Characteristic Of Junction Field Effect Transistor - Electrical4U

Contents

• Characteristic of N Channel JFET
• Transfer Characteristic of N Channel JFET
• Characteristic of P Channel JFET
• Transfer Characteristic of P Channel JFET

• 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.

There are two types of junction field effect transistor.N channel JFETP channel JFETLet us discuss the characteristics of n channel and p channel transistor separately one by one for better understanding.

Characteristic of N Channel JFET

We know that there is a channel of n-type semiconductor material in n channel JFET. The gate region of the n channel JFET is a highly doped p-type region. A voltage is applied across the channel i.e. between drain and source terminal. First, let us plot the values of the drain to source current for different applied drain to source voltages. At first case, we will short circuit the gate terminal with the source terminal and ground them commonly. Now we will slowly increase the drain circuit voltage VDD from zero.

The drain current increases linearly with the voltage because the channel has resistance, though not constant. As the N channel voltage becomes positive compared to the gate, the gate-to-channel PN junction becomes reverse-biased, forming a depletion layer. This layer is wider near the drain terminal. As the drain voltage increases, the depletion layer near the drain thickens faster than near the source, increasing channel resistance and decreasing the channel opening. This causes the drain current versus voltage curve to align more horizontally along the voltage axis.At a certain drain voltage, the depletion layers near the drain terminal touch each other, causing the curve to become almost horizontal. A narrow opening remains, allowing the drain current to flow. Further increasing the drain voltage increases the depletion layer’s thickness, extending the channel’s narrow opening. This is called channel modulation, which raises the channel’s resistance proportionally to the drain voltage increase, making the drain current almost constant. This point is called the pinch-off voltage, where the current is known as IDSS or Shorted Gate Drain Current. Beyond a certain drain voltage, the depletion layers break down, causing a sudden rise in drain current, known as the breakdown region. The initial increase in drain current is the linear or ohmic region, while the constant current portion is the active region.Now, we will disconnect the gate terminal from the grounded source and apply a negative voltage to the gate. This causes the gate-to-channel junction to become more reverse-biased, reducing the drain current for the same drain-to-source voltage. The entire curve for the applied negative gate voltage shifts below the zero gate voltage curve. Applying more negative voltage to the gate shifts the curve further downward, as shown in the characteristic figure of an N channel JFET.

Transfer Characteristic of N Channel JFET

The transfer characteristic shows the relationship between gate voltage and drain current, with the drain-to-source voltage kept at the pinch-off voltage. When the gate is at zero potential, the maximum drain current (IDSS) flows through the transistor. As the gate’s negative potential increases, the drain current decreases. At a certain negative gate voltage, the drain current becomes zero. This voltage, where the drain current is zero at pinch-off voltage, is called the gate-to-source cutoff voltage VGS(off).

Characteristic of P Channel JFET

In a P channel JFET, we apply a negative potential to the drain terminal. If both the source and gate terminals are grounded and we increase the negative potential of the drain from zero, we get a curve similar to the N channel JFET. Initially, the drain current, flowing from source to drain due to hole drift, increases linearly with the negative drain voltage. As the channel’s negative potential is higher near the drain, the junction near the drain becomes more reverse-biased, thickening the depletion layer. Pinch-off occurs at a certain negative drain voltage, flattening the curve. Increasing the negative drain voltage further causes avalanche breakdown, rapidly increasing the drain current. The curve shows a linear region at the beginning, an active region in the middle, and a breakdown region at the end. Applying positive voltages to the gate terminal increases reverse biasing, shifting the characteristic curve downward.

Transfer Characteristic of P Channel JFET

This is drawn between positive gate voltage and drain current. The pattern will be the same as in the case of n channel JFET but the polarity of the applied voltage and direction of the drain current differ.