Semiconductors and Diodes

1. Semiconductors and Diodes Bands, gaps, etc.

2. Semiconductor • Semiconductors are materials, which are neither good conductors (like copper, gold, silver) nor good insulators (like rubber and glass). • The most common semiconductors are silicon and germanium. • Silicon is directly under Carbon on the periodic table, Germanium is under Silicon.

3. Periodic Table

4. The Importance of Being Semiconducting • Much of a computer’s circuitry is made of semiconductors. • Before semiconductor (solid-state) transistors, there were vacuum tubes which were huge by comparison. • The ENIAC was made from tubes. • In addition to the obvious benefit of taking up less space, smaller devices tend to be faster and require less energy.

5. ENIAC By today's standards for electronic computers the ENIAC was a grotesque monster. Its thirty separate units, plus power supply and forced-air cooling, weighed over thirty tons. Its 19,000 vacuum tubes, 1,500 relays, and hundreds of thousands of resistors, capacitors, and inductors consumed almost 200 kilowatts of electrical power.

6. Why are they useful? • Semiconductors can be made to conduct better under the right circumstances; it is this control over the conductivity that makes them useful. • A semiconductor device’s ability to carry current can depend on the current or voltage applied. • They are NOT Ohmic (i.e they do not obey Ohm’s law V=IR). • Their properties may also depend on the whether or not light is shining on them, how much light, what color light, etc. This make them useful as light detectors used in scanners, photcopiers, laser printers, digital cameras, etc.

7. Doping • One way to change a semiconductor’s properties is to dope it. • Doping is adding another substance to a pure semiconductor. • Typically one dopes with an element that lies either to the right or left of the pure element on the Periodic Table.

8. Periodic Table

10. n and p doping • Doping with elements on the right increases the number of free (valence) electrons and is called n doping. • Doping with elements on the left decreases the number of free (valence) electrons and is called p doping. • The material added is called a dopant. • Both kinds of doping tend to increase the conductivity. Why?

11. Energy levels • The electrons in atoms do not have arbitrary amounts of energy. • There are very precise energy levels that the electrons are allowed to have. • (You might recall 1s, 2s, 2p, 3s, 3p, 3d, etc. from chemistry.) • The energy is said to be “quantized.”

12. Bands • When many atoms come together to form a solid, there are still quantized energy levels, just many, many more of them. • For some energy levels, the next highest or lowest energy level is so close, it’s almost continuous (sometimes called quasi-continuous). • The closely packed energy levels are said to form a band.

13. Gap • There are still some energies that the electrons are not allowed to have. • Thus there are jumps in energy between the highest energy in one band and the lowest energy in the next band. • This jump in energy is called the band gap or just the gap.

14. http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/energy_gap/energy_gap.htmlhttp://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/energy_gap/energy_gap.html

15. Pauli Exclusion Principle • Electrons have a property called spin which can only have two values: up and down. • The Pauli Exclusion Principle says that two electrons with the same spin cannot occupy the same level. • Thus there can only be two electrons per level — one with spin up, one with spin down.

16. Filling in the bands (not the gaps) • Imagine the energy levels are all empty, and we begin putting in the electrons. • The first two electrons go into the lowest level. • The next two go into the next lowest level, etc. By filling the lowest levels first, you reduce as much as possible the energy of the whole system. This is the system’s ground state.

17. Where does it all end? • A material’s conductivity depends on whether the last filled energy level of its ground state is in the middle of a band (a metal) or at the top of a band just before a gap (semiconductors and insulators). • The size of the gap determines whether the material is a semiconductor (small gap) or insulator (large gap). • (This is over-simplified.)

18. Introducing a current • To have current, one must move electrons around. • To move electrons around, one must give some of them more energy than they have in the ground state. • With more energy they go to higher energy levels. • An electron has to go into an energy level that has room, it cannot go into a level that is completely occupied.

19. Band versus Gap • If the highest energy level of the ground state (the Fermi level) falls within a band, then one only needs to add a little energy to cause a current (a conductor). • If the Fermi level falls in a gap, then one has to add a lot of energy to cause a current (a semiconductor or insulator).

20. Valence and Conduction Bands • If the Fermi level falls in the gap, the highest filled band is called the valence band and the lowest unoccupied band is called the conduction band. • In such a case, electrons must be promoted from the valence band to the conduction band in order to move around.

21. http://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/energy_gap/energy_gap.htmlhttp://people.seas.harvard.edu/~jones/es154/lectures/lecture_2/energy_gap/energy_gap.html

22. An Analogy • Assume a university offers 100-, 200-, 300- and 400- level courses. • All of the 100-level courses are of a similar difficulty, but 200-level courses are significantly harder, etc. • The university offers 10 classes at each level and each class has a strict cap of 20 students.

23. Analogy continued • Each level has an “occupancy” of 200 (10 classes  20 students). • The course levels (100, 200, etc.) correspond to the bands. • The change in difficulty between 100 and 200 corresponds to a gap.

