Comparison of the basic properties of Bipolar Transistors and FETs.

The basic difference between the two devices, is that Bipolar Junction Transistors BJTs are current operated devices and FETs are voltage operated devices.

Advantages and disadvantages of BJTs.

The main advantages of BJTs are that they have faster switching speeds and higher operating frequencies. ( BJTs historically have had greater power handling capabilities than FETs, but the power handling ability of MOSFETs has improved since their first introduction, particularly with the development of devices called "VMOSFETs").

The disadvantages are;

The fact that they are current driven means they consume more power and also generate more waste heat, (some components such as microprocessors require cooling when they are in use).
The heat generated, limits how closely BJTs can be "packed" onto integrated circuits, (they need more spacing to dissipate the heat, so you can't fit as many BJTs on an IC, compared to FETs).

Advantages and disadvantages of FETs.

Virtually no current flows in the gate of a FETs (IGFETS have insulated gates), this give them the following benefits;

They can be used to amplify low power signals (the signals don't need to expend energy to control the gate).
They present very high impedance, so measuring instruments based on FETs don't load the circuits they are connected to.
They can result in long battery life for portable devices.
The low current consumption* and therefore low heat generation enables denser packing of FETs in integrated circuits.

Disadvantages include slower switching speeds and lower frequency limits compared to BJTs. One particularly important aspect with MOSFETs that requires caution, is that static electricity can destroy or damage the thin insulation between the gate and substrate. Therefore handling precautions are required to eliminate static electricity.

(* When either BJTs or FETs are used to control large currents, then of course heat is generated. However in many applications in digital electronic circuits, transistors are just used in latching elements. Even in this application BJTs constantly require current flowing through them, while FETs do not. Therefore the difference in energy consumed, (and heat generated) in these circuits, is significant )