Terahertz (THz) quantum cascade lasers (QCLs) are electrically pumped and heterostructure based semiconductor laser sources with intersubband transitions of electrons in different layers of the quantum wells and barriers. The THz QCLs have high output power in THz region which make them important from application point of view. Recently intensive research has been carried out by researchers for obtaining efficient designs of THz sources. Most of the researchers have investigated the THz frequency range between 0.1 and 3 THz; however, the output power of the THz sources in the frequency range 3–5 THz is small because of transit time and resistance-capacitance effects. Nevertheless, the present review is focused for the development of efficient THz QCL sources in the frequency range from 3 to 5 THz where one of the major problem of thermal backfilling of the carriers has to be overcome by engineering the heterostructure.

太赫兹（THz）量子级联激光器（QCL）是电泵浦的基于异质结构的半导体激光源，在量子阱和势垒的不同层中具有电子的子带间跃迁。太赫兹QCL在太赫兹区域具有很高的输出功率，这从应用角度来看非常重要。研究人员最近进行了深入研究，以获得有效的太赫兹源设计。大多数研究人员已经研究了0.1至3 THz的THz频率范围。但是，由于传播时间和电阻电容效应，太赫兹源在3–5 THz频率范围内的输出功率很小。不过，