14. Semiconductors

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Session Overview

Prerequisites

Before starting this session, you should be familiar with topics from Structure of the Atom (Session 1 through Session 7), including:

• Photon frequency, wavelength, and energy
• Atomic absorption and emission of photons
• The Bohr model of the atom

Looking Ahead

Some material from this session spills over into the next (Session 15); make sure you've covered it before moving on to the Electronic Materials Self-Assessment page.

Session 19 discusses the behavior and location of dopants in crystal lattices, and Session 24 explains how to control the depth and concentration of dopants via diffusion. The Maxwell-Boltzmann curve describes other processes governed by thermal activity, such as crystal vacancies and defects (Session 19) and chemical reaction kinetics (Session 23).

Learning Objectives

After completing this session, you should be able to:

• Describe the mechanisms for forming charge carriers in a semiconductor, and how they behave in the presence and absence of an applied voltage.
• Calculate how many charge carriers exist at a given temperature for intrinsic and extrinsic semiconductors, and how much dopant must be added to produce a desired band gap or charge carrier density.
• Sketch the Maxwell-Boltzmann distribution at several different temperatures, and explain how it applies to electrons in semiconductors.
• Explain why donor levels don't form a continuous band structure in doped semiconductors.

Reading

Archived Lecture Notes #3 (PDF)

Lecture Video

Resources

Lecture Slides (PDF - 2.5MB)

Lecture Summary

In intrinsic semiconductors, electron-hole charge carrier pairs are promoted to the conduction band by ambient thermal energy, as described by the Maxwell-Boltzmann distribution. Carrier density is also affected by the presence of dopants, which change the width of the band gap and produce excess electrons or holes. Engineering impurities in semiconducting materials allows the production of electronic devices such as photovoltaics, light-emitting diodes, CD/DVD optical discs, and photosensors.

Homework

Problems (PDF)

Solutions (PDF)

For Further Study

Supplemental Readings

Heitler, Walter, and Fritz London. "Wechselwirkung neutraler Atome und homöopolare Bindung nach der Quantenmechanik." Zeitschrift für Physik A: Hadrons and Nuclei 44 (1927): 455-472. (Note: this article is in German.)

People

James Clerk Maxwell

Ludwig Boltzmann

William Shockley, John Bardeen, Walter Brattain – 1956 Nobel Prize in Physics

Other OCW and OER Content

The resources listed below are selected from a wide variety of sites covering semiconductors and their applications. Motivated users are encouraged to search MIT OpenCourseWare and other sites for more advanced material based on their specific interests.

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