in thyristors (scr) what exactly happens in the silicon crystals in terms of electrons & holes when gating current is applied? also how is it that a semiconductor device like thyristor can sustain high currents & voltages in kilo and mega ?
the semiconductor element of an SCR is a 4 layer P - N - P - N structure. as you will notice if we count from the left you see 3 regions equivalent to a PNP. and if we count from the right you see also 3 regions equivalent to NPN. the anode of the SCR is connected to the P end of the array, likwise the cathode is connected to N side of the array and the gate is connected to the common P region
now if a current is attempting to flow from the anode to the cathode both transistors will block the flow. (remember the principle of PN junction) now if we apply current into the base region of the NPN via the gate terminal, the NPN will now turn on, hence the "working current of the NPN will now withdraw the control current" from the PNP base thus turning it on.
even if the trigger current is remove, the PNP working current continous to flow into the base of the NPN keeping it turned on. so current continous to flow from anode to cathode until such time the main supply is cut off.
as to how it can handle high voltage/current, it's just a matter of material design which should not concern us so much unless you intend to make your own thyristors. what should concern us how this device work base on certain parameters as specified by the manufacturer. there are at least 7 specs ( power dissipation, conductivity, leakage current, breakdonw voltage, operating speed, gate trigger current, gate trigger voltage) that we should give due consideration when using this device.
i have attach a schem for a substitute SCR base on what i have cited. it can be likewise configured to be a triac.
Originally posted by nskelkar@Oct 27 2004, 09:06 PM in thyristors (scr) what exactly happens in the silicon crystals in terms of electrons & holes when gating current is applied? also how is it that a semiconductor device like thyristor can sustain high currents & voltages in kilo and mega ?
If you want a taste of some structural factors that contribute to various thyristor (and other power semiconductors) characteristics such as current density, switching time, etc, see Philips Semeiconductor's manual on Power Semiconductor Applications, Introduction to Power Semiconductors - Chapter 1. That link will take you to page 104 - Table of Contenets. Just go to page 10 - "Refinements to the Basic Structure" and start there.
