Question

what are the differences between BJT and FET?

Answer 1

Generally BJT's are considered current amplifier with Io = hfe Ii, but it also can be model as voltage to current gain, gm transconductance, there are two other ways of model it depending on analysis. FET's generally model by voltage to current gain, gm, and has input resistance consider infinite. Depending on circuitry different models apply especially for biasing and signal gain. For both there's a more exacting model used in computer simulations or by EE will verse in model.

Answer 2

The difference is as tabulated below: