Alternator issues with car?

Yikes, ok. I need help! I was sitting waiting for my daughter to get out of Kindergarten, and I had my radio and heater on but the car WAS running, then it died! Someone jump-started me and followed me to an auto parts place and it died right as I pulled in. My first thought: alternator. So they checked my battery, it was bad. I bought a new one, then they checked my alternator. The readings were good until it said Bad Diode. What is that? The auto parts guy said I need a new alternator. But when I went searching online, I saw there was an actual Diode aside from the althernator. So do I have to replace the whole alternator or just the Diode? This is so sucky! Please help! Can the althernator be done by someone other than a mechanic, or is it difficult to replace? 2000 Mazda MPV Thanks so much for your thoughtful answers!