1. Semiconductor Devices
Fun Electron Tricks
Semiconductor Devices

2. npn junction
Put another n-type semiconductor on the other side of the p-type semiconductor
No matter which way I apply potential difference, one p-n junction is reverse biased, and electrons entering the p-type region quickly combine with holes, creating more negative charge

3. MOSFET (Metal-Oxide-Semiconductor, Field-Effect Transistor)
If, however, I apply a positive potential to one side of the p-type semiconductor, without allowing another path for electrons to flow out of the device, I will create a channel for e- to get from one n-side to the other.
n-type
p-type
n-type

4. MOSFET
Now, if I bias the device in either direction, current will flow, electrons toward the positive potential, and conventional positive current toward the negative potential
Gate
n-type
p-type
n-type

5. MOSFET (Metal-Oxide-Semiconductor, Field-Effect Transistor)
The potential difference between drain and source is continually applied
When the gate potential difference is applied, current flows
Gate
Drain
Source
n-type
p-type
n-type

6. Bipolar Junction Transistor
Base
Emitter
Collector
increasing electron energy
increasing hole energy
n-type
p-type
n-type

7. Bipolar Junction Transistor

8. How do transistors fit in?
For now, view transistor as switch:
If switch is “on,” current can pass
If switch is “off,” no current can pass
We can use this simple device to construct complicated functions

9. NOT Gate - the simplest case
Put an alternate path (output) before a switch.
If the switch is off, the current goes through the alternate path and is output.
If the switch is on, no current goes through the alternate path.
So the gate output is on if the switch is off and off if the switch is on.
Input
Output
Dump
Switch

10. NAND - a variation on a theme
NAND gate returns a signal unless both of its two inputs are on.
Put an extra switch after a NOT device
If both switches are on, current is dumped.
Otherwise the current goes to the output.
Input
Input
Output
Switch
Switch
Dump

11. AND - slightly more complicated
AND gate returns a signal only if both of its two inputs are on.
Use the NAND output as input for NOT
If both inputs are on, the NOT input is off, so the AND output is on.
Else the NOT input is on, so the output is off.
Output
Switch
Input
Input
Switch
Switch
Dump

12. Interference of Waves and the Double Slit Experiment
Waves spreading out from two points, such as waves passing through two slits, will interfere l
Wave crest
Wave trough
Spot of constructive interference
Spot of destructive interference d

13. The Double-slit experiment for particles
Particles do not diffract; they either go through a slit or they don’t
Particles passing through a slit hit a screen only in a small area; if they all have the same initial velocity, they will all hit at the exact same point
Particles passing through two slits will form two maxima in front of the two slits

14. What Happens if Electrons Pass Through Small Openings?
What does that tell you about electrons?

15. The Plot Thickens
An experiment called the “photoelectric effect” also gives unexpected results!

16. The Photoelectric Effect, Pictorially
Light shining on a material may be absorbed by electrons in that material
The kinetic energy of the electron will be equal to the energy absorbed by the electron minus the energy needed to free it, provided the electron does not lose any energy in collisions
If an electron absorbs enough energy to break free of its bonds, it can leave the material
crest
trough

17. Wave theory predicts . . .
the energy of emitted electrons should depend on the intensity of light
electrons will need to soak up energy from wave for period of time before being ejected
the frequency of light won’t affect the maximum kinetic energy of electrons

18. The Photoelectric Effect, Experimentally
As a given color (frequency) of light enters the black box-like photoelectric head, it falls on a plate of electron-emitting material inside
Emitted electrons are collected on another plate nearby, producing an electric potential difference between the two plates (like a capacitor)
When the capacitor is fully charged and no more electrons can be added, the potential energy of the capacitor equals the maximum kinetic energy of the electrons trying to leave the original plate
The potential difference on the capacitor at this point is called the stopping potential Vs for the electrons, and it is proportional to the maximum kinetic energy of electrons emitted by the light:
K = eVs = Eabsorbed - F
Work function (energy needed to remove electron)

19. Do the Photoelectric Experiment
Upon what does the energy of emitted electrons appear to depend?

20. Experiment sees . . .
the energy of emitted electrons does not depend on the intensity of light
electrons are ejected immediately
the frequency of light does affect the maximum kinetic energy of electrons; kinetic energy is linearly dependent on frequency
intensity of light determines number of emitted electrons (photocurrent)

21. Einstein to the Rescue
Einstein suggested that light was emitted or absorbed in particle-like quanta, called photons, of energy, E = hf
If that energy is larger than the work function of the metal, the electron can leave; if not, it can’t:
Kmax = Eabs – F = hf - F
If an electron absorbs one of these photons, it gets the entire hf of energy.
crest
trough

22. Einstein’s Photoelectric Theory
eVs = Kmax = hf – F
Kmax µ f
Is this consistent with what you saw in the experiment?
Electrons are ejected as soon as a photon strikes the material.

23. Einstein’s Photoelectric Theory
eVs = Kmax = hf – F
If hf < F, no electrons are emitted; cutoff frequency
What should the slope of a K vs. f plot yield? Is that what you got?

24. The Conflict
Wave theory accurately describes interference and diffraction, along with other behavior of light, such as dispersion and refraction
The particle theory accurately describes photoelectric effect, black body radiation, and other experimental results
Is light a particle? Or is it a wave?
Is a platypus a duck? Or is it a beaver?
Am I my mother? Or am I my father?

25. The Resolution Light is not either a particle or a wave
Light exhibits wavelike properties when traveling
Light exhibits particlelike properties when interacting with matter
deBroglie suggested that traditional “particles”, like the electron, also exhibit wavelike properties
p=h/l, so large (macroscopic) momentum means small (undetectable) wavelength

26. The interpretation
Light and “particles” propagate through space as probability waves
I cannot say for certain where a particle is, where it was, or how it got to wherever it might have been
I can, however, say where it is most likely to be found, where it most likely was, and how likely it is that it took a particular path
This behavior is described by a wave function Y(x,y) which obeys Schrödinger’s equation

27. Before the next class, . . . Homework 20
Do Activity 19 Evaluation by Midnight Monday
Read Chapters 7 and 8 in Turton.
Do Reading Quiz