Problem 108 - Chemistry The Central Science In Si Units

Question

We will see in Chapter 12 that semiconductors are materials that conduct electricity better than nonmetals but not as well as metals. The only two elements in the periodic table that are technologically useful semiconductors are silicon and germanium. Integrated circuits in computer chips today are based on silicon. Compound semiconductors are also used in the electronics industry. Examples are gallium arsenide, GaAs; gallium phosphide, GaP; cadmium sulfide, CdS; and cadmium selenide, CdSe. (a) What is the relationship between the compound semiconductors' compositions and the positions of their elements on the periodic table relative to Si and Ge? (b) Workers in the semiconductor industry refer to "II-VI" and "III-V" materials, using Roman numerals. Can you identify which compound semiconductors are II-VI and which are III-V? (c) Suggest other compositions of compound semiconductors based on the positions of their elements in the periodic table.

Step-by-Step Solution

Answer

(a) Compound semiconductors combine elements from different groups near Si and Ge. (b) GaAs and GaP are 'III-V'; CdS and CdSe are 'II-VI'. (c) Possible compositions: AlP (III-V) and ZnO (II-VI).

Step 1: Understanding Element Positions on the Periodic Table

Silicon (Si) and Germanium (Ge) are elements in Group 14 of the periodic table. To understand the relationships for compound semiconductors, locate elements near the vertical group positioning of Si and Ge.

Step 2: Relating Elements to Group Numbers

For compound semiconductors, the Roman numeral groups ‘II-VI’ and ‘III-V’ are derived from the positions of elements in the periodic table. Elements from Group 13 (IIIB) and Group 15 (VB) form ‘III-V’ semiconductors. Elements from Group 12 (IIB) and Group 16 (VIB) form ‘II-VI’ semiconductors.

Step 3: Identifying III-V and II-VI Compounds

Each listed compound semiconductor can be checked against these group notations: - GaAs consists of Gallium (Group 13) and Arsenide (Group 15), classifying it as 'III-V'. - GaP, with Gallium (Group 13) and Phosphorus (Group 15), is also 'III-V'. - CdS consists of Cadmium (Group 12) and Sulfur (Group 16), making it 'II-VI'. - CdSe, with Cadmium (Group 12) and Selenium (Group 16), is also 'II-VI'.

Step 4: Suggest Other Compound Semiconductor Compositions

Recognizing these classifications, suggest compositions such as: 1. 'III-V': Aluminum (Al, Group 13) with Phosphorus (P, Group 15), AlP. 2. 'II-VI': Zinc (Zn, Group 12) with Oxygen (O, Group 16), ZnO. These suggestions follow the group classifications of 'III-V' (Group 13 with Group 15) and 'II-VI' (Group 12 with Group 16).

Key Concepts

Periodic Table Groups

The periodic table is a critical tool in understanding the properties and behaviors of elements. One of its core features is the grouping of elements into columns known as groups or families. Each group contains elements with similar chemical properties because they have the same number of valence electrons.

For example, elements in Group 14, like silicon (Si) and germanium (Ge), have four valence electrons, which is why they are semiconductors. Their ability to conduct electricity is flexible, depending on the conditions. Having valence electrons means they can gain or lose electrons easily, which is what allows them to be effective in electronic materials.

• Group 13, for instance, contains elements like gallium (Ga), which pairs with Group 15 elements such as arsenic (As) or phosphorus (P) to form compound semiconductors.
• Group 12 elements, like cadmium (Cd), combine with Group 16 elements such as sulfur (S) or selenium (Se) in other compound semiconductors.

Understanding these periodic table groups makes it easier to predict the properties of different semiconductors and their potential uses.

III-V Semiconductors

III-V semiconductors are named after the elements that compose them, which fall into Group 13 (formerly known as Group IIIB) and Group 15 (formerly Group VB) of the periodic table. These semiconductors like gallium arsenide (GaAs) are essential in electronic devices due to their efficient electrical conductivity and direct bandgap nature.

III-V semiconductors are crucial in high-frequency and optoelectronic applications such as LEDs, laser diodes, and some types of photovoltaic cells.

• Gallium (Ga) from Group 13 and Arsenic (As) from Group 15 form GaAs, a commonly used III-V semiconductor.
• Another example is gallium phosphide (GaP), where gallium combines with phosphorus (P) from Group 15.

These materials provide advantages over traditional silicon semiconductors, particularly in applications where rapid response or light emission is essential.

II-VI Semiconductors

II-VI semiconductors are formed from elements belonging to Group 12 (IIB) and Group 16 (VIB) of the periodic table. These materials, such as cadmium sulfide (CdS) and cadmium selenide (CdSe), possess distinct properties that make them suitable for a variety of optoelectronic and photovoltaic applications.

Unlike III-V semiconductors, II-VI semiconductors often have a wider bandgap, which is ideal for light-emitting applications across a spectrum from visible to infrared.

• Cadmium (Cd) from Group 12 pairs with sulfur (S), a Group 16 element, to form CdS.
• Similarly, cadmium selenide (CdSe) is another common II-VI semiconductor example, where Cd combines with selenium (Se) from Group 16.

These semiconductors excel in applications where light emission or detection is key, such as in photodetectors or solar cells.

Electronic Materials

Electronic materials are the backbone of the devices we use every day, serving functions from conduction to insulation to modulation of electrical signals. Semiconductors form a major part of this category because their conductance can be cleverly manipulated.

Among electronic materials, semiconductors are particularly special because they bridge the gap between insulators and conductors. They can be altered in many ways, including doping—where impurities are introduced to tweak electrical properties.

• Silicon, a Group 14 element, is widely used due to its ideal properties and abundance.
• III-V semiconductors are pivotal in devices requiring high-speed or optoelectronic capabilities.
• II-VI semiconductors are often sought after for their light emission and detection capabilities.

Continuous innovation in electronic materials leads to the advancement of technology, enabling faster processing, clearer displays, and more efficient energy usage.