Click here to purchase the entire book in PDF format. Diodes and Semiconductors Back in chapter 2.1, we saw that some substances have too few electrons in their outer valence shell. These electrons are therefore free to move on to the adjacent atom if we give them a nudge with an extra electron from a negative battery terminal - we then call this substance a conductor, because it conducts the flow of electricity. Figure 2.37: Diagram of a copper atom. Notice that there is one lone electron out there in the outer shell. If the outer valence shell has too many electrons, they don't like moving (too many things to pack... moving vans are too expensive, and all their friends go to this school...) so they don't - we call those substances insulators. There is a class of substances that have an in-between number of electrons in their outer valence shell. These substances (like silicon, germanium and carbon) are neither conductors nor insulators - they lie somewhere in between so we call them semiconductors(not ``insuductors'' nor ``consulators'') Figure 2.38: Diagram of a silicon atom. Notice that there are 4 electrons in the outer shell. Compare Figure at Figure 2.39. As you can see, when you put a bunch of silicon atoms together, they start sharing outer electrons - that way each atom ``thinks'' that it has 8 electrons in its outer shell, but each one needs the 4 adjacent atoms to accomplish this Figure 2.39: Diagram of a collection of silicon atoms. Note the sharing of outer electrons. Compare Figure 2.38, which shows the structure of Silicon, to Figures 2.40 and 2.41 which show Arsenic and Gallium. The interesting thing about Arsenic is that it has 5 electrons in its outer shell (1 more than 4). Gallium has 3 electrons in its outer shell. We'll see why this is interesting in a second... Figure 2.40: Diagram of a Arsenic atom. Notice that there are 5 electrons in the outer shell. Figure 2.41: Diagram of a Gallium atom. Notice that there are 3 electrons in the outer shell. Recipe : 1 cup arsenic 999 999 cups silicon Stir well and bake until done. Serves lots. If you follow those steps carefully (do NOT sue me because I told you to play with arsenic...) you get a substance called arsenic doped silicon. Since arsenic has one too many electrons floating around its outer shell, then this new substance will have 1 extra electron per 1 000 000 atoms. We call this substance N-silicon or an N-type material (because electrons are negative) in spite of the fact that the substance really doesn't have a negative charge (because the extra electrons are equalled in opposite charge by the extra proton in each Arsenic atom's nucleus. Figure 2.42: Diagram of N-type material. Notice that the extra ``unattached'' electron orbiting the Arsenic atom. If you repeat the same recipe, replacing the arsenic with gallium, you wind up with 1 atom per 1 000 000 with 1 too few electrons. This makes P-silicon or P-type material (P because it's positive... Well, it's not really positive because the Gallium atom has only 3 protons in it, so it's balanced.) Figure 2.43: Diagram of P-type material. Notice that the ``missing'' electron orbiting the Gallium atom. Now, let's take a chunk of N-type material and glue it to a similarly sized chunk of P-type material as in Figure 2.44. (We'll also run a wire out of each chunk...) The extra electrons in the N-type material near the joint between the two materials see some new homes to move into across the street (the lack of electrons in the P-type material) and pack up and move out - this creates a barrier of sorts around the join where there are no extra electrons floating around - therefore it is an insulating barrier. This doesn't happen for the entire collection of electrons and holes in the two materials because the physical attraction just isn't great enough. Figure 2.44: A chunk of N-type material glued to a chunk of P-type material. If we connect the materials to a battery as shown in Figure 2.45, with the positive terminal connected to the N-type material and the negative terminal to the P-type material through a resistor, the electrons in the N-type material will get attracted to the holes in the positive terminal and the electrons in the negative terminal will move into the holes in the P-type material. Once this happens, there are no spare electrons floating around, and no current can pass through the system. This situation is called reverse-biasing the device (we may as well start calling it by its real name - a diode). When the circuit is connect in this way, no current flows through the diode. Figure 2.45: A reverse-biased diode. Note that no current will flow through the circuit. If we connect the battery the other way, with the negative terminal to the N-type material and the positive terminal to the P-type material, then a completely different situation occurs. The extra electrons in the battery terminal push the electrons in the N-type material across the barrier, into the P-type material where they are drawn into the positive terminal. At the same time, of course, the holes (and therefore the current) is flowing in the opposite direction. This situation is called forward biasing the diode, which allows current to pass through it. There's only one catch with this electrical one-way gate. The diode needs some amount of voltage across it to open up and stay open. If it's a silicon diode (as most are...) then, you'll see a drop of about 0.6 V or 0.7 V across it - irrespective of current or voltage applied to the circuit (the remainder of the voltage drop will be across the resistor, which also determines the current). If the diode is made of doped germanium, then you'll only need about 0.3 V to get things running. Figure 2.46: A forward-biased diode with current flowing through it. Of course, we don't draw all of those -'s and +'s on a schematic, we just indicate a diode using a little triangle with a cap on it. The arrow points in the direction of the current flow, so Figure 2.47 below is a schematic representation of the forward-biased diode in the circuit shown in Figure 2.46. Figure 2.47: A forward-biased diode with current flowing through it just like in Figure 2.46. Note that current flows in the direction of the arrow, and will not flow in the opposite direction. Now, remember that the diode ``costs'' 0.6 V to stay open, therefore if the voltage supply in Figure 2.47 is a 10 V DC source, we will see a 9.4 V DC difference across the resistor (because 0.6 V is lost across the diode). If the voltage source is 100 V DC, then there will be a 99.4 V DC difference across the resistor. If the voltage source is 0.5 V DC, then there will be a 0 V DC difference across the resistor because the source voltage isn't big enough to open up the diode. If there's no current flowing through the resistor, there's no voltage difference across it.