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A Sziklai pair is a connection between two BJTs of the different polarity (one NPN and one PNP), which are often referred to as the input transistor and the output transistor.
The configuration is built by connecting the emitter of the input transistor with the collector of the output transistor and
feeding the collector of the input transistor to the base of the output one.
You end up with a compound device whose base terminal is the base of the input transistor, the collector terminal is the emitter of the output transistor and the emitter terminal is the junction point between the emitter of the input BJT and the collector of the output BJT.
The pair is essentially equivalent to a single BJT having a current gain β that is the product of the current gains of the single transistors.
The compound device behaves as a BJT of the same polarity as the input BJT. So if the input BJT is NPN, the pair will behave as an NPN device.
This kind of configuration can also be used with different kind of devices. In particular, it is common to use a FET as the input transistor, which produces a compound device whose characteristics are
a tradeoff between that of a FET and that of a BJT.
IGBTs are discrete devices built in that way.
Sziklai pair on Wikipedia.
