Description-
BD139 is an NPN transistor that has a gain value of 40 to 160. The maximum amount of current that could flow through the Collector pin is 1.5A.  It can be used to control (On/Off) bigger loads that consume less than 1.5A. It also has a very less saturation voltage.
 
Specifications-
-Transistor type  :  Medium power NPN transistor
-Continuous collector current  :  1.5A
-Collector-Emitter voltage  :  80 V
-Collector-Base voltage  :  80V
-Emitter Base Breakdown Voltage  :  5V
-DC current gain  :  40 to 160
-Operating Temperature Range : - 65 to 150°C
-Low saturation voltage
 
Applications / Suggested Uses -
-RF Amplifiers
-Switching circuits
-Amplification circuits
-Audio amplifiers
-Load driver circuits