Work in groups on these problems. You should try to answer the questions without referring to your textbook. If you get stuck, try asking another group for help.
- Explain band theory
- Identify the different types of semiconductors
- Propose appropriate elemental combinations to improve or diminish conductivity
A semiconductor is a material which exhibits increasing conductivity with increasing temperature. Semiconductors are important for computers, cell phones, and other popular technologies and are an area of active research today.
Metallic Conductors, Insulators, and Semiconductors
Model 1: An application of molecular orbital theory to solid materials results in what is known as band theory. When two atoms are brought together, each atomic orbital produces two molecular orbitals. When extended to n atoms, then n molecular orbitals are produced. In a solid, n is a very large number and, thus, the number of molecular orbitals is equally large. The result is a band of orbitals of very similar energy. We can represent these bands of orbitals using simple diagrams; shaded areas represent filled orbitals while the empty orbitals are not shaded. The space between the lower energy band and the higher energy band is called the band gap.
Critical Thinking Questions
- Based on the diagram in Model 1, what is the primary difference between an insulator and a semiconductor? (Hint: focus on parts a and d.)
- Referencing the diagram in Model 1,
- Compare parts d and e. When temperature increases from 0 K to room temperature, what happens to the electrons?
- If you further increase the temperature (above room temperature), what do you think will happen? Draw a diagram to illustrate your idea.
- Briefly explain your reasoning.
- Looking at the diagram in Model 1 and your answers to CTQ 2, would you expect the conductivity in a semiconductor to increase or decrease if the temperature is raised? Explain your reasoning.
Information: Conductivity increases with increasing temperature in a semiconductor. Pure materials that have semiconductor properties are called intrinsic semiconductors.
- Based on your answer to CTQ 1, would intrinsic semiconductors have relatively large or small band gaps? Why?
- Explain why increasing the temperature of an intrinsic semiconductor increases the conductivity.
- As the band gap increases, will conductivity in an intrinsic semiconductor increase or decrease? Why?
Model 2: Doped semiconductors are produced by replacing a few atoms of the bulk element with atoms containing either fewer or more electrons. If the dopant has more electrons, an n-type semiconductor is formed (“negative”, since extra electrons are added). If the dopant has fewer electrons, a p-type semiconductor is formed (“positive” holes are added).
- a) Draw a figure, similar to the one in Model 1 representing a p-type semiconductor.
b) Draw a figure similar to the one in Model 1 representing an n-type semiconductor.
- Below are examples of semiconductor materials. Identify each as p-type, n-type, or intrinsic.
- Silicon and gallium are common semiconductor materials. Suggest a dopant for each that would create a p-type semiconductor.
- Suggest a dopant for silicon and one for gallium that would create n-type semiconductors.
- Jens-Uwe Kuhn, Santa Barbara City College
- Jessica Martin, Northeastern State University