Semiconductors and Diodes Bands gaps etc PHY 202

Semiconductors and Diodes Bands, gaps, etc. PHY 202 (Blum) 1

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. PHY 202 (Blum) 2

Periodic Table PHY 202 (Blum) 3

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. PHY 202 (Blum) 4

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. PHY 202 (Blum) 5

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. PHY 202 (Blum) 6

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. PHY 202 (Blum) 7

Periodic Table B C N O Boron Carbon Nitrogen Oxygen Al Si P S Aluminum Silicon Phosphorus Sulfur Ga Ge As Se Gallium Germanium Arsenic Selenium In Sn Sb Te Indium Tin Antimony Tellurium PHY 202 (Blum) 8

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? PHY 202 (Blum) 9

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 1 s, 2 p, 3 s, 3 p, 3 d, etc. from chemistry. ) The energy is said to be “quantized. ” PHY 202 (Blum) 10

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. PHY 202 (Blum) 11

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 the lowest energy in the next band. This jump in energy is called the band gap or just the gap. PHY 202 (Blum) 12

http: //people. seas. harvard. edu/~jones/es 154/lectures /lecture_2/energy_gap. html PHY 202 (Blum) 13

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. PHY 202 (Blum) 14

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. PHY 202 (Blum) 15

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. ) PHY 202 (Blum) 16

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. PHY 202 (Blum) 17

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). PHY 202 (Blum) 18

Valence and Conduction Bands If the Fermi level falls in the gap, the highest filled band is called the valence band 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. PHY 202 (Blum) 19

http: //people. seas. harvard. edu/~jones/es 154/lectures /lecture_2/energy_gap. html PHY 202 (Blum) 20

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. PHY 202 (Blum) 21

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. PHY 202 (Blum) 22

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). PHY 202 (Blum) 23

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. PHY 202 (Blum) 24

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. PHY 202 (Blum) 25

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. PHY 202 (Blum) 26

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. PHY 202 (Blum) 27

$Doping improves conductivity We see in our analogy that introducing a fraction of students$

Doping improves conductivity We see in our analogy that introducing a fraction of students who take fewer 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. PHY 202 (Blum) 28

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. PHY 202 (Blum) 29

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 pdoped material. PHY 202 (Blum) 30

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. PHY 202 (Blum) 31

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 pdoped side, the n-doped side is left with a positive charge and the p-doped side gets a negative charge. PHY 202 (Blum) 32

Like a little capacitor There is an energy cost involved in separating charges. The p-n junction is like a little capacitor p-doped PHY 202 (Blum) -- +++ n-doped 33

Depletion Zone In a diode one brings together n-doped and p -doped material. Some of the “extra” electrons from the n side’s conduction band fill the empty levels in the p side’s valence band, forming a region in which the valence band is filled and conduction band is empty. This region (called the depletion zone) is a poor conductor. PHY 201 (Blum)

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? PHY 202 (Blum) 35

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. -- +++ p-doped n-doped -- +++ PHY 202 (Blum) 36

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. -- +++ p-doped n-doped -- +++ PHY 202 (Blum) 37

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. PHY 202 (Blum) 38

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 are used as simple displays and indicators. Organic LEDs (OLEDs) may be used to make displays, replacing the LCDs and CRTs that currently dominate. PHY 202 (Blum) 39

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. PHY 202 (Blum) 40

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. PHY 202 (Blum) 41

Transistors and Logic Gates References: http: //www. st-and. ac. uk/~www_pa/Scots_Guide/info/comp/active/Bi. Polar/page 1. html PHY 201 (Blum)

Transistors There are various kinds of transistors: bipolar, field-effect, etc. They differ in stability, energy usage, and so on, but they serve a similar purpose. They are used to amplify a signal or to act as a switch. It is as switches that they are used in computers. The change from vacuum tubes to transistors meant dramatic differences in the physical size of computers and ultimately dramatic differences in their speed, capacity and ubiquity. PHY 201 (Blum)

Bipolar transistors A bipolar transistor starts with two back-toback diodes (pn junctions). There are two kinds: NPN and PNP The middle region is usually small PHY 201 (Blum) N-doped P-doped

Third lead So far the device seems useless; two back-to-back diodes wouldn’t conduct in either direction. But we add a third lead (connection) directly to the middle portion. PHY 201 (Blum)

Not symmetric The transistor would seem to be symmetric with the two N-doped regions being the same, but actually these regions differ in their amount of doping and serve different purposes in the transistor. PHY 201 (Blum)

Collector, base, emitter Collector Base Emitter PHY 201 (Blum) In bipolar transistors, the various regions are referred to as the collector, base and emitter.

Collector, base, emitter PHY 201 (Blum)

Connecting the transistor C (N) B (P) E (N) PHY 201 (Blum) Imagine applying a potential difference (voltage) across the base-collector leads with the collector higher, this reverse biases that pn junction so there would be no current flow.

No flow There is no flow because of the depletion zone (the region in which the valence band is filled and the conduction band empty). Reverse bias voltages tend to make the depletion zone a bit larger. PHY 201 (Blum)

Connecting the transistor (Cont. ) C B E PHY 201 (Blum) Now consider applying a (smaller) voltage across the base-emitter leads with the base higher, this forward biases that pn junction so current will flow.

Flow Forward biasing a pn junction tends to eliminate the depletion zone (in this case putting electrons into the conduction band). Because the transistor has one shared depletion zone that has been eliminated by the base-emitter forward bias, both currents (collector-base and base-emitter) can flow. PHY 201 (Blum)

NPN in a circuit C B E PHY 201 (Blum) The arrow on an NPN points from base to emitter indicating the forward-bias direction that turns the transistor “on. ”

Characteristics of the Off State C B E Little to no current going into the base and a large voltage drop across the collector/emitter leads. PHY 201 (Blum)

Off Base-emitter circuit Little to no current flowing. Most of the voltage dropped across the baseemitter as opposed to the resistor in the circuit. Collector-emitter circuit Little to no current flowing Most of the voltage dropped across the collectoremitter as opposed to the resistor in the circuit. PHY 201 (Blum)

Characteristics of the On State C B E Current through the base lead, small voltage drop across collector/emitter leads. PHY 201 (Blum)

On Base-emitter circuit Current flowing. Most of the voltage dropped across the resistor as opposed to the base-emitter in the circuit. Collector-emitter circuit Current flowing. Most of the voltage dropped across the resistor as opposed to the collector-emitter in the circuit. PHY 201 (Blum)

Logic gates The on-off nature of diodes and transistors make them ideal for building logic gates. Logic gates have input which is interpreted as a logic value (0 or 1, low or high, false or true) and have output which can also be interpreted logically. PHY 201 (Blum)