The following presentation is a part of the level 4 module -- Electrical and Electronic Principles. This resources is a part of the 2009/2010 Engineering (foundation degree, BEng and HN) courses from University of Wales Newport (course codes H101, H691, H620, HH37 and 001H). This resource is a part of the core modules for the full time 1st year undergraduate programme. The BEng & Foundation Degrees and HNC/D in Engineering are designed to meet the needs of employers by placing the emphasis on the theoretical, practical and vocational aspects of engineering within the workplace and beyond. Engineering is becoming more high profile, and therefore more in demand as a skill set, in today’s high-tech world. This course has been designed to provide you with knowledge, skills and practical experience encountered in everyday engineering environments.

1. Four Layer Devices Electrical and Electronic Principles © University of Wales Newport 2009 This work is licensed under a Creative Commons Attribution 2.0 License .

3. A four-layer device has two names: Thyristor Silicon Controlled Rectifier or SCR It is a P N P N structure as shown below: Note this device is a three terminal device with an anode and cathode like a diode but with the addition of a gate connection. Four Layer Devices p p n n ANODE A GATE G CATHODE K

4. Its operation is best defined by splitting the device in the following way: Four Layer Devices p p n n ANODE A GATE G CATHODE K p ANODE A n CATHODE K n n p p GATE G

5. This can be thought of as two interconnected transistors – an NPN and a PNP as shown below. Q 1 is the pnp transistor and Q 2 is the npn transistor. The first thing to note is that with the anode negative with respect to the cathode the device will never conduct as conduction in the pnp is only from emitter to collector and in the npn only from collector to emitter Four Layer Devices ANODE A CATHODE K GATE G Q 1 Q 2

6. With the anode positive with respect to the cathode, there again will be no current flow as: I B1 = 0  I C1 = 0  I B2 = 0  I CE2 = 0  I B1 = 0. If now a voltage is applied to the gate: we will generate a base current for Q 2 (I B2 ) has a value this causes a current to flow through Q 2 (I CE2 ) its value will be larger than I B2 and will form the base current for Q 1 (I B1 ) this leads to a current flow through Q 1 (I EC1 ) this forms the base current for Q 2 . The net effect is a large current flow through the device from anode to cathode. Four Layer Devices

7. If the gate voltage is removed the flow from Q 1 keeps the device conducting and in fact the only way to turn it off is to take the anode cathode voltage to 0V. The device is essentially a diode whose forward bias switch on can be controlled by an input on its gate. Once turned on though it cannot be simply turned off. Symbol Four Layer Devices A K G

8. Characteristic If the anode cathode voltage is taken high enough the thyristor will fire by itself. The normal working voltage would be below this value and would also be below the reverse breakdown voltage also. Four Layer Devices

9. This resource was created by the University of Wales Newport and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. © 2009 University of Wales Newport Except where other wise noted, this work is licensed under a Creative Commons Attribution 2.0 License . The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher. The name and logo of University of Wales Newport is a trade mark and all rights in it are reserved. The name and logo should not be reproduced without the express authorisation of the University. Four Layer Devices