4 ms·
I so seldom think at this level I had a couple questions about the following passage that I'm hoping someone could answer: >"The transistor can be viewed as a
by bogomipz 8y ago
I so seldom think at this level I had a couple questions about the following passage that I'm hoping someone could answer:
>"The transistor can be viewed as a switch, allowing current to flow between two diffusion regions called the source and drain. The transistor is controlled by the gate, made of a special type of silicon called polysilicon. A high signal voltage on the gate lets current flow between the source and drain, while a low signal voltage blocks current flow."
If current flows from the source to the drain, is the source terminal hardwired to the power rail and always receiving some nominal voltage? Also does the voltage applied to the gate in order to turn the switch on come from the drain of a neighboring transistor? Basically wired in a series?
I always see MOSFET transistors depicted in isolation and then the conventional logic gate shapes used when depicting specific circuits like adder, mux etc. I find it tough to visualize how the gate, source and drain terminals in transistors as they are depicted in this picture physically interconnect to form the logic gate. Is there a separate metal layer for each of source, gate, drain and ground?
- acqq 8y agoIt's all good known: https://www.allaboutcircuits.com/textbook/digital/chpt-3/cmos-gate-circuitry/ https://www.allaboutcircuits.com/textbook/digital/chpt-3/cmo...
- bogomipz 8y agoWhat a great resource! Thanks.
- kens 8y agoOn a chip, there's really no difference between the source and drain. The source/drain can be connected to ground, another transistor or power as needed. Look at the diagram of an inverter in the article, and it should make it clearer how gates are constructed. A NOR gate is similar to the inverter, but with two input transistors in parallel, so either one can pull the output low. A NAND gate has two input transistors in series, so they must both be on to pull the output low. There's a single metal layer in the 8087 (but many metal layers in modern chips). The transistor can be wired however works best for the layout, with polysilicon or metal. If a transistor's source is connected to a neighboring drain, the silicon regions can just be merged and no wiring is needed.
- bogomipz 8y agoThanks for pointing that diagram out, I should have spent more time staring at it. Indeed it makes the gate construction clearer. Cheers.