You guessed it, diodes can also be used to build logic gates, but not all of them, only the AND and OR gates can be built using diodes only. However, by adding a transistor as an active element ...

The output is initially 4.5V for logic 1 and 1.5V for logic 0. Adding two 1N914 diodes and an AND gate ahead of the inverter create a two-input NAND gate. With the two diodes reversed and a 910 ...