- JFET Definition: A Junction Field Effect Transistor (JFET) is defined as a voltage-controlled semiconductor device that controls current flow using majority carriers.
- JFET Construction: JFETs consist of a channel of either p-type or n-type semiconductor material, surrounded by the opposite type of semiconductor.
- Types of JFET: There are n-channel and p-channel JFETs, determined by the type of semiconductor in the channel.
- Working Principle of JFET: The working principle of JFET involves controlling the drain current by applying a voltage to the gate, which affects the depletion layer and the channel’s resistance.
- Pinch-Off Voltage: At pinch-off voltage, the depletion layers in the channel almost touch, maintaining a constant drain current despite further increases in drain voltage.
A bipolar junction transistor uses both majority and minority carriers. Its collector current has an exponential relationship to base-emitter voltage, while base current provides the terminal drive in common circuit models. A junction field-effect transistor is a majority-carrier device whose reverse-biased gate junction controls channel width. Because normal gate current is only leakage, a JFET has high input resistance and is described as voltage controlled.
A JFET contains a continuous semiconductor channel between source and drain. Gate regions of the opposite conductivity type form a PN junction to the channel and are electrically connected as the gate terminal. An n-channel JFET carries electrons and uses p-type gate regions; a p-channel JFET has the opposite polarities and carrier type. Practical geometries vary, so the gate material need not literally surround every side of the channel.
The following explanation uses an n-channel JFET. A p-channel device follows the same field-control principle with voltage polarities and conventional current directions reversed.
The two ends of the n-type channel form the source and drain terminals. The p-type gate regions form reverse-biased junctions to that channel during normal operation.
In the shown circuit, the source and gate are grounded and the drain is made positive, so VGS = 0 and VDS is positive. The device must have a current-limiting load or supply arrangement that keeps it inside its ratings.
Channel voltage rises from source to drain. The gate-channel junction is therefore reverse biased, with the greatest reverse bias near the drain. Its depletion region is wider where the local channel-to-gate voltage is larger.
At small VDS, the channel behaves approximately as a voltage-controlled resistance and drain current rises with drain voltage. As VDS increases, the depletion region widens more near the drain, narrowing the conducting channel. At a particular condition the channel reaches pinch-off at the drain end and operation enters the constant-current, or saturation, region. Pinch-off does not make drain current fall to zero and restart in a cycle. Carriers cross the short high-field pinch-off region and drain current continues.
Beyond pinch-off, extra drain voltage appears mainly across the high-field region and the pinch-off point moves slightly toward the source. The ideal model gives nearly constant drain current for a fixed VGS.
A real JFET still shows some rise in current from channel-length modulation. Excess VDS can reach avalanche breakdown, so the saturation region must not be confused with unlimited safe voltage. The term pinch-off is also used inconsistently: drain-end pinch-off starts current saturation, while the negative VGS(off) condition reduces drain current almost to leakage.
Making the gate more negative than the source increases reverse bias of an n-channel JFET gate junction. The depletion region expands, the effective channel narrows and drain current falls at a fixed VDS in the saturation region.
At VGS = VGS(off), the drain current is reduced to its small off-state leakage. Gate current remains low only while the PN junction stays reverse biased. Device values such as IDSS and VGS(off) vary widely, so bias design must use the datasheet range rather than one nominal curve.
