
Evolution of computer storage – part one
Owen Green delves into the evolution of computer storage in this article, the first in our series of articles on this topic.
Take a journey through the evolution of data storage, from massive early hard drives to cutting-edge SSDs, and discover the science behind modern storage technology, including the ingenious floating-gate MOSFET, and learn how advancements in data storage have shaped the digital age.
In 2023, through a series of articles titled the ‘Evolution of storage devices’, I documented the story of James Howells – an unfortunate Welsh IT engineer who lost his hard drive holding the keys to 8,000 bitcoin. As well as telling James’ story, the series looked at advancements in hard disk drive (HDD) technology, beginning with a desk-sized machine developed by IBM, and ending with modern-day 3.5” drives.
Since then, James brought legal action against Newport City Council in an effort to gain access to Docksway landfill site, with the claim that he still owns the digital property on the hard drive. The Council had, up to this point, refused to allow James to carry out his search, explaining that they own the material in landfill and that such an excavation would violate environmental permits. Howells’ claim pushed for either the ability to carry out a search for the drive, or monetary compensation, and following Bitcoin’s recent price surge to all-time highs, he was presumably more determined than ever to recover the drive, with his inaccessible cryptocurrency wallet currently valued at around £600m.
In response, Newport City Council requested James’ claim be struck out, and a High Court hearing took place early in December 2024 to decide whether the matter would make it to trial.
Just recently, news broke that James’ claim has been thrown out, with the judge explaining that there appeared to be no realistic chance of success. James has gone on to say that he is “very upset”.
It remains to be seen whether this is the last we will hear of James, and although the HDD is/was the focal point of this story, there are plenty of other ways in which data may be stored. However, in the age of streaming services and cloud computing, these alternatives (think floppy disks and CD-ROMs) might begin to fade into relative obscurity.
The latest laptops and smartphones now almost solely rely on solid state drives, or SSDs – a type of no-moving-parts, non-volatile (i.e., permanent) storage.
Over the course of this series of articles, I’ll take a look at how SSD technology has evolved over time, starting here with the fundamental building block of modern SSDs – the floating gate MOSFET – before delving into the technology used in the most recent devices by the likes of Western Digital, Micron, Samsung, SK Hynix and Kioxia (formerly Toshiba Memory).
SSDs are essentially large banks of ‘flash’ memory chips – the same chips used in SD cards and USB drives. SSD technology thus stems from that of flash memory, which itself was built up from the floating gate MOSFET.
A regular MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) is an electronic device that acts as a switch or amplifier, controlled by an electric signal supplied to the gate terminal. It comprises two similarly doped regions of semiconductor material (the source and drain terminals) separated by an inversely doped central region (the substrate). In an n-type MOSFET, applying a positive voltage to the gate, which is electrically isolated from the substrate by an insulating oxide layer, creates an electric field. This electric field attracts electrons to the substrate region just beneath the gate, forming an inversion layer, or n-channel. Once the n-channel is formed, applying a voltage between the source and drain terminals allows electrons to flow through the channel, forming an electric current.
A floating-gate MOSFET (FGMOS) is similar to a regular MOSFET but includes an additional conductive layer between the gate terminal and the substrate, surrounded on all sides by insulating material. When a sufficiently high voltage is applied to the gate (i.e., higher than typical for normal MOSFET operation), strong electric fields are generated, enabling electrons to transfer to the floating gate through quantum-mechanical processes such as Fowler-Nordheim tunneling. The below image from patent GB1517925 shows an example of an FGMOS.
GB1517925
Once the high voltage is removed, the electrons become trapped in the floating gate due to the surrounding insulating material, allowing the floating gate to retain a net charge (even when power is removed from each terminal) and influence MOSFET operation for extended periods.
Whether net charge is retained or not can be thought of as a stored binary value. Large numbers of these devices can be connected together in order to store vast quantities of data.
One patent relating to improvements to the FGMOS, specifically in the way in which the floating gate is discharged, is US5493141A, which relates to a “Method and circuit for tunnel-effect programming of floating gate MOSFET transistors”. Generally, the patent provides an accurate method of discharging, i.e., erasing, the value of the floating gate using a bootstrap capacitor.
US5493141A
In a first stage, capacitor 3 is charged to voltage V1 by closing switch 5, and setting Vp to ground. At the same time, switches 7 and 8 are open, such that no current flows between the source terminal S and the drain terminal D.
In a second stage, switch 8 is closed to apply a pre-determined voltage VCGE to the gate terminal CG. This voltage VCGE effectively sets the desired threshold voltage of the MOSFET (a voltage at which the MOSFET begins to conduct). Switch 5 is opened and voltage Vp is linearly increased, causing voltage VD (i.e., the voltage on the top plate of the capacitor) to also increase (beyond the value of voltage V1). As voltage VD continues to increase, the voltage drop across the insulating layer between the drain terminal D and the floating gate FG soon becomes sufficient to cause electron tunneling, and electrons begin to move out of the floating gate FG towards the large positive potential at the drain terminal D.
As more electrons move out of the floating gate FG, the threshold voltage of the MOSFET begins to decrease. Eventually, enough electrons move out of the floating gate FG such that the voltage VCGE at the gate terminal becomes sufficient to turn the MOSFET on. Since the MOSFET is now in a conducting state, a current flows through the MOSFET between the source terminal S and the drain terminal D, quickly depleting the charge stored on the capacitor 3. This quickly reduces the voltage VD at the drain terminal D, automatically halting the movement of electrons from the floating gate FG, and ending the discharge procedure.
The method provides improved control of the final charge left in the floating gate and, hence, the final threshold voltage. This method of programming is, advantageously, largely independent of the characteristics of the wear of the tunnel oxide and the initial charge in the floating gate.
Unfortunately for James, the mere fact that data is stored differently on an SSD than in an HDD has no real bearing on whether his drive is able to be found, and with his claim now thrown out, the chance of recovery now appears very small. In any case, given that James’ keys were stored on an HDD, not an SSD, and are currently located somewhere in a Welsh landfill site, you could say he was always unlikely to find them in a ‘flash’.
In this series of articles, we use patents to show how far things have come in the evolution of data storage.
Owen Green delves into the evolution of computer storage in this article, the first in our series of articles on this topic.
We explore the ways in which digital data makes its way onto whirring metal platters, and developments made following the technology’s conception, in the second article in this series.
In our final article in this series, we take a look at some more evolutionary steps in the storage device timeline.
This is for general information only and does not constitute legal advice. Should you require advice on this or any other topic then please contact hlk@hlk-ip.com or your usual HLK adviser.
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