This lecture 12, we're going to learn further application of the MOSFET transistor. You probably heard about the DRAM, dynamic random access memory, and then also flash memory or USB memory. Then also, you heard about the modern display called the AMOLED, active matrix organic light-emitting devices. Those application also based on the MOSFET transistor. First, RAM, random access memory, such as dynamic random access memory or DRAM, and resistive random access memory or RRAM. These are DRAM chip architecture. A lot of a memory cell located in here and there's a row decoder and column decoder, they are controlling unit of the memory. This is the DRAM cell and each memory cell has the one MOSFET transistor called the transfer gate, or access transistor, or switching MOSFET. Those MOSFET gate is connected to the word line, one end of the MOSFET connected to the bit line. The other end of the MOSFET is connected to the DRAM capacitors. The potential of the one end of the capacitor is grounded, the other called a storage node. If a storage node is high potential, then you store the charging capacitor, maybe that's the digital one. If the storage node is the low potential zero, then the charge is not stored in the capacitor, that is the digital zero. Also, if you're replacing this capacitor with some resistive memory, then it is called a resistive random access memory, RRAM, or if you're replacing this capacitor with a magnetoresistive material, then that is called the MRAM. What is the role of the transistor here? This transistor actually work as the access gate. If you want to access this memory cell, whether this is the one or zero, or writing, reading, erasing, then you have to turn on this transistor. Then if you transistor on, then you can access this memory cell. If you transistor off, these remain as it is. RAM, random access memory, is called as a RAM because you can access exact address of each memory cell by using the MOSFET transistor. If you want to access this memory cell, you access on of the transistor of the n MOSFET here, then you can access. You can access any memory cell, then that is called the RAM. Not all memory is the random access memory, for example, of the fresh memory never called as fresh in RAM memory, they are just a fresh memory, they cannot access each memory cell which we're going to learn later. This capacitor actuary can store the charge or store the data very limited time. So each memory cell should refresh the charge loss every microsec. This is actually my research as during my KAIST affiliation. This is the resistive random access memory. Resistive random access memory, you're replacing the capacitor with a resistive memory store memory, and these resistive memory changing the resistance depending on the voltage applied between the top and bottom. If you apply the one end up the voltage of the resistive memory, depending on the path, it becomes the low resistances memory and it becomes the high resistance memory. Maybe if it's high resistance, that's a digital one, or low resistance, that's a digital zero. This RAM, resistive random access memory, also has to connect it with the transistor. In here, we use the transistor of the silicon MOSFET, and then to access each memory cell to form random access operation. This is the image is the one MOSFET transistor. Then there it is, my memory, changing the resistance depending on the voltage path, they can be high resistance and low resistance. You might have a question, why do we need transistor to access here? Maybe if you're accessing here, you're applying two voltages in here, then you can access. What is the purpose of the transistor? What happens if we don't have a transistor? These actually showing in this graph. These method approach can be applied any application like DRAM, RRAM, MRAM, or later on, active matrix display and passive matrix display. Why do we need transistor? Let's assume that there is a full memory cell, here is the low resistance, and here is the high resistance. These graph show that you don't have a transistor. Then you want to read high resistance, maybe that's a digital zero, so that you're approaching this memory cell with a select word line and the bit line here, then you can read high resistance here. However, as you can learn in the general physics, current is always following the low resistance instead of the high resistance. You have low resistance and high resistance, one ohm and 100 ohm. Most current will be flowing in low resistance, not high resistance. Current is flowing, instead of the high resistance, they flow in the low resistance region. You want to read the high resistance but the outcome is the low resistance, then your data is wrong. This path is called a sneak path. We need to prevent this sneak path. To do that, we install each memory cell with a transistor. So here, we have a transistor, transistor, transistor, transistor. Most path transistor is like a faucet of the water flow. If we're selecting here and here, and other transistor is off, then current cannot flow in low resistance, they are shut off. Only this word line is transistor on, therefore, current flowing in high resistance, we did correct information. That's why we need switching allotment for the large memory cell or large data array.
