Semiconductors and Electromagnetic Waves

1. Semiconductors and Electromagnetic Waves

2. 23.5 Semiconductor Devices Semiconductor devices such as diodes and transistors are widely used in modern electronics. “Technology has clearly revolutionized society, but solid-state electronics is revolutionizing technology itself”.

3. Semiconductors • Silicon is the most common material used as a semiconductor (germanium is also used). • It has 4 valence electrons and forms a stable lattice structure. • All electrons are used in the bonding process of the lattice, there are none free to move through the lattice structure, therefore pure Si is a poor conductor.

4. SEMICONDUCTORS 23.5 Semiconductor Devices The semiconducting materials (silicon and germanium) used to make diodes and transistors are doped by adding small amounts of an impurity element.

5. n-TYPE SEMICONDUCTORS 23.5 Semiconductor Devices • Small amounts of a material with 5 outer-shell electrons added to the silicon (e.g phosphorus). • Extra electron which diffuses through the silicon lattice structure and increases overall conductivity. • The n-type semiconductor is electrically neutral. Although the phosphorus contributed an electron that doesn’t fit into the lattice structure, the phosphorus atom is neutral.

6. 23.5 Semiconductor Devices p-TYPE SEMICONDUCTORS • Small amounts of a material with 3 valence electrons are added to the silicon (e.g boron). This leaves an electron “hole” which diffuses through the silicon lattice structure and increases conductivity. • Note that the p-type semiconductor is electrically neutral, just like the n-type material.

7. 23.5 Semiconductor Devices What do you get when you put an p-type and an n-type semiconductor together? overall neutral overall neutral

8. You get a p-n Junction • Mobile electrons from the blue, n-type material move left to fill the holes in the pink p-type material ( left, in Fig a). One may think of the square electron holes as moving right. • The layer at the end of p-type material becomes negative and vice versa. This results in an electric field, pointing from n-type material to p-type material (Fig b). • The resulting structure is called a diode. • No current flows because the diode is electrically neutral.

9. Connect a voltage source with a diode. • Consider attaching a battery so that positive terminal (gold) goes to the n-type side and the negative terminal goes to the p-type side. What happens? • Current flows in the direction of the E field (left). • Current flows against the electric field (right) • Current doesn’t flow

10. Connect a voltage source with a diode. • Consider attaching a battery so that positive terminal (gold) goes to the n-type side and the negative terminal goes to the p-type side. What happens? • Current flows in the direction of the E field (left). • Current flows against the electric field (right) • Current doesn’t flow, the positive charge is repelled by the positive layer in the n-type material.

11. 23.5 Semiconductor Devices There is an appreciable current through the diode when the diode is forward biased. Under a reverse bias, there is almost no current through the diode.

12. 23.5 Semiconductor Devices • The graph shows dependence of current on magnitude and polarity of voltage applied across an ideal p-n junction. • the arrow/bar is the symbol for diode (arrow shows the direction the diode allows conventional current to flow). • Reverse bias- regardless of how much voltage applied, no current flows. • Forward bias – after some “threshold” voltage applied (here slightly more than 0.5 volts), current rises at an exponential rate.

13. 23.5 Semiconductor Devices • A more realistic graph for a silicon diode. • When reverse-biased, a real diode lets in a very small amount of current. • If you apply enough reverse voltage (V), the junction breaks down and lets current through (shown at far-left unlikely in normal circumstances). • When forward-biased, the threshold voltage for silicon is about 0.7 volts. • A diode is a non-ohmic device; it does not obey Ohm’s Law. • If you apply more more voltage (bigger battery), the current through the diode will increase, but the voltage drop will always remain at the threshold value.

14. A solar cell is a diode. • Photons in sunlight hit the solar panel. • The energy ionizes atoms in the charge layers. • Electrons are ejected from their atoms, allowing them to flow through the material to produce electricity. • Due to the composition of solar cells, the electrons are only allowed to move in a single direction. As a result, the solar cell develops a positive and negative terminal, much like a battery.

15. 24.1 The Nature of Electromagnetic Waves This picture shows an electromagnetic wave, such as a light wave, or radio wave An EM wave is a transverse wave that does not need a medium, e.g. air, or water, to propagate. • In 1865, long before the experiment, the English physicist Maxwell correctly predicted that, in a vacuum: ε0 = 8.85 x 10-12 C2/(N m2 μ0 = 4π x 10-7 T m/A.

16. 24.3 The Speed of Light • The American physicist Albert Michaelson improved on attempts to measure the speed of light. • By placing his mirrors on top of 2 Southern California mountains, he obtained a value of c that was less than 0.0014% different that the currently accepted value. • He definitely got a A on that lab.

17. 24.2 The Electromagnetic Spectrum Like all waves, electromagnetic waves have a wavelength and frequency, related by:

18. 24.2 The Electromagnetic Spectrum Example 1 The Wavelength of Visible Light Find the range in wavelengths for visible light in the frequency range between 4.0x1014Hz (red) and 7.9x1014Hz (violet).

19. The Crab Nebula is a remnant of a star that underwent a supernova. This event was recorded in the year 1054 A.D (see Anasazi pictograph, below). The Crab Nebula is located at a distance of 6.0 x 1016 km away from the earth. How long ago did the supernova happen?

