How do the access transistors in an SRAM cell work? - Page 1
Forum section: electronics / beginners

author: NotAnt
author_rank: Newbie
timestamp: April 01, 2015, 12:29:32 am
content: For example, in the circuit above, where is the source of M5 and M6 connected to? because no matter where it is connected I don't understand how they could operate. If the source is connected to the inverters:how can they be turned on by simply asserting WL? wouldn't the transistors turn on or off based on the gate-source voltage? I don't understand how, for example, it would be possible to turn on M6 when there is a 1 stored in Q. In that case asserting WL would only bring the gate-source voltage to 0, how can that be enough to turn it on?The usual explanation of the operation of the SRAM cell relies on both access transistors being turned on. You can find an example here (http://goo.gl/vPoeRG). So I don't understand how the access transistors could be both turned on if the source was on the side of the bit lines (because you are bringing BOTH bit lines to HIGH during a read operation).Even if the source was on the side of the inverters I don't understand how they work, because then only one access transistor would be turned on during a write operation, the same that would be turned on during a read operation (because now the only thing that matters to turn on the access transistors is Q and -Q: remember that source is on the side of the inverters). If both reading and writing turned on the same transistors provided that the contents of the cell are the same, then what's the difference between reading and writing? I don't think it's just the sense amplifier. I would like for someone to clear these doubts.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2015, 03:22:54 am
content: The lower potential always behaves as the source terminal.Generally, MOSFETs on chip are symmetrical and can be made floating. A full MOSFET is a four terminal device (G, D, S and substrate), but in IC circuits, the substrate is rarely shown, as it is assumed connected to GND (N channel) or VDD (P channel). Which is probably the case here. (One case where floating substrate is needed is an analog switch; this application is similar, but as it's a more limited digital situation, it's not a problem.)Whichever terminal acts as source, this fact remains constant: if you keep raising the  gate voltage, eventually the channel will conduct. If one side is high and the other is low, a high gate will cause it to conduct (pulling down the high, or pulling up the low, as the case may be) and a low gate will leave it isolated. If both sides are high, a high gate won't be high enough (it would have to be doubly high, a state which binary logic doesn't include), but it won't matter because... both sides are high already!Remember that the body diode in a conventional, discrete, three terminal MOSFET is explicitly shown in the conventional symbol (the one with three line segments and an arrow to/from the center). The PN diode junction is explicitly from substrate to channel; if both source and drain voltages are free to move (with respect to substrate), then it doesn't matter which is which, and either one will exhibit diode behavior with respect to substrate. Which, in a system where no voltage exists below substrate voltage (or above VDD), you can safely ignore the diode as well.The substrate is omitted from theshorthandsymbols shown above; the oft seen "base with an emitter going at a right angle" symbol serves the same purpose, the "emitter" arrow suggesting polarity instead of the dotted/undotted gate. A poor choice, as historically, arrows have always indicated PN junctions (and the conventional current flow through them), where no such junction exists in the channel of the MOSFET.So, if you wanted to implement this with discrete parts, you'd be better off with JFETs than MOSFETs, because MOSFETs with a separate substrate terminal are very rare indeed.Tim

author: NotAnt
author_rank: Newbie
timestamp: April 02, 2015, 04:35:49 am
content: Thanks for your answer. Apparently my knowledge of MOSFETs is lacking, do you know where I could read about them? thanks.