Superconducting transmon qubits are an attractive platform for building a prototype quantum computer comprising ~100 qubits. In order to go beyond this and build quantum computers powerful enough to break secure encryption or simulate small molecules, it will be necessary to improve coherence and scalability. Gatemon qubits based on III-V semiconductors are a recently innovated voltage-controlled transmon that offers advantages with scaling, but their performance is currently limited by microwave losses. In order to improve performance we will study loss mechanisms in III-V structures comprising Al/Nb/InP by measuring the power, temperature and time-dependence of quality factors. Initial measurements will be performed at the National Physical Laboratory. We will also explore voltage-controlled semiconductors that can be grown on low-loss Si substrates, for instance, graphene or V-VI topological insulators. The strong spin-orbit interaction in the latter also makes it possible realise Majorana physics and and one aim is to test for topologically-protected relaxation times in a Majorana transmon-MRC.