What is the use of drain diodes in the circuit below?
The Supertex drivers (now Microchip IIRC) are quite strange beasts. In fact they are almost single-purpose chips.
The diodes are to protect the mosfets from the high voltage you have during resonance. In that 100V driver, you could have more than 200V on the transducer which would avalanche the fets (going over Vdsmax of 200V, and maybe doing damage). That diodes would add another 200V protecting the channel.