How a Transistor is Used in Memory
Brief Discussion About Transistor
"Transistor" made Electronic devices relatively compact, cheap, fast and affordable. It is building block for every semiconductor device.
See Here for How Transistor is Made.
Word Transistor is actually combination of two words i.e. Transfer of Resistance. Transistor was Invented in 1947 and firstly was used in hearing aid devices, because one of its function is amplification of signal when operated in that range. Around that time when transistor was invented, it was quite easy to disassemble the device and one could physically see the connections. But today transistors are operating at nano-metres and there can be accommodated billions of transistors on tiny chips.
Transistors is a three terminal semiconductor device which is mainly used for switching and can also be used for amplification. As shown in the IMAGE above, it can be noted that by controlling the "Trigger" on a smaller circuit, one can control a bigger circuit connected to this.
The whole Digital Electronics can be implemented by a single component "TRANSISTOR".
How Transistor is Used Memory
There are many types of memories being used today and earlier. There are two broad categories though. RAM i.e. Volatile memories and ROM i.e. Non-Volatile Memories.
RAM stores the computer's program during execution and today I will discuss How 1T-DRAM Can be Implemented by Using Transistor and Capacitor.
So here two important parameters i.e. 'Bit Line' and 'Word Line' can be explained as:
Bit Line determines the value of 'bit' of memory by comparing the value of element 5 (Capacitor) above or below a specific limit and transmits as bit being 'High' or 'Low' to an Amplifier. Bit Line is controlled by a Column Address multiplexer.
Word Line is controlled by Row Address De-Multiplexer and it further 'TRIGGERS' a Transistor. So when transistor gate/Base is triggered what ever is in the 'Component 4' (Capacitor) is transferred Bit Line.
So How is single element of DRAM working? When the Word Line is 1, it "triggers" transistor and whatever (0 or 1) is stored in Capacitor #4 is read by "Bit Line" and considered 0 or 1 accordingly. Component #4 (Capacitor) is like a leaking bucket of water, in DRAM structure we have to Refresh the DRAM otherwise capacitor#4 leaks out and result in all zeroes. So it is Volatile memory. Needs Battery Backup to get it running in good state.
So capacitor is holding up value and transistor is switching to retrieve that value from capacitor and send on Bit Line to CPU for processing that bit.
Here in the figure below is a READ circuitry for 16 Bit 1T-DRAM, So transistors are being controlled by Word Line of 'a0 and a1' bits. Depending on the value of 'a0 and a1'(0,1,2,3) any of the complete row will be selected. for example if a0=1 and a1=1 also, then Word Line selected will be #3 and all transistors in row#3 will be "triggered". All of them sending their values on Bit Lines (0,1,2,3).
Same a0 and a1 i.e a0=1 and a1=1 (#3) will also be fed to selection lines of Multiplexer as well and that multiplexer will gather data only from Bit line #3 because selection on multiplexer of column is so.
So as a result only Bit Value of element 3X3 will be obtained and sent to requesting device.
Here complete working except time synchronization of data retrieval and refresh rate for compensation of leaking Capacitor has been discussed.
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Latch is a solid state device which can be made out of transistors to Latch to a value,
Tristate Buffer is also electronics device which generates either 1, 0 or High impedance at output.
Multiplexer, depending upon the 'selection state' of Mux outputs value of that selection state. if value at "Selecting Control" is #3 then it will send #3's input value to output.
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