The diode-connected transistor is used for synchronous rectifier. I have just drawn bulk diode as below. The fist one is mosfet with its bulk-source, bulk-drain diodes. Second and third pictures are when B-S or B-D are connected together. When B-D is tied together, we have body effect and this increases threshold voltage. However, we get a diode from D to S. I think this diode is good because it helps make the diode-connected transistor more like a real diode.
With the case B-S connected, we don't suffer from body effect but we don't have the help of diode from D to S.
So, my question is which one is better in this case, connect bulk with source or drain?