> btw the sn7400 is already a fairly advanced ic; something like an uln2003 is closer to 'the simplest real world integrated circuit'
If the goal is to explain how logic is implemented in general, skipping bipolar transistors and TTL and jumping directly to MOS may be easier. The behavior of a FET is fairly easy to explain, especially if you don't care about the ohmic region (which you usually don't in logic ICs), and it's straightforward to step from there to a practical implementation of a simple gate like an unbuffered NAND -- the latter of which can be trivially assembled on a breadboard with as little as two FETs and a resistor for a NMOS implementation.
> especially if you don't care about the ohmic region (which you usually don't in logic ICs),
you have to care about the ohmic region to be confident you've safely steered clear of it; at least one fet moves through the ohmic region every time a mos gate's output transitions
rtl is the bipolar equivalent of nmos (see the analog simulation at http://tinyurl.com/ylnljbgz) but you do need base resistors if you're going to try to drive its inputs with voltage sources instead of the outputs of other rtl gates. but you can omit them when the inputs are connected to rtl outputs http://tinyurl.com/ywja8z28
the flip side of that is that, though you need a base resistor to provide a constant logic high to an rtl gate, you can provide a low just by leaving the input open, you don't even need a wire like you do for nmos
bipolar logic is also a lot harder for students to blow up if your lab power supply has a current limit on its output
If the goal is to explain how logic is implemented in general, skipping bipolar transistors and TTL and jumping directly to MOS may be easier. The behavior of a FET is fairly easy to explain, especially if you don't care about the ohmic region (which you usually don't in logic ICs), and it's straightforward to step from there to a practical implementation of a simple gate like an unbuffered NAND -- the latter of which can be trivially assembled on a breadboard with as little as two FETs and a resistor for a NMOS implementation.