Four Layer Devices

1. Four Layer Devices Electrical and Electronic Principles

2. 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. In addition to the resource below, there are supporting documents which should be used in combination with this resource. Please see: Green D C, Higher Electrical Principles, Longman 1998 Hughes E , Electrical & Electronic, Pearson Education 2002 Hambly A , Electronics 2nd Edition, Pearson Education 2000 Storey N, A Systems Approach, Addison-Wesley, 1998 Four Layer Devices

3. ANODE A p n GATE G p n CATHODE K 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

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

5. ANODE A Q1 GATE G Q2 CATHODE K This can be thought of as two interconnected transistors – an NPN and a PNP as shown below. Q1 is the pnp transistor and Q2 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

6. With the anode positive with respect to the cathode, there again will be no current flow as: IB1 = 0  IC1 = 0  IB2 = 0  ICE2 = 0  IB1 = 0. If now a voltage is applied to the gate: we will generate a base current for Q2 (IB2) has a value this causes a current to flow through Q2 (ICE2) its value will be larger than IB2 and will form the base current for Q1 (IB1) this leads to a current flow through Q1 (IEC1) this forms the base current for Q2. The net effect is a large current flow through the device from anode to cathode. Four Layer Devices

7. A G K If the gate voltage is removed the flow from Q1 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

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