Advantages and Disadvantages of JFET:
Junction field-effect transistors combine several merits of both conventional (or bipolar) transistors and vacuum tubes. Some of these advantages and disadvantages of JFET are enumerated below :
1. Its operation depends upon the flow of majority carriers only. It is, therefore, a unipolar (one type of carrier) device.
On the other hand in an ordinary transistor both majority and minority carriers take part in conduction and, therefore, ordinary transistor is sometimes called the bipolar transistor. The vacuum tube is another example of a unipolar device.
2. It is simpler to fabricate, smaller in size, rugged in construction and has longer life and higher efficiency. Simpler to fabricate in IC form and space requirement is also lesser.
3. It has high input impedance (of the order of 100 MΩ), because its input circuit (gate to source) is reverse biased, and so permits high degree of isolation between the input and output circuits. However, the input circuit of an ordinary transistor is forward biased and, therefore, ordinary transistor has low input impedance.
4. Another advantage of JFET is it carries very small current because of reverse-biased gate and, therefore, it operates just like a vacuum tube where control grid (corresponding to gate in JFET) carries extremely small current and input voltage controls the output current. This is the reason that JFET is essentially a voltage driven device. Ordinary transistor is a current operated device since input current controls the output current.
5. An ordinary transistor uses a current into its base for controlling a large current between collector and emitter whereas in a JFET voltage on the gate (base) terminal is used for controlling the drain current (current between drain and source). Thus an ordinary transistor gain is characterized by current gain whereas the JFET gain is characterized as the transconductance (the ratio of drain current and gate-source voltage).
6. JFET has no junction like an ordinary transistor and the conduction is through bulk material current carriers (N-type or P-type semiconductor material) that do not cross junctions. Hence the inherent noise of tubes (owing to high temperature operation) and that of ordinary transistors (owing to junction transitions) is not present in JFET.
7. It is relatively immune to radiation.
8. It has -ve temperature coefficient of resistance and, therefore, has better thermal stability.
9. It has high power gain and, therefore, the necessity of employing driver stages is eliminated.
10. It exhibits no offset voltage at zero drain current and, therefore, makes an excellent signal chopper.
11. It has square-law characteristics and, therefore, it is very useful in the tuners of radio and TV receivers.
12. It has got high frequency response.
The main Disadvantages of JFET is its relative small gain-bandwidth product in comparison with that of a conventional transistor and greater susceptibility to damage in its handling. Other disadvantages of JFET are low voltage gains because of small transconductance and higher cost than that of BJTs.