How to make falling time shorter for mosfet switching for high voltage and high frequency ultrasonic pulser
The large fall-time is most likely caused by the drain-source capacitance of the mosfet (700pF in datasheet). When turning off you have essentially a 10k resistor in series with 700pF of capacitance which takes time to charge up.

If you only need round about 20mA I would choose a smaller mosfet with less capacitance.