24. The Semiconductor University • The students at this university must all take four courses. • This is analogous to Silicon’s having four valence electrons. • Say there are 100 students at the university, they must take 400 courses all together (100 students  4 courses each).

25. The lazy students • The students all register for the easiest courses first, but they fill up. • The students end up only taking 100- and 200- level courses.

26. An afternoon job • A student wants to take an afternoon job, but has a schedule conflict. • In order to have some flexibility in his/her schedule, such a student would have to take a substantially harder course (the only open courses are in the 300 and 400 level). • The schedule flexibility is like conductivity (the ease of motion or lack thereof). • So far our analogy is of a pure semiconductor, next let’s look into doping.

27. The exchange program • Now say a percentage of the 100 students belong to an exchange program and that they only take three courses. • In such a case, not all of the 200-level courses would be filled. • And the student wanting the afternoon job would not have to take a significantly harder course (assuming he/she was already taking one of the harder 200-level courses). • So there is more flexibility (higher conductivity). • This is analogous to p-doping.

28. The three-year program • A different possibility is that a percentage of the 100 students belong to an 3-year program and take five courses. • In such a case, some of the 300-level courses would be filled. • If the student wanting the afternoon job already had a 300-level course, he/she would not have to take a significantly harder course. • So there is more flexibility (higher conductivity). • This is analogous to n-doping.

29. Doping improves conductivity • We see in our analogy that introducing a fraction of students who take fewer courses courses improved flexibility. Likewise, introducing a fraction of students who take more courses improved flexibility. • Either type of doping increased conductivity. • But if all we wanted was better conductivity, we would just use good conductors. So there must be more to it.

30. At this juncture (or is that junction?) • We can change the conductivity, so what? • The interesting phenomena occurs when a p-doped semiconductor meets an n-doped semiconductor. • This is called a p-n junction.

31. Two nearby universities • To pursue the analogy, imagine two universities in the same city. The p-doped University has an exchange program (students taking fewer courses), n-doped University has a 3-year program (students taking more classes). • Some students taking 300-level courses at N.U. notice there are open slots in the 200-level courses at P.U. • There are unoccupied low-energy levels in the p-doped material.

32. The penalty • For 300-level-taking N.U. students who live in between the universities, there is a benefit to taking 200-level classes at P.U., but for others there is a penalty (a longer commute). • In the real materials the penalty is charge separation. Before an migration, the n-doped and p-doped materials were electrically neutral; after the migration p-doped material will end up with a net negative charge (those extra electrons). • Recall with batteries and capacitors it required energy to separate charges.

33. Violating Neutrality • Although one might think of n-doped materials as having “excess” electrons, they are neutral. • The extra electron are compared to the number of electron the pure semiconductor has, but the doping material also has an “extra” proton. • Similarly p-doped materials may be thought of as “deficient” in electrons, but they too are neutral. • But when electrons from n-doped go over to the p-doped side, the n-doped side is left with a positive charge and the p-doped side gets a negative charge.

34. - - • - - + + + + + + p-doped Like a little capacitor • There is an energy cost involved in separating charges. • The p-n junction is like a little capacitor n-doped n-doped

35. A loss in flexibility • With some N.U. students now taking P.U. classes, there is a loss in schedule flexibility (conductivity) especially among students who live between the universities (at the p-n junction). • There is a loss in conductivity at the p-n junction. • So our increase in conductivity is lost, but what have we gained?

36. - - • - - + + + + + + p-doped Applying a voltage • Connecting a p-n junction to a battery as shown below adds positive charges on the n-doped side making the region of poor conductivity larger. • This is called reverse bias. • Current doesn’t flow in this direction. n-doped + + + 

37. - - • - - + + + + + + p-doped Applying a voltage II • Connecting a p-n junction to a battery as shown below adds positive charges on the p-doped side making the region of poor conductivity smaller (oversimplified). • This is called forward bias. • Current does flow in this direction. n-doped  + + +

38. Diode • Thus current at p-n junction flows readily in one direction and poorly if at all in the other direction. • Devices with this property are called diodes. • A p-n junction is a diode. • It’s like a valve in the heart that only allows blood to flow in one direction. • We will see in the lab some uses of diodes.

39. LED • Some diodes are configured such that when electrons cross the junction they must give off a photon (light). • Thus they glow when they are conducting. • They are known as light emitting diodes or LEDs. • LEDs are used as simple displays and indicators. Organic LEDs (OLEDs) may be used to make displays, replacing the LCDs and CRTs that currently dominate.

40. Photoconductors • An electron below the gap can be given the boost it needs to jump the gap by absorbing the energy from a photon (light). • Once in the conduction band, the excited electron is free to move around. • If the boost is provided by visible light, the material is called a photoconductor.

41. Night and day • A photoconductor is a poor conductor in the dark but a reasonable conductor in the light. • Photoconductors are the key technology in photocopiers, laser printers, scanners and digital cameras.