[NanoMachine]

The stability decreases with decreasing bond dissociation energy. Lower energy bonds split more easily.

NH₃ ~ 339 kJ/mol
PH₃ ~ 297 kJ/mol
AsH₃ ??? kJ/mol
SbH₃ ~ 240 kJ/mol

NH₃ has a much higher boiling point than the others because of hydrogen bonding. That's also why H₂O has a much higher boiling point than you'd expect.