Q9Analog Electronics

An N-Channel JFET (Junction Field Effect Transistor) consists of an N-type semiconductor bar with two ohmic contacts — the Source (S) and Drain (D). Two P-type regions are diffused on opposite sides of the N-channel to form the Gate (G) terminals (connected together). The N-type material between the gate P-regions forms the conducting channel.

When $V_{DS}$ is applied (drain positive w.r.t. source), electrons flow from source to drain through the N-channel. The gate-source junction is reverse biased by $V_{GS}$ (negative for N-channel), creating a depletion region that narrows the conducting channel. A more negative $V_{GS}$ creates a wider depletion region, reducing channel width and hence $I_D$.

For a given $V_{GS}$, the drain characteristics show three regions: (1) Ohmic/Linear region — for small $V_{DS}$, $I_D$ rises linearly. (2) Pinch-off region (Saturation) — beyond pinch-off voltage $V_{DS}=V_P+V_{GS}$, $I_D$ becomes nearly constant (saturation). (3) Breakdown — at very large $V_{DS}$, avalanche breakdown occurs.

The transfer characteristic (Shockley's equation) relates $I_D$ to $V_{GS}$ in saturation:

At $V_{GS}=0$: $I_D=I_{DSS}$ (maximum drain current). At $V_{GS}=V_P$ (pinch-off): $I_D=0$.

Pinch-off voltage $V_P$ is the value of $V_{GS}$ at which the depletion regions from both sides of the gate touch and completely block the channel, reducing $I_D$ to zero (in transfer characteristics). For an N-channel JFET, $V_P$ is negative (e.g., $-4\text{V}$). It is also defined as the drain-source voltage at which $I_D$ enters saturation when $V_{GS}=0$.