- JFET Definition: A Junction Field Effect Transistor (JFET) is defined as a type of transistor that uses an electric field to control the flow of current.
- Characteristics of N Channel JFET: The N channel JFET features a channel of N-type semiconductor material and a highly doped P-type gate region.
- Pinch-Off Voltage: The pinch-off voltage is the point where the drain current becomes almost constant due to the narrowing of the channel by the depletion layer.
- Transfer Characteristics of JFET: The transfer characteristics of JFET show the relationship between gate voltage and drain current, crucial for understanding how gate voltage controls the device.
- P Channel JFET Operation: The P channel JFET operates similarly to the N channel but with opposite polarities for voltages and currents, showing how different types of JFETs work.
Junction field-effect transistors have two channel polarities:
N-channel JFET
P-channel JFET
The output characteristic plots drain current against drain-source voltage for several gate-source voltages. The transfer characteristic plots drain current against gate-source voltage at a stated drain-source voltage. Both plots need their voltage polarities and test conditions.
Characteristic of N Channel JFET
An N-channel JFET has an N-type channel between source and drain and P-type gate regions that form reverse-biased PN junctions with the channel. To measure its output curves, hold VGS at a selected value and vary drain-source voltage. For the VGS = 0 curve, the gate and source are at the same potential while the drain supply VDD increases from zero.
At small VDS, the channel behaves approximately as a voltage-controlled resistor, so drain current initially rises with drain voltage. Channel potential is highest near the drain. The gate-drain junction is therefore more reverse biased than the gate-source junction, and its depletion region extends farther into the channel.
When the channel reaches drain-end pinch-off, the JFET enters its saturation region, also called the active region. Pinch-off does not stop drain current. Instead, the pinch-off point shifts toward the source as VDS rises. An ideal model gives constant current, while a real curve usually has a positive slope because of channel-length modulation. With VGS = 0, the saturation current is called IDSS, and a datasheet specifies the VDS and temperature used to measure this Shorted Gate Drain Current. At still higher VDS, avalanche breakdown causes a steep current rise and may damage the device. The output graph therefore has an ohmic region, a saturation region and a breakdown limit.
Making the gate negative relative to the source increases the reverse bias of an N-channel device. The depletion regions widen, so each more-negative VGS curve has a lower drain current. The required VDS at the start of saturation also changes with VGS.
Transfer Characteristic of N Channel JFET
The transfer curve relates saturation-region drain current to gate-source voltage at a stated VDS. At VGS = 0, the current is IDSS under the specified test conditions. Making VGS more negative reduces current. At the gate-source cutoff voltage VGS(off), current reaches the small off-state test value rather than an absolute mathematical zero. The familiar square-law curve is an approximation; IDSS and VGS(off) can vary widely between individual devices, so circuit design must use datasheet limits.
Characteristic of P Channel JFET
A P-channel JFET uses the opposite voltage polarities and conventional-current direction. Its drain is negative relative to its source, and a positive gate-source voltage increases reverse bias. The magnitude of drain current first rises in the ohmic region, then changes only slightly after drain-end pinch-off, and rises sharply if the device enters breakdown. More-positive VGS values reduce the magnitude of drain current and move the output curves toward cutoff. The graph axis may show negative current or its positive magnitude, so its sign convention must be checked.
Transfer Characteristic of P Channel JFET
The P-channel transfer curve relates drain-current magnitude to positive gate-source voltage in the saturation region. It has the same general depletion-mode form as the N-channel curve, but voltage polarities and conventional-current direction are reversed. IDSS, cutoff voltage and the measurement conditions remain device-specific. 
