The left-hand section of a junction transistor is called the emitter. The emitter is one of the three main regions of a transistor, along with the base and collector. These regions are formed by the junctions between two differently doped semiconductor materials.

The emitter is typically heavily doped with impurities to create an excess of charge carriers, either electrons or holes, depending on the type of transistor (NPN or PNP). It is located on the left side of the transistor symbol in circuit diagrams.

The emitter is responsible for injecting majority charge carriers into the base region. In an NPN transistor, the emitter is made of N-type material, which means it has an excess of electrons. These electrons flow into the base region, which is made of P-type material with a deficiency of electrons (holes). On the other hand, in a PNP transistor, the emitter is made of P-type material and injects holes into the N-type base region.

The current flowing through the emitter is the sum of the base current and the collector current. The emitter current is typically larger than the collector current, as it provides the majority of the charge carriers needed for amplification. The emitter current is controlled by the base-emitter junction voltage and the properties of the transistor.

The emitter is an important component of the transistor as it helps control the amplification of signals. By varying the base-emitter voltage, the current flowing through the emitter can be controlled, which in turn affects the current flowing through the collector. This allows the transistor to act as an amplifier or a switch in electronic circuits.

In summary, the left-hand section of a junction transistor is called the emitter. It is heavily doped with impurities and injects majority charge carriers into the base region, controlling the amplification of signals in the transistor.