We proposed and demonstrated a resonant-tunnelling hot electron transistor (RHET) in 1985, and developed a multi-emitter RHET in 1993, enabling us to make an SRAM cell and multi-input logic gates using a single transistor. Through the development of these quantum tunnelling transistors, we have created the technology needed to fabricate precisely controlled quantum well structures, and have mostly understood the physics behind quantum well structures. Although this technology has been successfully used to develop heterostructure transistors and quantum well laser diodes, RHETs cannot yet be used in practical applications. One reason is that the RHET should be cooled down to about 77 K and its functionality is insufficient to replace conventional semiconductor devices. To make these RHETs practical for future use in cryoelectronic systems, integration with complementary-HEMT logic circuits or development of new architecture circuits will be essential. There are also other important areas of research for us. One is to develop room temperature quantum functional bipolar transistors. Another is to develop quantum box devices based on the RHET technology, searching ultrasmall limit of electron devices. These research will be also useful in the development of photonic devices, such as quantum dot lasers and new photonic memory devices.