AT-1 bonds (Additional tier 1 bonds) are :

🔸️ Perpetual bonds [ No fixed maturity tenure ] have fixed Interest rate (But interest rate is usually set high, because AT-1 bonds are very risky)

🔸️ Unsecured bonds : Not backed by any mortgage/physical asset or source of revenue are listed on exchange Issued to fulfil (capital adequacy Ratiol under basel 3 Norms. 

🔸️ AT-1 bonds can be written down, if common Equity tier-1 capital of bank falls below 5.5% of Risk weighted assets