The following circuit act as :

The circuit diagram in the question is of SCR. 

Important Points Darlington Pair: CC – CC

• Darlington pair is a special arrangement of two standard NPN or PNP bipolar junction transistors (BJT) connected together
• The Emitter of one transistor is connected to the base of the other to produce a more sensitive transistor with a much larger current gain
• It is useful in applications where current amplification or switching is required; The voltage gain is unity in this case

The symbolic diagram of the Darlington pair is shown below

It has high current gain β (equal to the product of current gain of individual transistors)