How Does SSD (Solid-State-Drive) Works In Laptops/Smartphones?

How Does SSD (Solid-State-Drive) Works In Laptops/Smartphones? 

A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data persistently.

It is hard to believe that all your photos, videos, messages and apps can be stored in palm of your hand and to most of us it’s a mystery how so much information can fit in such a small space but it might not seem so surprisingly when you see the complexity inside your smart-phone- the inside of this one terabyte solid-state drive commonly found in laptops or computers. However as seeing the outside of this memory storage microchip tells us little about how these smart-phones and solid-state drives can store tens of thousands of photos and files. Let’s explore deeper.

You have to zoom in the nano-scopic view where you will see the structure called V-NAND that hold all the data in your smart-phone and computer. Here’s where the real magic happens, every picture or message and bit of information gets saved as quantity of electrons inside the memory cells which are called “CHARGED TRAP FLASH”.

These insanely small and intricate structures seem very complex and yes they are. I am not saying this marvel of engineering are simple but you have to trust me. So let’s get started.

I am going to use a real life example and explore how it works when you save a picture to your smart-phone or computer. First of all any picture is made up of pixel and each pixel has a color if you zoom in any picture you will see individual pixel (in shape of a square). The color of every pixel is defined by a combination of three numbers ranging from 0 to 255 each representing red, green or blue, for example the number would be 55 53 55 or 124 53 52 etc. each number from 0 to 255 is represented by eight bits in binary or eight ones and zeros because you know computer works in binary. So three colors red blue and green and eight bits each means each pixel takes 24 bits to define its color and picture forms a grid of colored pixels. So let’s turn this into a grid of values – kind of like a spreadsheet in excel but called an array instead of spreadsheet. This array of bits is what your computer cares about and not coincidently. It’s also the information that the camera on my smart-phone recorded when I took the picture. If you want to see the pixel of any image just open it into an image editing program like paint or 3D paint etc. and zoom in and if you want the green, red or blue or rgb values just use the eye dropper. Click on the pixel and then click on the color edit option and you will see the three values for red, green and blue and the resulting color. 


Let back to chapter. If you take of picture of 4032 tall and 3024 wide you will get a picture of 3024 x 4032 = 12,192,768 which is approx equal to 12 Megapixels which relates the resolution of the 12 MP cameras on my smart-phone.  Next by doing some multiplication we calculate that an array of this size where each pixel is defined by 24 bits only requires 293 million bits or a unique set of 293 million zeros or ones that’s a ton of bits.

So let’s figure out how your smartphone or the solid state drive seamlessly stores one of them. If you zoom into in a simplified nanoscopic view as we had earlier it’s here that you can see the memory cells that are used in every single one of your smartphones or tablets as well as inside the solid state drive in your computer. This is the basic unit of long term memory storage and it is called charge trap flash memory. So how does it work?

Well in each cell we can store information by placing different levels of electrons onto a charge trap which is key component inside the memory cell. Older technology could store two different levels of electrons a lot of electrons or very few electrons which were used to store a single bit as a 1 or a 0. However engineers have been developing more finely tuned capabilities for trapping and measuring different amounts of electrons or charges onto the charge trap. Most memory cells in 2020 can hold 8 different levels but newer technology can have 1 different levels of electrons. This means that a single cell instead of holding only one bit as a lot of electrons or no electrons can now hold 3 or more bits. The key to the charge trap is that it is especially designed so that after it gets charged with electrons it can hold onto those electrons for a decade which is how information is saved or written to the solid state drive. I mean – it’s a called a charge trap for a reason. It traps electrons, or charge for years on end, and in order to  read the information the electron charge level is measured and the amount of charge on the charge trap is unchanged. However in order to erase the contents of a memory cell, all the electron charges forcibly removed from the charge trap returning it to its lowest level and leaving no excess electron charges behind. 


Let’s move on and explore how these memory cells are organized so that we can store more than just 3 bits of information. In the actual figure you can see that the memory cells are stacked vertically. This is where the vertical part in Vertical NAND or VNAND comes from. This stack of memory cells which is technically called a string is composed of 10 charge trap flash cells layered numerically. When information is written to or read from a string only one cell can be activated at any given time and to do that we use separate control gates attached to every layer in the string. It works like this: the bottom control gate first says “ Hey you, charge trap 1 what’s your electron charge level at?”. Then the bottom cell sends that information through the center of the string up to the information highway at top which is technically called a “bitline”.  Then the next control gat for 2nd layer asks for the charge level in the 2nd cell, and so on up the string, each cell sending their information up to the highway or bitline. The same kind sequence happens when a charge are being added to a charge trap which is how information is written to memory cell. The main thing is that only one layer in the string is either written to or read from at any given time.

Let’s move on in complexity, next we duplicates this strings 32 times and this gets us a page of strings and the entire page of strings is called as a row. When we duplicate the string we also duplicate the bitline 32 times however rather than duplicate the control gates, we are going to have every cell in the same page share a common control gate. This makes it such that when information is written to or read from a row an entire page composed of 32 adjacent cells, all in the same layer are activated at the same time.

Let’s step up in complexity again. Next we duplicate these rows 6 times until we get a block but we are going to do it 12 times so we can see 2 blocks so we have a column, row, string, cell and layer next we have a page and finally we have a block. We are going to connect the tops of each string in a column together so they all share the same bitline and our bitline is looking more like a highway now. In addition we have to add acontrol gate that selects between rows so that only one row is using bitline at same time. These are called bitline selectors. As discussed these bitlines are like highways and the selectors at the top act as traffic lights that mediate the flow of information so that only a single row can use the highway or is active at a time. Similarly the control gates attached to each layer acts as traffic light for the layer. With bitline selectors along the tops of each row and control gate selectors along each layer. The solid state drive can read from or write to a single page at any given time. Additionally in order to connect the bitline selectors and control gate selectors there are wires that drops down from above and run perpendicular to the bitlines.


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