HPT technology has seen recent rapid progress and is now one of the main options for wireless communication, power amplifiers, and frequency synthesizer applications. HPT devices have demonstrated higher emitter injection efficiency compared with conventional homojunction phototransistors, with the ability to create high internal gain. Unlike homojunction phototransistors, HPTs don’t require a heavily doped emitter with lightly doped base contact to give better emitter injection efficiency. In fact HPTs can reach injection efficiencies close to unity because reverse injection from the base is blocked by the hetero-barrier.

As photo detectors HPTs can be stable devices with internal gain. Unlike avalanche photodiodes, they can provide larger photocurrent gain without the need for high bias voltages. Most reported HPTs are based on InGaAs or AlGaAs compounds and cover the near-infrared (NIR) spectral region, with some limitations in the tunability of the cutoff wavelength. For MWIR optical receivers a type-II superlattice (T2SL) material system and design is a promising alternative.

T2SLs are a developing material system that has led to the development of high-performance MWIR HPTs. T2SL-based HPTs are also a possible solution to the urgent demand for sensitive MWIR photodetectors for MWIR free-space optical communication as well as high-speed sensitive MWIR imagers.

Vertical scaling seemed to be a major driving force for the evolution of bipolar transistors and HPTs. In the case of vertical scaling, collector and base scaling have the primary effect on the performance of these devices. Recently, under the leading supervision of Prof. M. Rzeghi at the center for quantum devices of Northwestern University, high-performance HPT devices based on type-II InAs/AlSb/GaSb superlattices were successfully demonstrated. By targeting high gain, the research team has shown a MWIR HPT device with the saturated responsivity of 8845 AW−1 and 9528 AW−1 at 77 K and 150 K. By scaling the base thickness down to 40 nm, the T2SL-based HPT devices exhibited remarkable enhancement in saturated current gain, with 17,690 AW−1 at 77 K, and 19,050 AW−1 at 150 K.