LTSpice is a free electronics simulation program from Analog Devices (originally made by Linear Technologies).
It can be downloaded from their website: https://www.analog.com/en/design-center/design-tools-and-calculators.html
If the component you want to use in your circuit is not available you can import one.
LTSpice models are available in two types:
- Model (.mod, model)
- these are simple components such as transistors and diodes
- SubCircuit (.sub, subckt)
- these are more complex parts made up of simpler Spice parts (diodes, transistors, resistors etc)