The Harmonic Perturbation subnode is available from the context menu (right-click the parent node) or from the Physics toolbar in the Contextual group.
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Care must be taken when computing quantities such as the transconductance and output conductance for a MOSFET. For example, if the drain is a Metal Contact with terminal name 1, then evaluating the global terminal current semi.I0_1 for the Harmonic Perturbation (with the Compute differential check box selected) gives the (complex) perturbation in the drain current, δIdrain. The output conductance is obtained by taking the absolute value of the perturbation and dividing by the amplitude of the voltage perturbation applied (for example 0.01[V]). Thus the output conductance (dIdrain/dVdrain) is obtained by evaluating the quantity: abs(semi.I0_1)/0.01[V]. It is not possible to evaluate the output conductance by evaluating the perturbation part of the expression: abs(semi.I0_1)/abs(semi.V0_1) because COMSOL Multiphysics linearizes the expression entered, computing the quantity: δIdrain/Vdrain-(Idrain/Vdrain2)δVdrain.
Alternatively, for complicated expressions, use the lindev operator, for example: lindev(semi.I0_2)/lindev(semi.V0_2). In this case, the Compute differential check box must be cleared.
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In the COMSOL Multiphysics Reference Manual:
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