- JFET Definition: A Junction Field Effect Transistor (JFET) is defined as a type of transistor that controls current flow.
- Ohmic Region: The ohmic region is where the JFET behaves like a resistor, allowing current to flow with minimal power loss.
- Cutoff Region: In the cutoff region, the JFET stops current flow, acting as an off switch with no power loss.
- Gate Current: The gate current in a JFET is always zero, preventing power loss from the gate signal.
- JFET as a Switch: Using a JFET as a switch efficiently controls power with minimal loss, operating in ohmic and cutoff regions.
Both N-channel and P-channel JFETs can switch analogue signals. A JFET is a normally-on depletion device: zero gate-source voltage opens the channel, while a reverse gate-source voltage moves it toward cutoff.
An ideal closed switch has zero resistance, and an ideal open switch has zero leakage. A real JFET has finite on-resistance, off-state leakage, capacitance and voltage limits.
Low switch loss therefore depends on the device, signal level and circuit. In the on state, loss follows channel resistance and current. In the off state, leakage and the blocked voltage determine static loss.
The useful switching regions are the ohmic region for the on state and cutoff for the off state. Saturation, also called the active region in JFET amplifier discussions, is not the normal low-resistance on state.
The output characteristic shows where these operating regions occur. Circuit design must also keep the gate junction reverse biased and remain within drain current, gate voltage, power and breakdown ratings.
At small drain-source voltage, a conducting JFET operates in its ohmic or triode region. Drain current changes approximately in proportion to drain-source voltage, so the channel behaves as a resistor. Its resistance is not zero and varies with gate voltage, signal voltage, temperature and the selected device. The on state must keep the full signal range inside this region; pinch-off voltage alone does not establish a universal low-loss limit.
For an N-channel JFET, making the gate negative relative to the source widens the depletion regions and reduces channel current. At VGS(off), drain current reaches the small off-state value defined by the datasheet test conditions. A more-negative control voltage can improve isolation only while the gate-source and gate-drain junctions stay within their reverse-voltage ratings. A P-channel device uses opposite polarities.
The reverse-biased gate junction draws very little DC current, but its current is not always zero. Datasheets specify gate leakage, and that leakage usually rises with temperature. The gate also has capacitance, so the driver must supply transient current when the control voltage changes. A gate-return resistor provides a defined state and discharges stored charge.
A JFET can therefore serve as a voltage-controlled switch when the circuit accounts for on-resistance, off leakage, gate capacitance, device spread and absolute maximum ratings. Small analogue signals are a common use. Power efficiency still depends on the device ratings and operating conditions.
Two common arrangements are:
Shunt switching, where the JFET diverts the signal away from the output
Series switching, where the JFET opens or closes the signal path
