Why Admittance, symbol Y, is defined as the reciprocal of impedance: Y = 1/Z, measured in siemens (formerly mhos). Just as impedance is the complex sum of resistance and reactance, admittance is the complex sum of conductance G and susceptance B. Working in admittance makes parallel circuits easy, because admittances simply add in parallel the way impedances add in series.
Watch out The inverse of reactance alone is susceptance, not admittance; admittance is the inverse of the whole complex impedance. The field effect transistor choices belong to a different topic entirely (transconductance and on-resistance).
Flip the whole thing: 1/Z = Y (admittance); flip just the reactive part: 1/X = B (susceptance).