N-Channel JFET: Structure, Operation, and Characteristics Explained

The device structure of the N-Channel JFET is depicted in Figure 1.

Figure 1

As illustrated in Figure 1, the structure of the n-channel JFET comprises an n-type substrate diffused with p-type material on both sides. The n-region, termed the channel, forms a pathway between the Source and Drain. The two p-regions are electrically interconnected, and the shared terminal is referred to as the Gate. Note 1: For a p-channel JFET, reverse all semiconductor types, i.e., replace n-channel with p-channel and replace p-type diffusions with n-type diffusions. Similarly, reverse bias voltages. Note 2: The JFET is a symmetrical device, meaning the Source and Drain terminals are interchangeable. However, designating one terminal as Source and the other as Drain is practical.

Why the name JFET?

The fundamental operation of the device relies on the biasing of the pn junction. In this case, the junction is reverse biased, controlling the channel width and thus the current flow between Drain and Source. The pivotal role of the pn junction in the device's operation leads to the name Junction Field-Effect Transistor (JFET).

Working of the N-Channel JFET

Figure 2 displays the electrical connections of the n-channel JFET.

Figure 2
Assuming the voltage applied to the Gate terminal is 0 (i.e., VGS=0), and the voltage VDS is finite with the polarity shown in Figure 2. Under these bias conditions, there is a current flow (ID) from Drain to Source. Case 1: Now, consider the scenario where VGS becomes negative. The depletion region of the Gate-Channel widens, reducing the channel width and increasing the channel resistance. The JFET operates like a resistance controlled by VGS. As VGS becomes more negative, the depletion width increases until it occupies the entire channel, a state known as threshold voltage or pinch-off voltage Vp. Case 2: In this case, maintain a constant negative VGS below the pinch-off voltage and increase the voltage between Drain and Source (VDS). The VDS appears as a voltage drop across the channel and as seen from the figure 3, increasing along the channel from Source to Drain. As VDS increases, the depletion width expands, leading to channel pinching off. This effect causes non-linear characteristics in the drain current-drain voltage (ID vs VDS) relationship. When VGD falls below the pinch-off voltage Vp, the channel is pinched off at the Drain end (see figure 4), and the Drain current saturates.
Note 3: The JFET is a depletion-type device, as the Gate-Channel pn junction is reverse biased, and the Gate has control over the Channel. Applying a positive voltage to the Gate forward biases the Gate-Channel pn junction, causing the Gate to lose control over the Channel. Note 4: The maximum applicable VGS is 0 V; practically, VGS may go up to 0.3 V as the pn junction remains in the cut-off region at this small positive voltage. Note 5: The Gate Current (Ig) = 0.

Figure 3 Effect of Vds on the depletion region when Vgs is constant

Figure 4 Effect of further increase in Vds on the depletion region when Vgs is constant


Here are a few common N-Channel Junction Field Effect Transistors (JFETs): 2N5457, J113, BF256A, MPF102, 2N4117

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