Drain Charecteristics of FET

Drain characteristic of FET

The curve between drain current I_D and drain source voltage V_DS of a FET at constant gate source voltage V_GS is known as drain or output characteristics of FET. The below fig shows the circuit for determining the output characteristics of FET. Keeping V_GS fixed at some value, the drain – source voltage is changed in steps. Corresponding to each value of V_DS the drain current I_D is noted. A plot of these value gives the output characteristics of FET. Repeating similar procedure, output characteristics at other gate _ source voltage can be drawn. The fig shows a family of output characteristics understand the nature of the drain characteristics. Let us consider the characteristics for V_GS = 0, the channel between the gate junctions is fully open. On application of small voltage of V_DS, the n-type bar simply acts as a semiconductor and the drain current I_D increase linearly with V_DS. As the voltage V_DS is progressively increased a value of V_DS is reached at which the channel is pinched off. With further increases of V_DS, the current I_D begins to level off and approaches a constant value. The value of V_DS at which the channel is pinched off i.e. all the free charges from the channel are removed, is called pinch off voltage V_P. Further increases of V_DS causes the avalanche breakdown.

Now let us assume that the gate is supplied with negative voltage. In this case the junction is reverse biased. Hence the conducting portion of the channel is reduced. The resulting drain characteristics are similar to the one for V_GS = 0, except that the pinch off occurs for smaller values of V_DS. The breakdown also occurs at lower V_DS. If the gate voltage is increased further negatively biasing the gate junction, we obtain characteristics similar to the one obtained above. All such characteristics will show a reduction in pinch off voltage smaller where the drain current is proportional to the V_DS and lower breakdown voltages.

(a) Circuit for Determining the Drain Charecteristics

(b) Drain Charecteristics