periodic table. What makes GaAs unique is its superior electron mobility compared to silicon, allowing for higher frequency operation and efficiency, which is crucial for applications such as LEDs and solid-state lasers.

GaAs has a direct bandgap, which means it can efficiently emit light; this is the reason it is widely used in optoelectronics. In terms of doping, GaAs can be turned into either n-type or p-type semiconductor by incorporating respective dopants, making it extremely adaptable and functional for various electronic components.

Valence Electrons

Valence electrons are the electrons present in the outermost shell of an atom, and they play a pivotal role in semiconductor doping. These are the electrons involved in chemical reactions, bond formation, and, particularly for semiconductors, they significantly affect electrical conductivity.

Intrinsic semiconductors like silicon have four valence electrons, making them tetravalent. Doping involves altering the number of valence electrons in a semiconductor to control its conductivity. The essence of creating n-type or p-type semiconductors is all about manipulating the availability of valence electrons through the introduction of dopants, thereby tailoring the electrical properties of the semiconductor to fit specific technological needs.