I have an N-MOSFET array IC and I don't need to use all of them, but I do need to use a diode. Is it OK to use the body diode of one of the MOSFETs as a regular diode to decrease component count and footprint? Can I just tie source to gate and use the MOSFET in place of a diode in the circuit?
If the MOSFETs in the array are isolated from each other and the body diode rating is not exceeded, the voltage rating of the MOSFET is suitable, and the specs (current rating and reverse recovery time, especially) of the body diode are acceptable it should be fine.
It might not be a good choice in something like a switching regulator where trr is important.
The below figure shows a typical diode-connected MOSFET (for NMOS). It performs a rectifying function similar to a diode. It is not necessary to connect bulk and source as in the figure, it is an artifact of the lack of a true 4-terminal device in the schematics editor for electronics.stackexchange. The source terminal which is tied with bulk in the figure could be at a voltage higher than bulk.
Bulk is typically shared among many devices and the lowest voltage is used for bulk. In bulk CMOS and NMOS technologies the NMOS transistor body is made of P-type material (excess positive charges), while source and drain are made of N-type material (excess negative charges). There are therefore also some "parasitic" PN-junctions you could exploit: Bulk(P)-Source(N) and Bulk(P)-Drain(N) are real PN junctions - however this is normally not done! For ICs the body material is usually shared by many transistors, and fixed (by specification) to 0 Volt. It then only makes sense if the voltage you need on bulk (for the P-side of the diode) is 0 Volt. Source implants usually have very high number of dopants. Therefore the diodes are somewhat weak - you need a higher forward voltage to send the same amount current through them - compared to a device with more moderate doping.