Advantages and Limitations
Advantages of JFET
1. It is a unipolar device. The current conduction through the device is only through majority carriers either holes in P channel or electrons in N channel.
2. FET’s are less noisy compared to BJT’s as the operation of the device depends on majority carriers and no recombination currents are required for its operation.
3. It exhibits high input resistance typically of the order of mega ohms. In JFET this is due to reverse biasing both the junctions of JFET thereby reducing drastically the conductance of channel. In MOSFET this is aggravated due to silicon dioxide layer provided at the gate terminal this is the most important advantage of FET over BJT.
4. It is simpler to fabricate and occupies less space in integrated form.
5. It is an excellent signal chopper as it exhibits no offset voltage at zero drain current.
6. Field Effect Transistors are more stable than BJT with respect to temperature variations. This is due to decrease in drain current (analogous to collector current in BJT) as temperature increases.In JFET as the temperature increases the majority carrier concentration decreases due to increased recombination with thermally generated minority charge carriers. Hence the drain current which is majority carrier current decreases.
The drain current in MOSFET depends on threshold voltage and mobility of charge carriers. As temperature increases threshold voltage decreases, threshold voltage is the minimum gate voltage (analogous to Base cut in voltage in BJT) at which drain current starts flowing (decrease in threshold voltage due to increase in charge concentration of minority carriers). Drain current increases with decreasing threshold voltage. Also mobility of charge carriers decreases with increasing temperatures. This is due to increased lattice vibrations leading to increased collisions of charged carriers with the lattice. Drain current is directly proportional to mobility of charge carriers, so drain current decreases with increased temperatures with the dependence of mobility on temperature is taken into consideration. But taking into account the effect of both decreased threshold voltage and decreased mobility with increased temperatures, the effect of latter is dominant than the former. Hence drain current decreases with increased temperatures.
7. It is relatively more immune to radiation.
8. It is a voltage-controlled device, and shows a high degree of isolation between input and output. But this is not true as frequency increases due to feedback provided by internal capacitance’s of FET.
Limitations of JFET
1. FET’s theoretically are ideal voltage amplifiers with high input resistance and low output resistance. But it is seldom used in amplifier circuits due to its low gain bandwidth product compared to Bipolar Junction Transistors.
2. Faster switching times can be achieved in BJT compared to FET by preventing the devices from going into hard saturation. In FET internal junction capacitance’s are responsible for higher delay times.
3. The performance of FET deteriorates as frequency increases due to feedback by internal capacitance’s.