Fun Electron Tricks Semiconductor Devices npn junction Put another n-type semiconductor on the other side of the p-type semiconductor No matter which.

Fun Electron Tricks

Semiconductor Devices

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

MOSFET

• 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-typen-type

(Metal-Oxide-Semiconductor, Field-Effect Transistor)

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

n-type p-typen-type

Gate

MOSFET

• The potential difference between drain and source is continually applied

• When the gate potential difference is applied, current flows

Source Drain

n-type p-typen-type

Gate

(Metal-Oxide-Semiconductor, Field-Effect Transistor)

Bipolar Junction Transistor

n-type p-type n-type

Emitter BaseCollector

increasing electron energy

increasing hole energy

Bipolar Junction Transistorhttp://hyperphysics.phy-astr.gsu.edu/hbase/solids/trans.html#c1

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

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.

OutputDump

Input

Switch

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.

OutputDump

InputInput

Switch Switch

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.

Dump

Input Input

Switch Switch

Switch

Output

Interference of Waves and the Double Slit Experiment

• Waves spreading out from two points, such as waves passing through two slits, will interfere

d

Wave crestWave troughSpot of constructive interference

Spot of destructive interference

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

What Happens if Electrons Pass Through Small Openings?

What does that tell you about electrons?

The Plot Thickens

An experiment called the “photoelectric effect” also gives

unexpected results!

The Photoelectric Effect, Pictorially

• Light shining on a material may be absorbed by electrons in that material If an electron absorbs

enough energy to break free of its bonds, it can leave the material

cresttrough

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

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

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 - Work function (energy needed to remove electron)

Do the Photoelectric Experiment

Upon what does the energy of emitted electrons appear to depend?

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)

Einstein to the Rescue

• Einstein suggested that light was emitted or absorbed in particle-like quanta, called photons, of energy, E = hf

cresttrough

If an electron absorbs one of these photons, it gets the entire hf of energy.

If that energy is larger than the work function of the metal, the electron can leave; if not, it can’t:

Kmax = Eabs – = hf -

Einstein’s Photoelectric Theory

eVs = Kmax = hf –

• Kmax f• Is this consistent with what you saw in the experiment?

• Electrons are ejected as soon as a photon strikes the material.

• Is this consistent with what you saw in the experiment?

Einstein’s Photoelectric Theory

eVs = Kmax = hf –

• If hf < , no electrons are emitted; cutoff frequency

• What should the slope of a K vs. f plot yield? Is that what you got?

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?

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/, so large (macroscopic) momentum means

small (undetectable) wavelength

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 (x,y) which obeys Schrödinger’s equation

Before the next class, . . .

• Homework 20

• Do Activity 19 Evaluation by Midnight Monday

• Read Chapters 7 and 8 in Turton.

• Do Reading Quiz