Explain the Depletion region and drain characteristics of n channel JFET.
- The cross-sectional diagram above shows an N-type semiconductor channel with a P-type region called the Gate diffused into the N-type channel forming a reverse biased PN-junction and it is this junction that forms the depletion region around the Gate area when no external voltages are applied. JFETs are therefore known as depletion-mode devices.
- This depletion region produces a potential gradient which is of varying thickness around the PN-junction and restricts the current flow through the channel by reducing its effective width and thus increasing the overall resistance of the channel itself.
- Then we can see that the most-depleted portion of the depletion region is in between the Gate and the Drain, while the least-depleted area is between the Gate and the Source. Then the JFET’s channel conducts with zero bias voltage applied (ie, the depletion region has near-zero width).
- With no external Gate voltage ( VG = 0 ), and a small voltage ( VDS ) applied between the Drain and the Source, maximum saturation current ( IDSS ) will flow through the channel from the Drain to the Source restricted only by the small depletion region around the junctions.
- If a small negative voltage ( -VGS ) is now applied to the Gate the size of the depletion region begins to increase reducing the overall effective area of the channel and thus reducing the current flowing through it, a sort of “squeezing” effect takes place. So by applying a reverse bias voltage increases the width of the depletion region which in turn reduces the conduction of the channel.
- Since the PN-junction is reverse biased, a little current will flow into the gate connection. As the Gate voltage ( -VGS ) is made more negative, the width of the channel decreases until no more current flows between the Drain and the Source, and the FET is said to be “pinched-off” (similar to the cut-off region for a BJT). The voltage at which the channel closes is called the “pinch-off voltage”, ( VP ).
- Ohmic Region – When VGS = 0 the depletion layer of the channel is very small and the JFET acts like a voltage-controlled resistor.
- Cut-off Region – This is also known as the pinch-off region where the Gate voltage, VGS is sufficient to cause the JFET to act as an open circuit as the channel resistance is at maximum.
- Saturation or Active Region – The JFET becomes a good conductor and is controlled by the Gate-Source voltage, ( VGS ) while the Drain-Source voltage, ( VDS ) has little or no effect.
- Breakdown Region – The voltage between the Drain and the Source, ( VDS ) is high enough to causes the JFET’s resistive channel to break down and pass uncontrolled maximum current.
