Differentiate between JFET and BJT. With the help of characteristics curves and neat sketches explain its switching characteristics.

JFET (Junction Field-Effect Transistor)

• A JFET is a majority-carrier device.
• It is a voltage-controlled device.
• The output current is controlled by the input voltage.
• The gate-source voltage VGS controls the width of the channel.
• When VGS is increased, the depletion region expands, reducing the channel width and, hence, the drain current.
• The drain current is approximately proportional to the square of the gate-source voltage.
• JFETs are normally operated in the depletion mode.
• They are used in low-power, low-noise applications, such as audio amplifiers and analog switches.

BJT (Bipolar Junction Transistor)

• A BJT is a minority-carrier device.
• It is a current-controlled device.
• The output current is controlled by the input current.
• The base current IB controls the emitter current IE.
• The collector current IC is approximately equal to the emitter current IE.
• BJTs are normally operated in the active mode.
• They are used in a wide variety of applications, including amplifiers, switches, and oscillators.

Switching Characteristics

The switching characteristics of a transistor are determined by its ability to turn on and off quickly. The turn-on time is the time it takes for the transistor to reach its full on-state when a gate or base signal is applied. The turn-off time is the time it takes for the transistor to reach its full off-state when the gate or base signal is removed.

JFET:

• The JFET has a relatively slow turn-on time because the depletion region must be fully expanded before the drain current can flow.
• The JFET has a relatively fast turn-off time because the depletion region can be collapsed quickly by reducing the gate-source voltage.

BJT:

• The BJT has a relatively fast turn-on time because the emitter-base junction can be forward-biased quickly by increasing the base current.
• The BJT has a relatively slow turn-off time because the emitter-base junction must be reverse-biased to stop the flow of current.

Characteristics Curves

The characteristics curves of a transistor show the relationship between the input and output currents and voltages. The most important characteristics curves are the output characteristics and the transfer characteristics.

Output Characteristics:

• The output characteristics show the relationship between the drain current and the drain-source voltage for a given gate-source voltage.
• The JFET output characteristics are a family of curves that are parabolic in shape.
• The BJT output characteristics are a family of curves that are linear in shape.

Transfer Characteristics:

• The transfer characteristics show the relationship between the gate-source voltage and the drain current for a given drain-source voltage.
• The JFET transfer characteristics are a family of curves that are approximately parabolic in shape.
• The BJT transfer characteristics are a family of curves that are exponential in shape.

Conclusion

JFETs and BJTs are two different types of transistors that have different characteristics and applications. JFETs are majority-carrier devices that are voltage-controlled, while BJTs are minority-carrier devices that are current-controlled. JFETs have a relatively slow turn-on time and a relatively fast turn-off time, while BJTs have a relatively fast turn-on time and a relatively slow turn-off time.