Alternator "tridiode?"

[Yarisduder]

My electric system dummy light came on the other day, so I figured it was the battery since I was told it is weak. So replaced the battery, and they tell me that the "tridiode" on the alternator which "sends the signal to the dash light" is faulty. The symptom it causes is that the electric light comes on when the car starts and goes off shortly afterward, but the system still charges fine. They say to fix it I'd need a new alternator, but since it's charging it's a waste of money to do that.

So my questions:
1 - Is this "tridiode" even a real thing?
2 - Can just the "tridiode" be replaced?
3 - Or is the real problem a corroded contact?

Thanks.