Special Semiconductor Devices

The SCR is the most important special semiconductor device. This device is popular for its Forward-Conducting and Reverse-blocking characteristics.
                SCR can be used in high-power devices. For example, in the central processing unit of the computer, the SCR is used in switch mode power supply (SMPS).
                The DIAC, a combination of two Shockley Diodes, and the TRIAC, a combination of two SCRs connected
anti-parallelly are important power-control devices. The UJT   is also used as an efficient switching device.


The silicon-controlled rectifier or semiconductor controlled rectifier is a two-state device used for efficient power control.

                            SCR is the parent member of the thyristor family and is used in high-power electronics.Its constructional features, physical operation and characteristics are explained in the following sections.


                        The SCR is a four-layer structure, either p–n–p–n or n–p–n–p, that effectively blocks current through two terminals until it is turned ON by a small-signal at a third terminal.
                        The SCR has two states: a high-current low-impedance ON state and a low-current high-impedance OFF state.
                        The basic transistor action in a four-layer p–n–p–n structure is analysed first with only two terminals,and then the third control input is introduced.


          The physical operation of the SCR can be explained clearly with reference to the current–voltage characteristics.
          The forward-bias condition and reverse-bias condition illustrate the conducting state and the reverse blocking state respectively.
            There are two different states in which we can examine the SCR in the forward-biased condition:
                (i) The high- impedance or forward-blocking state
                (ii) The low-impedance or forward-conducting state
      At a critical peak forward voltage Vp, the SCR...