20. The Crab Nebula is a remnant of a star that underwent a supernova. This event was recorded in the year 1054 A.D (see Anasazi pictograph, below). The Crab Nebula is located at a distance of 6.0 x 1016 km away from the earth. How long ago did the supernova happen? - 7300 years ago from 2010

21. 24.4 The Energy Carried by Electromagnetic Waves Electromagnetic waves, such as the microwaves shown below, carry energy, much like sound waves

22. Light waves and infrared rays also carry energy

23. Greenhouse gases • A greenhouse gas (sometimes abbreviated GHG) is a gas in an atmosphere that absorb and emit radiation in the thermal infrared part of the spectrum • Greenhouse gases (CO2, CH4 H2O) in the atmosphere absorb infrared radiation and then re-emit it in all directions

24. Most solar energy is in the form of shortwave radiation (e.g. light, uv rays) • Earth absorbs this energy and re-emits as longwave radiation (infra-red, “heat”) • Greenhouse gases (CO2, CH4 H2O) in the atmosphere absorb infrared radiation • This natural process allows the Earth to maintain an average yearly temperature of about 150 C (600 F).

25. Electromagnetic Energy Density (0) • Electromagnetic waves fluctuate in a sinusoidal manner. When the electric field has a maximum strength, so does the magnetic field. Both are zero at the same instant. • The energy carried by the wave fluctuates in the same manner. • A way to quantify the energy of an electromagnetic wave is to measure the total energy density carried by an electromagnetic wave (energy per unit volume).

26. Electromagnetic Energy Density (1) • This can be divided into electric energy density and magnetic energy density: • where E is the magnitude of the electric field at some instant, and ε0 = 8.85 x 10-12 C2/(N m2 )

27. Electromagnetic Energy Density (2) • Similarly, the magnetic energy density is: • where B is the magnitude of the magnetic field at some instant and μ0 = 4π x 10-7 T m/A. • In 1865, long before Michelson’s experiment, the English physicist Maxwell correctly predicted that:

28. Electromagnetic Energy Density (3) • The total energy density is a combination of the electrical energy stored in the electric field and the magnetic energy stored in the magnetic field. • This value changes as the electric and magnetic fields fluctuate.

29. Electromagnetic Energy Density (4) • Since each term is equal to one half of the total energy density at a given instant of time: Setting these two terms equal shows that E = cB in the electromagnetic wave.

30. Average energy density and rms values • The energy density fluctuates between zero and some maximum value, when E and B are at their maximum values. • To obtain an average value of energy density, we use rms (root mean square) values for E and B in the calculations: • Erms = and Brms =

31. Example The rms value of the magnetic field in an electromagnetic wave is 3.3 x 10-6 T. • What is the maximum strength of the wave’s electric field? • What is the average energy density, u, of the wave?

32. Example The rms value of the magnetic field in an electromagnetic wave is 3.3 x 10-6 T. • What is the maximum strength of the wave’s electric field? 1400 V/m • What is the average energy density, u, of the wave? 8.67 x 10-6 J/m3.

33. EM Wave Intensity • Intensity – defined previously for sound waves as power to area ratio: Intensity = P/A. • Intensity is inversely proportional to the square of the distance from the source of the wave. • Recall power is the amount of energy transported per second.

34. EM Wave Intensity Note the volume through which the energy passes is ctA, and your book uses S for intensity

35. EXAMPLE The electric field of a laser beam Note that a laser sends out light in a fixed beam, not out in all directions, so the appropriate area to use is the area of the circular beam, and not the surface area of a sphere.

36. EXAMPLE The electric field of a laser beam and therefore = 690 V/m

37. 24.5 The Doppler Effect and Electromagnetic Waves • Electromagnetic waves also can exhibit a Dopper effect, but it • differs for two reasons: • Sound waves require a medium, whereas electromagnetic • waves do not. • For sound, it is the motion relative to the medium that is important. • For electromagnetic waves, only the relative motion of the source • and observer is important. • c) use plus if observer and source are moving together, minus if they are moving apart. • d) vrel is a magnitude and therefore always positive.

38. 24.5 The Doppler Effect and Electromagnetic Waves Example 6 Radar Guns and Speed Traps The radar gun of a police car emits an electromagnetic wave with a frequency of 8.0x109Hz. The approach is essentially head on. The wave from the gun reflects from the speeding car and returns to the police car, where on-board equipment measures its frequency to be greater than the emitted wave by 2100 Hz. Find the speed of the car with respect to the highway. The police car is stationary.

39. 24.5 The Doppler Effect and Electromagnetic Waves source frequency fs = 8 x 109 Hz frequency “observed” by speeding car reflected frequency observed by police car Replace f’0with term for f0 on the right side of equation: Replace f0with fs on the right side of the equation and expand the square:

40. 24.5 The Doppler Effect and Electromagnetic Waves Continuing: but we can make the assumption that vrel<< c, so the last term becomes 2:

41. Doppler weather radar uses the Doppler shift of reflected radar signals to measure wind speeds and gauge the severity of a storm. This picture is off the coast of Florida.

42. Red shifts and blue shifts: The Big Bang • For light coming from astronomical objects, this Doppler equation is no longer correct, but it is still true that the light coming from an object moving closer has a higher frequency, while the light coming from a receding object has a lower frequency. • We say light has been “blue-shifted” for an object moving closer, and “red-shifted” for an object moving away. • The light coming from the stars and galaxies around us is red-shifted, leading to our present belief that the galaxy is expanding. • Extrapolating back in time brings us to a point when the universe was contained in a volumeless point that “exploded”, aka The Big Bang.