Hi, new here. Those are power transistors. Mostly found on power supply circuit boards. The backs are usually tin plated cu and used as a heat sink and ground for the chip. These are almost always screwed to a secondary heat sink. In the old days the sinks were copper but now mostly ext. al. Tore down some old servers the other day and the power supplies had cu secondary heat sinks. Gallium is in high demand in the IC industry.