A memory cell comprises asymmetric retention elements formed of bipolar junction transistors integrated with a CMOS transistor. The BJT transistors of the retention element may be vertically stacked. In one embodiment, the N region of two adjacent NPN BJT transistors may be connected to ground and may form a common emitter of the NPN BJT transistors while the P region of two adjacent PNP BJT transistors may be connected to high voltage and may form a common emitter of the PNP BJT transistors. For further compactness in one embodiment a base of one transistor doubles as a collector of another transistor. The retention element may have only a single bit line and a single write line, with no negative bit line. In some embodiments, a single inverter and only three transistors may form the retention element. Memory space may be cut approximately in half.