Micro Electronics
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We've built transistors - now what? This is the level at which we build transistors into gates: fragments of circuitry that correspond to simple pieces of logic. This is also the level at which we worry about delivering enough power to our transistors, ensuring electrical signals propagate to them fast enough to keep the system working in time, and disposing of the resulting heat before our expensive chip melts a hole in our lovely new iPhone.
So we now need to make circuits from the FETs we described in Semiconductor Physics. Here we show how to make a NAND gate.
The FET circled in blue is an on/off switch. When a signal is present on input A the switch is "on", connecting the links at the top and bottom. Similarly, when a signal is present on input B, the net result is that the output line is connected to Vss (ground). Thus when A and B are both "On", the output is connected to Vss and is therefore "Off".
The FET circled in red has a little circle on its input, showing that the sense of the switch is reversed. Thus when a signal is present on A, the switch is "Off." Similarly for the other.
So if either A or B is Off, the connection between the output and Vss (ground) is broken, but there is a connection between Vdd (positive) and the output.
If A is off, the connection is made from the output to Vdd (positive) and broken to Vss (ground) so the output is "On".
If B is off, similarly, so the output is again "On".
If A and B are both on, the output goes off.
Thus we have a NAND gate:
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By forcing either A or B to 1 we have a NOT gate, although in that case the circuit can be simplified, and by adding extra FETs we can make an n-way NAND gate. In a similar fashion we can construct OR gates, and more.
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