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p-n-p-n Switching Devices The basic pnpn device consists of four back to back alternate p and n regions. Thus it has three p-n junctions: j1, j2 and j3. The two terminals are known as anode and cathode according to the region to which they are connected. The junctions can be forward bias junctions or reverse bias. When the anode is positive with regard to cathode the device is in forward bias and reverse configuration is known as the reverse bias. However, the forward bias state has two modes of operation: the forward blocking mode and the forward conducting mode. Switching between these two modes takes place at the onset of a critical peak forward voltage. In the forward blocking mode (v positive) the j1 and j3 junctions are both forward biased while j2 is reverse biased. But after a critical forward peak voltage, the device goes into forward conducting mode. In forward conduction mode (v positive) the device exhibits low impedance state and allows current to flow. All the three junctions j1, j2 and j3 are forward biased. Whereas in reverse blocking mode the junctions j1 and j3 are reverse biased while junction j2 is forward biased. The reverse biased junction leads to blocking of current through the device terminal. Hence no current flows in such a reverse blocking mode. The working of pnpn as a switching device can be visualized as a combination of two transistors. One is a pnp BJT formed by junctions j1 and j2 and the other is npn BJT formed by junctions j2 and j3. In this configuration, the base of pnp forms the collector of npn and the base of npn forms the collector of pnp. Thus the base current of pnp equals collector current of npn and vice-versa. For the device to get into the operating mode there are some triggering mechanisms. Thermal agitation: charge carriers are produced by thermal effects. Optical injection: by EHP generation through optical methods, one can increase the current through the device. However these methods are complex and require external tools. Voltage triggering: it is a simple triggering method. When voltage increases the critical peak voltage or a steady state triggering voltage, complete switching takes place at this voltage. Semiconductor controlled Rectifier (SCR) While a pnpn diode is switched by controlling the voltage across its terminal, the SCR is switched by controlling through a third terminal. The SCR is used in power switching applications like dimming and can handle current which ranges from a few mA to hundreds of amperes. The structure of a three terminal SCR is similar to the pnpn diode, except that it has a third metal contact attached to one of the base regions. This third pin is known as gate. The gate current helps in sweeping the carriers across junctions, thus increasing the injection efficiency. Hence the switching state is achieved at lower values of Vp. Once the device switches to forward conducting state, gate current no longer has any effect on the working and the device can remain in the conducting state for an infinitely long duration. Hence SCRs can be widely used in switching applications. Switching devices like the SCR or pnpn can function for switching applications only in the forward blocking mode. The bilateral switching device can be switched to both the forward and reverse biased mode by making suitable modifications in the structure. These can be used in the switching application, which involves a-c waveforms. If a bilateral switch is properly designed it shows symmetrical characteristics in the forward and reverse bias regions.