In darlington pair, two transistors connected together so that the current amplified by the first is amplified further by the second transistor. The overall current gain is equal to the two individual gains multiplied together:
Darlington pair current gain, hFE = hFE1 × hFE2
(hFE1 and hFE2 are the gains of the individual transistors)
Since only small base current is required to switch the transistor on, the Darlington pair has a very high gain (approx. 10000.
A Darlington pair basically consists two transistor connected in series. The three leads (B, C and E) showed in the figure are equivalent to the leads of a standard individual transistor. To turn on two transistors TR1 and TR2 at the same time there must be 0.7V across base-emitter junctions of both the transistors. To put it simply, 1.4V is required to turn two transistors on at same time.
Darlington pairs are available as complete packages in the market but you can make up your own from two transistors; TR1 can be a low power type, but normally TR2 will need to be high power. The maximum collector current Ic(max) for the pair is the same as Ic(max) for TR2.