A semiconductor operates as a switch if it can have two distinct states (figure 1):

Figure 1

• the ON state (or passing state) in which the semiconductor opposes a neglectable voltage drop (ideally zero) to the flowing of the current and acts like a connexion between its terminals, as a closed switch would do.

• the OFF state (or blocking state) in which the semiconductor opposes a very high impedance (ideally infinite), which excludes all connection between its terminals, as an open switch would do.

The way in which a semiconductor switches from a state to the other depends on the semiconductor's type.

• a diode is in the ON state if the current that flows through it is positive. It switches to the OFF state if the current tends to become negative. It remains in the OFF state as long as the voltage to its terminals is negative (figure 2).

Figure 2

 

• the state (ON or OFF) of a transistor depends on the command that is given to it (figure 3).

  

  Figure 3

  But we have to take into consideration that:

  • the voltage to its terminals cannot become negative when it is OFF
  • the current that flows through it cannot become negative when it is ON

• a thyristor is in the OFF state if the voltage across its terminals is negative, and it remains in the OFF state if this voltage becomes positive. But in this moment it can be switched ON by using a command pulse. It remains then in the ON state until the current that flows through it tends to become negative, moment when, spontaneously, it returns in the OFF state (figure 4).

  

  Figure 4