1. To Address These Questions, We Will Study:
Basic Quantum Mechanics
28 and 30 January 2015
What Is An Energy Band And How Does It Explain The Operation Of Classical Semiconductor Devices and Quantum Well Devices?
To Address These Questions, We Will Study:
Introduction to quantum mechanics (Chap.2)
Quantum theory for semiconductors (Chap. 3)
Allowed and forbidden energy bands (Chap. 3.1)
Also refer to Appendices: Table B 2 (Conversion Factors), Table B.3 (Physical Constants), and Tables B.4 and B.5 Si, Ge, and GaAs key attributes and properties.
We will expand the derivation Schrӧdinger’s Wave Equation as summarized in Appendix E. 1

2. Classical Mechanics and Quantum Mechanics
Mechanics: the study of the behavior of physical bodies when subjected to forces or displacements
Classical Mechanics: describing the motion of macroscopic objects.
Macroscopic: measurable or observable by naked eyes
Quantum Mechanics: describing behavior of systems at atomic length scales and smaller . 2 2

3. Photoelectric Effect Tmax νo ν Metal Plateν νo
Metal Plate
Incident light with frequency ν
Emitted electron kinetic energy = T
The photoelectric effect ( year1887 by Hertz)
Experiment results
Inconsistency with classical light theory
According to the classical wave theory, maximum kinetic energy of the photoelectron is only dependent on the incident intensity of the light, and independent on the light frequency; however, experimental results show that the kinetic energy of the photoelectron is dependent on the light frequency.
Concept of “energy quanta” 3 3

4. Energy Quanta
Photoelectric experiment results suggest that the energy in
light wave is contained in discrete energy packets, which are
called energy quanta or photon
The wave behaviors like particles. The particle is photon
Planck’s constant: h = 6.625×10-34 J-s
Photon energy = hn
Work function of the metal material = hno
Maximum kinetic energy of a photoelectron: Tmax= h(n-no) 4

6. Electron’s Wave Behavior
Electron beam
Nickel sample
Detector
Scattered beam θ
θ =0
θ =45º
θ =90º
Davisson-Germer experiment (1927)
Electron as a particle has wave-like behavior 6

7. Wave-Particle Duality
Particle-like wave behavior (example, photoelectric effect)
Wave-like particle behavior
(example, Davisson-Germer experiment)
Wave-particle duality
Mathematical descriptions:
The momentum of a photon is:
The wavelength of a particle is:
λ is called the de Broglie wavelength 7

8. The Uncertainty Principle
The Heisenberg Uncertainty Principle (year 1927):
It is impossible to simultaneously describe with absolute accuracy the position and momentum of a particle
It is impossible to simultaneously describe with absolute accuracy the energy of a particle and the instant of time the particle has this energy
The Heisenberg uncertainty principle applies to electrons and states that we can not determine the exact position of an electron. Instead, we could determine the probability of finding an electron at a particular position. 8

9. Quantum Theory for Semiconductors
How to determine the behavior of electrons in the semiconductor?
Mathematical description of motion of electrons in quantum mechanics ─ Schrödinger’s Wave Equation
Solution of Schrödinger’s Wave Equation energy band structure and probability of finding a electron at a particular position 9

10. Schrӧdinger’s Wave Equation
One dimensional Schrӧdinger’s Wave Equation:
Wave function
, the probability to find a particle in (x, x+dx) at time t
, the probability density at location x and time t
Potential function
Mass of the particle 10