Chapter 29 Solid State Electronics

Objectives

• 29.1 Compare and contrast n-type and p-type semiconductors

• 29.1 Describe electron motion in conductors and semiconductors

• 29.2 Describe how diodes limit current to motion in only one direction

• 29.2 Explain how a transistor can amplify or increase voltage changes

Conduction in Solids

• Review– Conductors (easily allow electrons or electrical

charge to move)– Insulators (resist electron and electrical charge

movement)– Electrons are only allowed to occupy certain

energy levels– Under most conditions, electrons are found in the

lowest available energy level (ground state)

Conduction in Solid

• What happens to the energy levels as atoms come close to one another?

• The energy levels are raised or lowered as they interfere with one another

• (a) the interatomic spacing between two atoms is reduced

• (b) the electron wave function of five atoms interact

• (c) in the formation of a continuous band of allowed energies the number of interacting atoms is very large.

Terms

• Forbidden Gaps: The bands of energy which no electrons possess

• Valence Band: The outermost band that contains electrons

• Conduction Band: The lowest energy band that is not filled to capacity

Conduction Band

• If a potential difference (voltage) is placed across a material, then the electrons are able to accelerate and gain energy– If the potential energy is large enough, the gap is

bridged and the electrons go from one atom to the next (current)

How many free electrons?

• How many valence electrons determines that amount of free electrons– Sodium (1st Column, Alkali Metal) has one free

electron. It has 11 electrons total, but only 1 electron is in the valence band

– Most metals provide only 1 or 2 free electrons

Insulator/Semiconductor/Conductor

• How big is the gap?– Insulator will be larger (typically 5 to 10 eV)– Semiconductor (1 eV or less)– Conductor (what gap?!)

The values are for individual atoms

• As the atoms come together, the gaps between the bands reduces

• Reason: Electrons in different atoms push the other electrons closer

Insulators

• At room temperature, no electrons have enough kinetic energy (as a result of their thermal energy) to jump the forbidden gap.

• Even small electric fields are usually not sufficient to cause them to conduct electricity

• Very HIGH ionization energies

Semi-Conductors

• At room temperature, a few electrons are able to jump the forbidden gap

• A small electric field is able to make more electrons jump the gap, so the current increases

• As temperature increases, conductivity goes up so you have more current

Metals

• Room temperature is more than enough energy to move electrons around. – What forbidden gap?! The electron goes and

pleases as he wants• Different than semi-conductors, increasing

temperature means less current due to increase in collisions

Holes

• When an electron jumps, it leaves a positive hole where the electron used to be

Holes

• An electron from another atom can jump over, but ultimately leaves a hole where he was– Musical Chairs

• The negative goes one way, positive other way

Types of Semi-Conductors

• Intrinsic: Are pure and conduct electricity in very tiny quantities at room temperature (very high resistance)

• Extrinsic: Impurity atoms added which increase the conductivity by either adding more electrons (n-doping) or more holes (p-doping)

P-Doping and N-doping

• P = Add a hole (less electrons than your semi-conductor)

• N = Add an electron (more electrons)

P- Doping and N-Doping

Periodic Table Connection

How conductive do you want it?

• Vary the amount of impurities• Even just a few different atoms can change the

conductivity by factors as large as a 1000

Thermistors

• Resistance depends highly on temperature– Used as a very sensitive thermometer– Used to compensate for temperature variations of

components in an electrical circuit– Used to measure power of light sources

Diodes

• Consists of a joined p-doped and n-doped semiconductor

• Boundary between is called a pn-junction diode

Diodes

• The free electrons on the n-side are attracted to the p-side (holes)

• Thus, the electrons move easily from the n-side to the p-side, but not the other way

Diodes

• Region in the middle with no charge carriers is called the depletion layer– So, two ends are great conductors with middle

being an insulator

No Current

Purpose for diodes

• Current in one direction (turns AC into DC)

Transistor

• pnp • npn• Middle is base• ends are collectors

and emitters

Purpose for Transistor

• Used as off/on switches• Modify voltages (amplify)• Store information (as an off/on switch)