Question

what are the difference between diode and bjt

Answer 1

diode is a two terminal device in which one takes in power and one lets it out so power cannot flow backwards for a diode but bjt often has 3 terminal devices in which power output is different for different set of connections. diodes are mainly used as protection device because power flows in only one direction it helps to keep the device safe from overloading and transistors are mainly used as switches(generally an FET has a faster switching time than a BJT).
another thing is that you can use a BJT as a diode(ex. current mirrors) by shorting B or C terminals bcause it will give you a high forward current rating but reverse voltage of the diode will be poor.