Construction
How Does a JFET Work
A voltage is applied to the gate, which is reverse-biased relative to the source. It is known as the gate-source voltage.
Gate (G): It significantly controls the current flow through the JFET.Source (S): Carriers enter the channel through this region.Drain (D): Carriers leave the channel through this region.
The source and drain are located at the ends of the JFET and are connected by the channel. The gate is situated around or in the middle of the channel. The schematics and symbols of the two types of JFET are shown in the diagram below. This creates a depletion region within the channel that is void of charge carriers and non-conductive. The width of the depletion region increases with the magnitude of the reverse bias voltage. In addition to the gate-source voltage, there is a drain-source voltage that creates an electric field, driving the current through the channel from the drain to the source. As the gate-source voltage increases, the depletion region expands, narrowing the conductive channel. This narrowing restricts the flow of current from the drain to the source. If the gate-source voltage is sufficiently high, the depletion regions from either side of the channel can meet. This phenomenon is known as “pinch-off”, and the channel severely restricts current flow. Beyond this point, the JFET enters saturation mode, where further increases in drain-source voltage do not significantly increase current flow.
Types
I-V Characteristics
Applications
- N-Channel JFET An N-Channel JFET has a channel composed of N-type semiconductor material, meaning that the majority charge carriers within the channel are electrons. N-channel JFETs typically exhibit higher electron mobility than P-channel JFETs, making them suitable for high-frequency applications.
- P-Channel JFET A P-Channel JFET features a channel made of P-type semiconductor material where the majority charge carriers are holes. While P-channel JFETs may not have the same electron mobility as N-channel devices, they are often used in low-power applications and can be advantageous in certain circuit configurations. Therefore, due to its superior electron mobility, N-channel JFET is preferred for high-speed and high-frequency applications. P-channel JFET finds its niche in low-power circuits where hole conduction is beneficial.
Output or Drain Characteristics
The output characteristics of a JFET show the relationship between the drain current (ID) and the drain-source voltage (VDS) while keeping the gate-source voltage (VGS) constant. These curves illustrate how ID changes with different VDS values, providing insights into the transistor’s behavior under various operating conditions. Regions of Operation The JFET exhibits varying characteristics at different stages of operation depending on the input voltages. Its characteristics in the different regions are described below.
- Active or Saturation Region: The JFET operates as a constant-current source. Here, ID becomes relatively independent of VDS and is primarily controlled by the gate-source voltage VGS.
- Ohmic Region: VDS is small, and ID increases linearly with VDS. The JFET behaves as a resistor whose value is controlled by VGS.
- Cut-off or Pinch-off Region: VGS is sufficiently negative (for n-channel JFETs) or positive (for p-channel JFETs) to deplete the channel of carriers, effectively turning off the device. In this region, ID is minimal or zero, as the channel is fully pinched off.
Transfer Characteristics
The transfer characteristics describe the relationship between ID and VGS while keeping VDS constant. The transfer characteristics graph typically shows a curve that starts from ID at VGS = VP (pinch-off voltage). It rises as VGS becomes less negative (for n-channel) or less positive (for p-channel), demonstrating the inverse relationship between VGS and ID. As VGS approaches zero, ID reaches its maximum value, known as IDSS (drain current with gate shorted to source). The graph below shows the I-V Characteristics of an N-channel JFET.
