digital logic - IC takes no input (or high) as high and low input as low on breadboard

I was working with IC 7400 (or LM74LS00 / NAND) (and IC 7486 - XOR) to design and verify combination circuits and logic gates.

Consider the lower end of breadboard as LOW (connected to GROUND) and upper end as HIGH (+5V).

When I connect the input to GROUND (like putting the wire on lower end on breadboard), it is considered as LOW and works as LOW; and when I connect the input to HIGH, it works as HIGH (normal behavior).

But when I remove the wire from both HIGH and GROUND, it still behaves as HIGH.

Why it happens so?

Answer

The input stage of a TTL device acts as a reverse-biased diode. As such, any high impedance at the input stage, whether due to expansion of the depletion zone of the B-E junction or due to an entirely disconnected pin, will appear as a high input.