Now, we are going to learn BJT, Bipolar Junction Transistor. Actually, this BJT is a whole huge chapter of the [inaudible] number, and then a lot of theory and phenomenon occurs in BJT. However, we're going to learn very shortly because though, as you know, BJT is invented by the Shockley. An initial stage of semiconductor, BJT is worked as a boss switching element, and the amplification. However, modern electronic switching element is totally occupied by the MOSFET transistor. Therefore, BJT is widely used in amplification. That's why we are going to learn [inaudible] and MOSFET because application is analog and then a little different than switching element. So BJT has a three terminal: collector, base, and emitter. This is MPN. When MPN, the arrow goes out. If this is the PMP, then arrow goes inside and then you making a silicon BJT transistor. You put it inside and you have a three terminal showing in here. The inside this BJT, there's a semiconductor of a PMP, and then there's the emitter and the one contact of n is the base and then there's a collector contact. Important fabrication instead this based regions to be extremely narrow, less than the diffusion length of the whole. Then applying voltage for the BJT is there. You applying the forward bias between the emitter and base, and diversed bias between the collector and base. Because this is the PN. You're applying positive voltage to a, then this is the forward bias. Then this is the PN, you applying negative bias to the collector this is the reverse. Now, so what happen if it works like this. Then first, forward bias. This is the pn, and then you're applying positive voltage. Then you're applying negative voltage. Measure [inaudible] horse, go over to the end-time region and then choose current is flowing by the diffusion current. What about the reverse bias of the MP? Even though there's a huge diverse electric field is occur between the pn junction, the minority carrier of hole is very limited in end-time region. Therefore, current cannot be flowing in diverse current. So this is the same thing about the pn junction by the drift current. So what happen if you have a both, PM, PM for the barriers and reverse barriers. Then what did happen is the major majority carrier holes go over to the end-time region of the base. Then there's a huge electric field in reverse bias of the MP. They, those majority carrier hole comes to the n, which becomes the minority carrier hole in base n region. They swept down to the collector because of the reversed drift current. Important thing is that those majority carrier comes in a base region. They don't recombine with the majority carrier of electron in n-type base region, and directory go to the correct region. So I_E, emitter current, is adding of the I_C and slightly of the current going on I_B. But most of the current go to the emitter to director directly, so I_E is almost equal to the I_C. To express this with a Venn diagram, here is the P pole emitter, N-type semiconductor base, P of the collector. Fermi energy is located below the E_i in emitter, above the E_i for base, below the E_i for collector. If you're applying the voltage, the dotted line is the voltage you apply. So for the bias, reducing the barrier, and then reverse bias increasing the barrier. What did happen is that majority carrier, hole, go to the base region N by the reduced forward bias. They are without the combination because the width of the base region is less than the diffusion length of minority hole. Therefore, they are not be combine with the electron and they go directly to the collector by the huge diverse electric field. Most of the electron carrier, a hole carrier in emitter go to the collector, but there's a slight bit of the current flowing to the I_B, which means that there's a slight of the electron is the supply from the I_B. What is the source of the base current? So those base current can be used in a slight recombination of the hole diffusion to the end-time region. There will be slight, almost none, but the slight recombination occurs in base region. Also, this is the forward bias, small bit of the electron will go to the hole region by injection, diffusion current. So that's the source of the base current. There's a two factor occurs in this BJT, which is the Alpha and Beta. Alpha is the current transfer ratio, and Beta is current amplification factor. Beta is very important and that is defined the amplification of the BJT. Then Alpha is I_C over I_E. Then Beta is I_C over I_B. Basic concept of the BJT amplification is that Beta is important, means that there's a slight bit changing of the I_B can control the huge current changing in I_C. BJT structure showing in here, p-type substrate [inaudible] , and there's the P-pole emitter. So emitter is connected, and then base is N, and then collector is the bottom of the P is connected. So that's called emitter base collector. So how it used in amplification? This amplification is used widely. For example, of your speaker or radio, speaker is analog, so you're changing the nob, volume, then the speaker volume is increasing. It's not digital, one and zero, right? Your LED light bulb can be changing from the brighter, if you're changing the volume of your light intensity. So to do that, you're changing the huge current of LED or speaker radio. It is very difficult to changing the current level using the huge changing of the changing resistance here. Instead of directly connecting the huge resistance changing in your actual device, LED or speaker, they're using the BJT in the middle. As I said in BJT is the current at emitter and then collector. Emitter and collector are controllable by the relatively small voltage changing in i_B because of the huge amplification factor. If you go to the electronic shop, you can buy BJT and then let's say that you tell him that, "I want BJT of the Beta ratio, amplification ratio is 100." Then you have a speaker and then you connecting this BJT here, and then socket connection showing here. There is a various connection here but this connection is also same as we learned here. We learned the emitter base is the forward bias and then base collector is the reverse bias. Here, emitter base is five voltage for the bias, and then you connecting the emitter to base is 10 volt. Let's say that this is zero volt, and then this is the 10 volt. Let's say that this is the five volt, so 0, 5, 10, to emitter a collector to base is zero, five, therefore, these are the reverse bias. You're applying the lower voltage to the piece, the reverse bias. Exactly the same function here. Now, let's see whether you can change. A slight change in i_B can choose current changing in your product. This p-n junction is forward bias. Forward bias, diode is on, very low resistance, almost no resistance here. I_B current is five voltage is apply and then changing resistance in here, let's say that we set it up at 50 kiloohm then i_B current is 0.21 milliampere. What about the i_C current? That's important for your product. Since this Beta is 100, therefore, this i_C is 10 milliampere, 100 times. Then i_E emitter current is i_B plus i_C, therefore, 10.1 milliampere. So 10.1 milliampere, 0.1 milliampere, 10 milliampere. Now, Alpha ratio, transfer ratio is i_C over i_E, 10 milliampere over 10.1 milliampere, 99 percent. What about the V_CE? The voltage applied here because some of them is a reverse bias. Therefore, there should be some potential applied in V_CE. To calculate V_CE, the V_CC,d 10 volt equal to the V_CE plus i_C i_R. Therefore, V_CE is V_CC minus i_C minus i_R. V_CC is the 10 volt, i_C is 10 milliampere, i_C is 500 ohm. Therefore, V_CE is five volt. If you're changing slight current in i_B by changing the slight change of a resistance, then choose current in i_C because of the Beta ratio. If you're changing the slight change in current, i_C can be huge, 100 time changing. That's the application of the BJT.
