Now we are learning the most fun fact of the p-n junction, which is the application of the p-n junction. P-n junction is used in 50 percent of most path. In addition, p-n junction itself is very very useful device itself in our daily life. For example, solar cell and LED, and camera photodetector, etc. So let's learn how p-n junction is used in our daily life. Before we go, just one more theory about the p-n junction and junction capacitance of the p-n junction. Let's go directly to the conclusion. Junction capacitance of the p-n junction is the C_j is Epsilon times A per W. A is cross section area and W is depletion width. Junction capacitance is formed in p-n junction because there is space charge. Because there's a space charge there is capacitance. Normally capacitance C_j is dQ over dV, which is the charge changing over the voltage changing. Then we have to know the Q. Q equal qAx_nN_d. To know the x_n, we first calculate the W, either forward bias or reverse bias. Then we can calculate the W. If we know the W then x_n equal up on the side of N_a over N_a plus N_d W. Then insert this x_n to here, then q becomes like this and C_j, dQ over dV becomes like this, this equation. So if I ask you the junction capacitance under forward bias, then all you need is, you already know the A and W at the forward bias using this equation, then you know junction capacitance. Reverse bias plus the reverse bias. So first application we're going to learn is rectifier. In your home, your power supply from the outside is the AC voltage but most often, your light and consumer electronics operating in DC. Then you convert AC signal to the DC using rectifier and resistance and capacitance. Rectifier is by p-n junction. If you go through the AC signal to the p-n junction, you only flowing forward bias and no current flowing at the reverse bias, so you remove the reverse voltage. So you only remain with the forward voltage and then using that register and capacitance, this becomes a normalize and becomes like the DC voltage. Next, photodetector. Photodetector is used in your camera. So light comes in. You have a filter of red. Color should be auditor changeable, I'm going use the same one, green color, blue color. Normally this is called the RGB. Then because of the color filter, only red light comes in, green, blue comes in, and you put the photodetector of a p-n junction here. Then detecting the color intensity of RGB, then you can depict the color of the outside, because the RGB can express all color. In your camera has this camera called the CMOS Image Sensor, CIS. So what is the operating mechanism of a photodetector? So light comes in in the depletion region then it's bombarded with the silicon lattice and they generating electron-hole pair. Then electron go to the N side, hole go to the p side by the electric field of the space charge. Then if reverse current is increasing, you can detect how much light comes in. Important thing about the p-n junction photodetector, this is operate under reverse bias. Also, a wider depletion region is preferable because the wider we can detect more light. So most photodiode have PIN instead of the p-n junction, I is intrinsic. So between the p-n junction there, you put an intrinsic semiconductor to elongate that depletion region. Next, LED, everyone has LED pointer. So LED important is operating in a forward bias, this is obvious. So you press the button and then you are applying the voltage under forward bias and light comes out. If you're applying forward bias on the p-n junction, majority electron will go to the other side of the hole by diffusion. This minority electron diffuse from the n-type region recombine with the hole inside the p near the junction area. Then the combination of the PN, electron-hole pair, emitting the light. Normally, this light emitting device is the direct semiconductor and indirect semiconductor like silicon cannot emit the light. After my class, sometimes my students says that they couldn't remember the detail and then some student told me that the combination occurs in depletion region, that's not true. They are recombined near the junction region. Solar cell. Solar cell, if sunshine is received by the PN junction, then you can obtain the energy, that's the solar cell. So obviously importantly, solar cell operate under no applied voltage, if you're applying voltage and giving the power and getting the energy, this is still useless without a prime voltage and you getting the energy. Under no applied voltage, if lights comes in, then electron-hole pair is generating and those charge is separating the PN region with open circuit voltage V_oc. This is the energy, is generating normally called photovoltaic effect which is got the Nobel Prize.
