Q. Describe small signal FET.What are its characteristics?

Small signal FET is the FETs used for amplification. The linear characteristic of FET is used for this purpose The input of small signal FET is very small.The small signal FET cannot be used for switching  function of FET since it requires large input signal. The characteristics of small signal FET is shown in the figure. The linear portion of that diagram is used for the amplification purpose.

The source drain characteristics give ID against V_Ds, with V_GS as parmeter. For V_GS = 0 and I_D = 0, the channel between the gate junction is entirely open. In response to a small applied voltage V_Ds,the n-type bar acts as a simple simple semiconductor resistor and the current Id increases linearly with VDs.With increasing current the ohmic voltage drop between the source and the channel region reverse biases the junction,and the conducting portion of the channel begins to constrict. Because of ohmic drop along the length of the channel itself,the construction is not uniform .Eventually,a voltage VDs is reached at which the channel is pinched off.At this the current Id begins to leave off and approach a constant value.

If a gate voltage VGS is applied in the direction to provide additional reverse bias,pinch off will occur for smaller value of VDs and the maximum drain current will be smaller.