The description :

• The transistor is an electronic component with three active electrodes which is used in most electronic circuits, it allows to control a current or a voltage on the output electrode This is a bipolar junction transistor named BD139.
• A bipolar transistor is a type of transistor that uses both electrons and holes as charge carriers.

It has 3 terminals, which are:

• Transmitter.
• Collector.
• Based.

Specifications:

• Type: bipolar NPN
• Model#: BD139
• Current: 1.5 A.
• Voltage: 100 V.
• Dissipated power: 12.5W
• Junction operating temperature range: -65 – 150 ° C
• Technical sheet : https://datasheetspdf.com/pdf-file/127840/STMicroelectronics/BD139/1