step graded and linear graded diodes

plz someone explain me what is abrupt junction , linear junction, linear graded and step graded diode.?what type of diode varactor diode is ? linear graded or step graded diode?

These are all different design variables of dopants across the junction that affect the inverse slope of reverse bias, Vr vs Capacitance ratio relative to 0V.

Capacitance is maximum at 0V and drops on a log scale or inverse power ratio as Vr increases. This power ratio ranges from 2 to 3.