In Lecture 6, we are learning very interesting moment of a p-n junction, on the forward bias and reverse bias. The p-n junction, if you are applying the voltage or current, then p-n junction is operating. For example of the LED, you are applying the voltage, then LED light comes out. Those are the p-n junction on the forward bias. Let's understand those p-n junction under non-equilibrium conditions. First, p-n junction under forward bias and reverse bias. Forward bias, you're applying p-n junction and positive voltage to the p and negative voltage to the n, so that current flowing from the p to n. Reverse bias, you are applying the negative voltage to the p and positive voltage to the n, there's no current flowing either directions. If you are applying the voltage, most of the voltage drop occurs in depletion region. This is easily understood by the general physics. If you apply 10 volt to the serially connected resistance, only 10 volts, then one volt applied here, nine volts applied proportional to the resistance. If you look at the p-n junction, neutral region is very highly conductive, low resistance. However, those depleted region is highly resistive area, so most voltage drop occurs in this p-n junction region. In forward bias, the barrier built-in potential is reduced. In reverse bias, this built-in potential is increased. Also this is the non-equilibrium condition where you're applying voltage, Fermi energy is no longer constant anymore. But why the forward bias barrier is lower and barrier of reverse bias is increased. This is also easily understood by the general physics. In general physics, in electrostatic potential, if you're applying the negative voltage, then this size of a potential becomes lower. In forward bias, you are applying negative voltage to the n-type semiconductor, then negative voltage should go down and much increasing the barrier, but band diagram is not based on the positive voltage of the electrostatic potential, but band diagram is based on the negative charge electron, so they are opposite. If you're applying the negative potential to the n-type region, this will raise up the band diagram of n-region, so that barrier becomes lower with a V_0 minus for the voltage. If you're applying the positive voltage to the n, which case of reverse bias, then positive voltage should increase the N-time region go up in electrostatic potential, but band diagram is based on the electron, then they will lower the potential of the N-time region means increasing the barrier to V_0 plus reverse bias. In forward bias, more holes can be moved to the N-region, current flowing from the p to n reverse bias very low carriers moving in either case. Let's understand this concept. This is the band diagram, P-type semiconductor and N-type semiconductor. In N-type semiconductor, there are a lot of electrons existing just above the E_c region. Carriers are distributed as like this, following the Boltzmann distribution. If you remember, the carrier concentration, there is the density of states multiplied by the Fermi-Dirac function, then integration of these two multiplies from the E_c to infinite. There's the electron carrier concentration of a majority electron, and same thing for the minority hole. Density of states minus 1 minus a P_e integration becomes minority carrier hole. When you do integration, instead of the Fermi-Dirac function, you use the Boltzmann approximation because the E minus E_f becomes larger than 3K_t, then Fermi-Dirac and Boltzmann is almost equal. So carriers existing as most of the carriers above the E_c unless electrons are higher than E_c region. In a Boltzmann distribution, most of the electron in this region and higher energy is less electron following the Boltzmann distribution. In this graph, the electron has the only E_c energy cannot go over the built-in potential and go back. Only those three electron that has a higher energy than built-in potential, where following the Boltzmann distribution, this region, they can go to the P-region from N-region by diffusion. Same thing for the hole in P-time region. A lot of the holes are located just near the E_v. Less electrons are existing much far from the E_v region, but less than the built-in potential. Those holes comes back. But the holes that has a higher energy than built-in potential can go over to the N-time region. Same number of the electron that goes to the P-region will go back to original N-region by drift the current. If the number of the electron goes to the P-region by diffusion and the number of the electron go back to the N-region by drift, they are equal under equilibrium, current is zero. Same thing for the hole. But for the bias, barriers are reduced. This is the equilibrium and forward bias in blue and carrier barrier is reduced by the Q built-in potential minus applied voltage. Then much high concentration of electron has a higher energy than built-in potential. These high number of electrons will diffuse to the P-region. However, same number of three electron go back to the original position by the drift current, therefore, current flowing from the P to N. Electron in P-time region is minority carrier therefore, there is not many electrons here so the drift current is limited. But the diffusion current that has a higher energy than the barriers of a built-in potential minus forward bias is a high number. Very high number that follow the Boltzmann distribution exponentially increasing. Those will go to the other side, current flowing from P to N, same thing for the hole. A lot of the holes that has higher energy than barriers deduced by the forward bias can go to the other side by diffusion and imitate the minority carrier, go back to the original by drift, therefore, current flowing from the P to N. So new current is flowing on forward bias. Forward bias more detail. So carrier diffusion current is depending on the both energy which delayed the barrier and concentration of the carriers that has higher energy than barriers. Only electron that has a higher energy can go to the other side by jumping the barrier. When you deduce the barrier, the carrier that has a higher energy will be increased by the exponentially because they're following the Boltzmann distribution, which is the proportional to the exponential. So applying voltage exponentially increasing the carrier that go to the other side, increasing current exponentially. Drift current is insensitive to the barrier height. If you think about the current equation, drift current is proportional to the carrier concentration and velocity, which is the mobility times electric field. If you're decreasing electric field, they might decreasing or increasing potential barrier, they might influence the electric field, however, the carrier concentration, carrier number is very limited because they are minority carrier. So current of the drift current is very limited, therefore, drift current is insensitive to the height of the potential. In other words, drifted current depends not by the how fast carrier move, which is proportional to the electric field, but by how open. How open means minority carrier very limited number. So current is insensitive to the height of the potential or independent of the applied voltage. Let's look at the reverse bias. This is equilibrium, and this is reverse bias. When you're applying reverse bias, barriers originally here is increased by the built-in potential plus the reverse bias. Then only small number of the electron can have higher energy than this increase the barrier, so very limited diffusion current of electron is generated. Also limited the number of minority electrons in P time region flowing. So there is almost no current flowing at reverse bias. Also, this current that has a higher energy than increase the barrier, is decreasing by the exponentially of the Boltzmann distribution.
