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A semiconductor switch needs to block voltage 
of only one polarity 
during OFF state as shown in figure (i) and carry current in both directions during ON state as shown in figure (ii). Which of the following switch combination(s) will realize the same?
Fig. (i)
Fig. (ii)
Since, the switch can block only one polarity of voltage called as Unipolar Switch.
But, since current can flow in both directions, so the switch is bidirectional.
Whenever anti-parallel diode is connected to a switch then reverse current can also flow so switch becomes Bidirectional.
as shown current can flow in both directions.
BJT blocks forward voltage but diode cannot allow blocking reverse voltage.
Switch is unipolar & bidirectional.
Diode connected in series does not allow current flow in both directions.
BJT blocks forward voltage while series diode blocks reverse voltage
∴ switch is unidirectional & bipolar.
Diode connected in series does not allow current flow in both directions.
BJT blocks forward voltage & series diode blocks reverse voltage. hence switch is unipolar.
∴ switch is bipolar & unidirectional
This switch is combination of previous switch & antiparallel diode.
∴ switch becomes bidirectional & unipolar.