The field effect transistor controls the current flow between drain (D) and source (S) via the voltage applied to the gate (G). The special thing about this component is that the connection between gate and source has a very high resistance. Field effect transistors are called MOSFET" (metal-oxide semiconducting field effect transistor) or "MOS for short. There are different types of MOS'. The one used in this brick is n-channel normally blocking. This means that a threshold voltage must be present at the gate so that current flows between drain and source. Since it is an n-channel MOS, the voltage at the gate contact must be positive to the source contact. For easier handling, the gate contact is available on both sides of the brick.