1. Physics I Honors
Electronics

2. Vacuum Tubes (early 1900’s)
were the forerunners to modern electronic devices. Vacuum tubes were used in radios and TVs to:
mix signals
detectors (separate useful signals from a complex signal)
amplify weak signals
rectifiers (convert DC to AC of desired frequency)
wave shapers (change voltage waveform into desired shapes)
oscillators (change AC to DC)

3. Vacuum Tube Drawbacks Use too much power Size Warm up delay
Frequency limitation
Vacuum tubes are used some today as CRTs (cathode ray tubes to display information for oscilloscopes, older TV and computer monitors).

5. History
Edison put a plate in his early light bulbs to absorb heat and reduce burnouts – the 1st diode.
This “Edison Effect” was used to receive radio signals (did not work well).
DeForest improved by placing a wire grid between plate and hot filament (a triode).

6. + plate
- cathode
Actual Edison light bulb or DIODE
DIODE symbol
Symbol for DeForest triode

7. Diode Characteristics
Allows current to travel in only 1 direction
Thermionic Emission – emission of electrons from hot surface. Tungsten used – operates well at high temps.
By changing plate current , current flow and temp of operation can be changed.
Power diodes – big in size to dissipate heat; used in power supply circuits
Signal diodes – smaller, used as signal detectors

8. Diode Operation
Input
Output

9. Cathode Ray Tube (CRT) – special tube designed to display information
Electrons are emitted from cathode, accelerated to high velocity and focused on a fluorescent screen electrostatically or magnetically.
Beam is swept across picture tube 15,730 times per second. Where beam hits, light is emitted from fluorescent chemicals.
Uses: test equipment (oscilloscope), computer, radar, microwave oven.

10. Band Theory of Solids
Electrons in an atom are found in ground state in what is called a valence band. If energy is applied to the atom, those electrons may go to a higher energy level (conduction band) and be conducted away to another atom. Energy gaps (energy needed to conduct electrons): Conductors – very low Insulators – high Semiconductors – in between

11. The energy needed to move electrons to a nearby atom are shown above for Carbon and Silicon. Silicon takes less energy than does carbon. Neither of these elements is a good conductor.
An electron volt (eV) is the amount of energy needed to move 1 electron across a potential difference of 1 volt.

12. Insulators
Electrons in an insulator like that indicated at right must be given a large amount of energy to be pulled into the conduction band.
An insulating material’s valence band is filled to capacity and the conduction band is empty.

13. Semiconductors
Electrons can move more freely in semiconductors than in insulators, but not as freely as in conductors. The energy needed to move an electron in a semiconductor as indicated at right is much less than for an insulator. Some electrons can conduct as a result of their thermal kinetic energy alone (intrinsic semiconductors).

14. Conductors
Materials with overlapping, partially filled bands are conductors, as indicated by the figure at right. There is no gap between the valence and conduction bands.

15. Making semiconductors (s-c’s) conduct better.
Silicon (Si) and Germanium (Ge) are the most common s-c’s. They are both group IV elements and they each have 4 valence electrons. If an electric field is applied to a semiconductor, electrons will move through the solid and leave a hole – an empty energy level in the valence band. The atom now has a + charge and becomes neutral again when an electron moves into the hole.

16. Doping Semiconductors
Dopants are elements that can either donate or accept electrons. As a result of doping, conductivity is greatly increased. Now they are called extrinsic s-c’s. Arsenic (As) has 5 valence electrons and when used as a dopant for Si produces an n-type s-c. The 5th electron is the donor electron. Gallium (Ga) has 3 valence electrons and when used as a dopant for Si produces a p-type s-c. When Ga replaces a Si atom, one binding electron is missing resulting in a hole.

17. n-type semiconductor
The energy of this donor electron is so close to the conduction band that thermal energy can easily move the electron from the dopant atom into the conduction band, as shown in the figure.
Conduction in n-type semiconductors is increased by the availability of these extra donor electrons to the conduction band.

18. p-type semiconductor
When a gallium atom replaces a silicon atom, one binding electron is missing, creating a hole in the silicon crystal, as shown in the adjoining figure.
Electrons in the conduction band can easily drop into these holes, creating new holes.

19. Electronic Devices
Diodes – a sandwich of n-type and p-type s-c’s. The boundary between them is called a p-n junction.

20. PN Junction
When P-type and N-type silicon are placed in contact with one another it forms a PN junction. At this junction an interesting phenomenon occurs, one that is the foundation of solid-state electronics.
imagesco.com

21. A basic PN junction creates a diode
that allows electricity to flow in one direction
but not the other. We can see in the diagram
of a diode that the N type material has free
electrons shown as black dots and the P type
material has holes shown as white dots.
circuitstoday.com

22. Reverse-Biased Diode electronicsandyou.com
The region around the junction is left with neither holes nor free electrons. This region is called the depletion layer since it contains no charge carriers. No current will flow.

23. Forward-Biased Diode
Now charge carriers are pushed toward the junction. If the voltage is high enough (0.6 V for a Si diode), electrons reach the p end and fill the holes, the depletion layer is eliminated, and current flows.
One major use for a diode is to convert AC voltage to DC voltage. When doing this, the diode acts as a rectifier.
asashop.org

24. In the left diagram we have connected an external power source; a battery with a light and current meter that indicate current flow. The negative terminal of the battery is connected to the N-type silicon. Like charges repel, so the free electrons are pushed toward the PN junction.
This is said to be forward biased.
Now the battery is connected to the diode so that the negative terminal of the battery connects to the P-type silicon . The negative terminal attracts the positive holes in the P-type silicon and the positive terminal of the battery attracts the free electrons in the N-type; no current flows. This is said to be reverse-biased. f

25. Transistors
Semiconductor devices composed of Ge or Si and used as a substitute for vacuum tubes in electronic applications. Small in size, require no heater or filament power.

26. Transistor Components
Crystal of Si or Ge acts as a cathode
Platinum catwhisker – anode
Permits large flow of e in forward direction and very small flow in reverse direction

27. Junction Transistors

28. To further improve on the efficiency of the transistor, the collector is made physically larger than the base for two reasons: (1) to increase the chance of collecting carriers that diffuse to the side as well as directly across the base region, and (2) to enable the collector to handle more heat without damage. A very basic relationship exists between these two currents:
IE = IB + IC

29. JFET – junction field effect transistor

30. MOSFET – metal oxide semiconductor FET
Thousands of these are used in computers for amplifying or switching electronic signals.

31. Transistors are the building blocks of microprocessors, the computer chips that run our desktops and laptops. They act like simple on-and-off switches, allowing electronic data to travel from point to point, store information and deliver the commands we make. Initially, transistors were large, and they were constructed one at a time. But between the years 1958 and 1959, two inventors, Jack Kilby at Texas Instruments and Robert Noyce at Fairchild Camera, found a way to put all of the necessary parts of a computer chip into one tiny package, something called the integrated circuit

32. computer.howstuffworks.com

33. sciencedaily.com
sciencedaily.com

34. Photovoltaic Effect Provides electrical energy for: Space vehicles
Solar batteries on earth
Telephone lines
Water pumps
Recharge storage batteries
Light meter on camera