Q.Explain the characteristics of n-channel JFET with a circuit diagram.

The JFET is tested in a circuit with variable gate and drain supplies, from which two families of curves — drain characteristics and transfer characteristics — are obtained. …

[!TLDR]

From the test circuit, the drain characteristic ($I_D$ vs $V_{DS}$ at constant $V_{GS}$) shows ohmic, saturation/pinch off and breakdown regions with $I_{DSS}$ at $V_{GS}=0$; the transfer characteristic ($I_D$ vs $V_{GS}$ at constant $V_{DS}$) follows Shockley's equation.

Circuit: The n-channel JFET is connected with two variable DC supplies. The gate is fed from $V_{GG}$ through a gate resistor $R_G$, which fixes and reverse biases the gate-to-source voltage $V_{GS}$. The drain is fed from $V_{DD}$ through a drain resistor $R_D$, which fixes the drain-to-source voltage $V_{DS}$. The drain current $I_D$ is measured in the drain branch, and the source with the negative terminals of both supplies is tied to a common ground. The JFET shows two main characteristics.

(a) Drain characteristics: These are the curves of drain current $I_D$ against drain-to-source voltage $V_{DS}$ at a constant gate-to-source voltage $V_{GS}$. First $V_{GS}$ is set to zero and $I_D$ is noted for various $V_{DS}$; the procedure is repeated for other $V_{GS}$ values, giving a family of curves. On any one curve three regions appear:

• Ohmic region: for small $V_{DS}$, $I_D$ increases almost proportionally with $V_{DS}$, because the channel behaves like a simple resistor.
• Saturation / pinch off region: as $V_{DS}$ increases the depletion regions widen and the channel narrows, so $I_D$ rises only slightly and then stays almost constant; the maximum current on the $V_{GS}=0$ curve is called $I_{DSS}$.
• Breakdown region: at large $V_{DS}$, $I_D$ increases rapidly due to avalanche breakdown. Higher reverse $V_{GS}$ values give lower curves (smaller saturation current). …