One of the most challenges for future wireless communication networks is energy consumption. Heterogeneous networks, which are built to handle the increasing need for high data traffic, are one of the most promising strategies to address these challenge in future cellular networks. Network coverage may be increased by adding more base stations, but this comes with a high power cost. In the two-tier network concept, small cells (SCs) work along with main cell stations (MCs) to provide wider coverage. Some SCs experience low traffic rates due to user equipment's (UEs) mobility, yet they still consume a considerable amount of energy. To save energy and increase energy efficiency (EE), some SCs must be turned off. This study extends the operation modes for BSs and introduces a bio-inspired mechanism to select the optimal operation mode for each SC. A bias factor is used to regulate power consumption, with SCs operating in one of four modes: On, Standby, Sleep, or Off. The EE maximization problem is defined under a set of limitations and a Variant Power Mode Selection (PSO-VPMS) based on Particle Swarm Optimization Algorithm is proposed in this study. The simulation findings show that the suggested algorithm scheme offers a high EE...................... Keywords: ...........two-tier network; energy efficiency (EE); bias factor; Particle Swarm Optimization (PSO).