Now, let's learn about the flash memory. Also, flash memory as the nonvolatile memory means that the data you stored in flash memory never go away, even if you turn up the switch. That's why you can use your camera or smartphone, although your smart phone off, data remain. Later on, you can use those data. Before we learn the flash memory, actually, flash memories started with a very interesting phenomenon called cold-hot electron effect. These hot carrier effects is some very negative phenomenon when we deducing the size of the transistor. Let's say that this transistor, its the channel length is long channel, then those hot carrier effect is not occur. But if you're reducing the channel length much farther, less than the one the micron, what happens is that voltage is applied and if you're reducing the channel length, then electric field between the source and drain is proportional to the distance, therefore electric field is reduced. Our electric field is increasing, if you maintain constant voltage and reducing the distance. To prevent this, semiconductor engineer is reducing the drain voltage to deducing the electric field in source drain. However, due to some system related circuit constrain, you cannot reduce the drain voltage at certain level. Let's say that threshold voltage is 0.5 and the on current operate is 0.8, you cannot reduce the drain voltage less than the 0.4 or 0.5. Then the drain voltage is maintained in certain voltage and the distance between the source drain is kept downward, then electric field is increasing. In very high electric field between the source drain region, especially in drain region, electron is in sources hugely accelerated and bombard with the silicon lattice at the drain region, then those bombardment creating the electron whole pair ehp from the silicon lattice. Then you apply positive voltage to the gate. Those electron by the positive gate voltage, they go into the silicon oxide layer, and then positive charge expelled and then they go to the p-type silicon. These positive charge in p-type silicon will degrade the device lifetime, also, electron in silicon oxide changing the threshold voltage. This is important. Since if you have a negative electron in the silicon oxide layer because of the hot-carrier effect, what happens is that even if you're applying positive voltage to the gate, they are blocked by the negative charge. To turn the transistor on, you're applying more positive voltage to compensate those negative charge, so threshold voltage will be increasing from the original threshold voltage. Actually, this is the bad effect and many semiconductor engineer thinking about how to remove this hot-carrier effect, but some smart engineer think differently. Let's use it because they are changing the threshold voltage, even if power is off, we can store the data, even if you turn up the transistor, turn up the power. This is how they made it. This is the normal MOSFET transistor, so threshold voltage is always constant. Now, you're building the control gate, this is the control gate, and then you're apply positive voltage. There is the blocking oxide and there is the another gate between below the control gate called a floating gate. Control gate, and source, and drain. If you are applying very high electric field, then the electron will be accelerated and bombarded with the silicon lattice, generated electron will jump up to the proton gate because of the control gate positive voltage. Then whether you store the charge in the floating gate or not, threshold voltage is changing from the low the threshold voltage without the charge and the high threshold the voltage with charge. If you're using the real voltage here, some data store is the transistor off there for low current, the huge current flowing where the charge is now stored, and here, maybe this is the digital 1 and digital 0. These will remaining, even if you turn up to power. That's why it's called non-volatile memory, opposite to the volatile. There are two type of the flash memory, a NOR type flash memory, a NAND type flash memory. Originally, the idea comes from the Toshiba, both of the NOR and NAND, but the first commercialized company is the Intel, and then they pursued NOR type flash memory. Now, major semiconductor flash memory industry are dominated by the NAND flash memory. NOR type is similar to the other RAM, so you can actually access each memory cell with the word line and bin line. Then the type flash memory is very interesting such as showing here. These are the [inaudible] one and MOSFET access transistor, once you access this transistor, all of the 16-bit of the NAND flash memory is connected serially. Then therefore, you cannot only access this one memory cell because these are all connected together. So how can we control this NAND flash memory? You can control the NAND flash memory as a block here. So if you want to change the data in this, then you first transistor on, access this block, and then you first read, or writing, or erasing, it's all same, well, access to the word line 1 of the transistor, and you're changing the data, and then changing here, changing here, or reading here, and serially go through every 16-bit. So you can block changing or block storing, block reading as a whole. Why the NAND flash memory is dominating the non-volatile memory nowadays. If you look at the cross-sectional graph, NOR type, each memory cell has a drain and source, but then the type, each memory cell sharing this source drain and drain becomes next to transistors, the source. Same thing. Sharing the source and drain in two transistor. Therefore, you can deduce the size. That's why you can build a huge gigabyte memory in your chip for your smartphone. Let's learn the two operation of the program operation and erase the operation of the flash memory. Program means that, here's the control gate and here's the source and drain, now you want to store the charge to the floating gate to changing the threshold voltage. You're applying zero voltage and high drain voltage, and then high control gate, electron-accelerated and bombarded silicon lattice, and ehp is generated, electron-jumping to control gate through the tunneling oxide. This phenomenon is called the hot-carrier effect or hot-carrier injection from tunnel to the floating gate. So program is done by the channel hot electron injection. What about the erase operation? Erase operation is removing the carrier in floating gate to the silicon substrate. These can be done using power law Fowler-Norheim tunneling between the floating gate and bulk. So here, floating potential, floating potential to the source and drain, you are applying huge negative voltage between the control gate and substrate, so minus 8 and 10 means minus 18 volt. Those electron is tunneled through the p-type silicon to erase the data.
