We report on the design; modelling and analysis of heterostructure barrier varactor (HBV) frequency quintuplers with output frequencies in the sub-millimetre wave region. The HBV is a symmetric varactor; thus only odd harmonics are generated and no DC bias is required. By incorporating several barriers in the device; the HBV is also capable of handling higher power levels than conventional varactors. This makes the HBV superior to the conventional Schottky varactor for high order frequency multiplier circuits. We present analytical models; which can be used to calculate parameters such as optimum doping concentration; layer structure; device area and series resistance for HBVs; as well as to predict the performance with respect to conversion efficiencies and output power levels. These parameters are then further optimised by harmonic balance simulations in commercial microwave EDA tools; for which we have developed accurate device models. We investigate the influence of embedding impedance levels for optimum conversion efficiency by means of analytical expressions and harmonic balance simulations. Theoretical calculations predict a maximum diode conversion efficiency to 500 GHz for a planar; six-barrier InGaAs HBV of more than 30 %.