Physics I Honors

Electronics

Vacuum Tubes (early 1900’s)were the forerunners to modern electronic devices.

Vacuum tubes were used in radios and TVs to: 1. mix signals2. detectors (separate useful signals from a

complex signal)3. amplify weak signals4. rectifiers (convert DC to AC of desired

frequency)5. wave shapers (change voltage waveform into

desired shapes)6. oscillators (change AC to DC)

Vacuum Tube Drawbacks

1. Use too much power2. Size3. Warm up delay4. Frequency limitationVacuum tubes are used some today as CRTs

(cathode ray tubes to display information for oscilloscopes, older TV and computer monitors).

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).

Actual Edison light bulb or DIODE

+ plate

- cathode

DIODE symbol

Symbol for DeForest triode

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

Diode Operation

Input Output

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.

Band Theory of SolidsElectrons 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 lowInsulators – highSemiconductors – in between

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.

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.

SemiconductorsElectrons 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).

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.

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.

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.

n-type semiconductorThe 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.

p-type semiconductorWhen 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.

Electronic Devices

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

PN JunctionWhen 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.

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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

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.

Forward-Biased Diodehttp://www.electronicsandyou.com/electronics-basics/diode.html

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.

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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.

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.

Transistor Components

Crystal of Si or Ge acts as a cathodePlatinum catwhisker – anodePermits large flow of e in forward direction

and very small flow in reverse direction

Junction Transistors

• 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

JFET – junction field effect transistor

MOSFET – metal oxide semiconductor FET

Thousands of these are used in computers for amplifying or switching electronic signals.

http://computer.howstuffworks.com/wirths-law.htm

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

computer.howstuffworks.com

sciencedaily.comsciencedaily.com

Photovoltaic Effect

Provides electrical energy for:1. Space vehicles2. Solar batteries on earth

a. Telephone linesb. Water pumpsc. Recharge storage batteries

3. Light meter on camera