EGA211 Semiconductor Technology - Lecture 1 Notes

Lecturer: Professor Karol Kalna
- Room: B202 Engineering East
- Email: k.kalna@swansea.ac.uk
Lectures/Exercise classes:
- 2 lectures per week:
  - Wednesday 10am-11am
  - Friday 10am-11am
Lab Work:
- Tuesday 3 sessions × 10am-11am
- Tuesday 3 sessions × 10am-12pm
- Location: EEE B107 (The EEE Lab)
Office hour:
- Friday 11am-12pm
- Location: Engineering East, room B202
Assessment:
- Project Report: 40%
- Exam Paper: 60%
Notes:
- Canvas: slides, lecture capture, exercise classes
Feedback:
- Written comments on the assessment project
- Office hours
Resources:
- Recommended books, web information, YouTube, comprehensive notes
- EGA211 Semiconductor Technology

Exam Mark Requirement:
- Must achieve at least 40% mark from the assessment project report for the exam mark to count.
- If less than 40% is achieved on the assessment project report, the entire module will fail with a mark of zero.
Assessment Project Report Mark Requirement:
- Must achieve at least 40% in the exam component for the mark from the assessment project report to count.
- If less than 40% is achieved on the exam, the module mark will be just the exam mark.
Laboratory Classes:
- 6 laboratory classes of 2 hours are compulsory.
- Students must attend at least 5 of all classes.
- Missing laboratory classes can be excused only with a valid Extenuating Circumstances request.
- Failing to attend one of the 5 compulsory classes will result in zero mark for the 40% project assessment.
- Zero mark from assessment will mean zero mark on the module, which means that the student failed the module.

Definition: A semiconductor is a solid whose electrical conductivity can be controlled over a wide range, either permanently or dynamically.
Importance: Semiconductors are vital technologically and economically in our society.
Silicon: The most commercially important semiconductor.
Semiconductor devices: Electronic components made of semiconductor materials, essential in modern electrical & electronic devices.
- Applications range from energy distribution and communication networks to smart domestic appliances, cars, computers, digital audio/video players, and mobile devices.

Atomic Number: 14
Melting Point: $1,410^{\circ}C$
Density: $2.33 g \cdot cm^{-3} = 2330 kg \cdot m^{-3}$
Tetravalent metalloid: Silicon is less reactive than its chemical analog carbon.
Abundance: Silicon is the second most abundant element (after oxygen) in the Earth's crust, making up 25.7% of the crust by mass.
Industrial Uses: Many (find out using mobile devices).
Principal component of most semiconductor devices.
Widely used due to:
- Remaining a semiconductor at higher temperatures.
- Ability to be p-type and n-type doped.
- Native oxide is easily grown in a furnace.
- Forms a better semiconductor/dielectric interface than almost all other material combinations.
Periodic Table (Dmitri Mendeleev, 1869):
- "In a dream,” he quoted, “I saw a table where all the elements fell into place as required. Awakening, I immediately wrote it down on a piece of paper. ”

Elemental Crystalline Form:
- Grey colour and a metallic lustre which increases with the size of the crystal.
- Rather strong, very brittle, and prone to chipping (similar to glass).
Reactivity:
- Relatively inert element.
- Reacts with halogens and dilute alkalis.
- Most acids (except for a combination of nitric acid and hydrofluoric acid) do not affect it.
Temperature Coefficient of Resistance:
- Pure silicon has a negative temperature coefficient of resistance.
- The number of free charge carriers increases with temperature.
Piezoresistive Effect:
- The electrical resistance of single crystal silicon changes significantly under the application of mechanical stress.

Semiconductors are very similar to insulators.
Difference between semiconductors and insulators:
- Insulators have larger band gap energies.
- Electrons must acquire a larger kinetic energy to be free to flow in insulators.
Semiconductors @ Room Temperature:
- Very few electrons gain enough thermal energy to leap the band gap, which is necessary for conduction.
Electrical Properties:
- Pure semiconductors and insulators, in the absence of applied fields, have roughly similar electrical properties.
Semiconductors bandgaps:
- Allow for many other means, besides temperature, to control their electrical properties.

Doping Definition: The addition of a small percentage of foreign atoms in the regular crystal lattice of silicon or germanium produces dramatic changes in their electrical properties, producing n-type and p-type semiconductors.
Pentavalent Impurities:
- Impurity atoms with 5 valence electrons produce n-type semiconductors by contributing extra electrons.
- Phosphorous may be added by diffusion of phosphine gas ( $PH_3$ ).
Trivalent Impurities:
- Impurity atoms with 3 valence electrons produce p-type semiconductors by producing a hole or electron deficiency.
- It is typical to use $B<em>2H</em>6$ diborane gas to diffuse boron into the silicon material.

Definition: A silicon crystal is different from an insulator because at any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position, leaving an electron deficiency (a “hole“).
Voltage Application: If a voltage is applied, then both the electron and the hole can contribute to a small current flow.
Band Theory of Solids: Explains the conductivity of a semiconductor.
Band Model: States that at room temperatures there is a statistical probability that electrons can reach the conduction band and contribute to electrical conduction.
Intrinsic vs. Doped Silicon: The term intrinsic here distinguishes between the properties of pure "intrinsic" silicon and the dramatically different properties of doped n-type or p-type semiconductors.

N-type Semiconductor:
- The dopant contributes extra electrons, dramatically increasing the conductivity.
P-type Semiconductor:
- The dopant produces extra vacancies or holes, which likewise increase the conductivity.
Electrons in Conduction Band:
- Electrons excited to the conduction band also leave behind unoccupied states in the valence band which can move along to carry positive charge - holes.
Both the conduction band electrons and the valence band holes contribute to electrical conductivity.
Hole Movement:
- The holes themselves do not actually move, but a neighbouring electron can move to fill the hole, leaving a hole at the place it has just come from.
- In this way the holes appear to move and behave as if they were actual positively charged particles.