- 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 be used as switches. Before exploring how JFETs operate as switches, let’s discuss the basic properties of a switch and compare them with JFET operations. This will make it easier to understand.
When a switch is ON, it acts as a short circuit with zero voltage drop, so there is no power loss. When a switch is OFF, it acts as an open circuit with zero current, so there is no power loss in this case either.
Now if we use a JFET as a switch, we have to establish that there is also very low power loss across the JFET during both in conducting and none conducting mode.
This is only possible if the JFET is being operated in its ohmic and cut off region.
Before discussing the main topic, let’s review the basic concepts of the ohmic and cutoff regions in JFET characteristics.
When the gate terminal voltage is zero and we slowly increase the drain voltage, the drain current increases linearly until it reaches a certain voltage. This voltage is called the pinch-off voltage, where the drain current becomes constant. Before pinch-off, the JFET acts like a simple resistor. The region before pinch-off is called the ohmic region. Since the pinch-off voltage is low, the power dissipation in the ohmic region is minimal, almost zero. Thus, a small signal through a JFET keeps the drain-to-source voltage below pinch-off, reducing power loss.
Now if we decrease the gate potential from zero, the current flowing through the channel for a certain drain to source voltage also gets decreased. This is because reverse biasing of the gate to channel pn junction gets increased with the decrease in gate terminal voltage from its zero potential level. After a certain negative gate voltage, the drain current becomes zero. This negative gate voltage at which the drain current becomes zero is called cut off gate voltage and hence for any applied gate voltage below the cut off voltage, there is no current flowing through the device and the device will behave as an off switch. So in the cutoff region of operations, there is no significant power loss in the transistor.
Here another point to be noted that in JFET, the gate current is always zero irrespective of its conducting or none conducting mode. So there is no power loss caused by the gate signal.
Therefore, a Junction Field Effect Transistor acts as a switch when operated in its ohmic and cutoff regions.
Let us see some practical applications of JFET as a switch. There are two types of switching operation of a JFET.
Shunt Switching
Series Switching
