THYRISTORThyristors or silicon controlled rectifiers (SCR) are find many uses in electronics, and in particular for power control. Thyristors or silicon controlled rectifiers, SCRs have even been called the workhorse of high power electronics. Thyristors are able to switch large levels of power are accordingly they used in a wide variety of different applications. Thyristors even finds uses in low power electronics where they are used in many circuits from light dimmers to power supply over voltage protection. The term SCR or silicon controlled rectifier is often used synonymously with that of thyristor - the SCR or silicon controlled rectifier is actually a trade name used by General Electric for a thyristor. Thyristor discovery The idea for the thyristor was first described by Shockley in 1950. It was referred to as a bipolar transistor with a p-n hook-collector. The mechanism for the operation of the thyristor was analysed further in 1952 by Ebers. Then in 1956 Moll investigated the switching mechanism of the thyristor. Development continued and more was learned about the device such that the first silicon controlled rectifiers became available in the early 1960s where it started to gain a significant level of popularity for power switching. Thyristor applications Thyristors, SCRs are used in many areas of electronics where they find uses in a variety of different applications. Some of the more common applications for thyristors are outlined below: AC power control (including lights, motors,etc). Overvoltage protection crowbar for power supplies. As a result the thyristor symbol shows the traditional diode symbol with a control gate entering near the junction. the thyristor will remain in conduction until the voltage across the anode and cathode is removed . Once fired. and then cut it off at the required time. cathode and gate. In operation. especially within AC scenarios. It will then require current in the gate circuit to fire the thyristor again. As might be expected the gate is the control terminal while the main current flows between the anode and cathode. Control elements in phase angle triggered controllers. As can be imagined from the thyristor symbol shown below. The next half cycle will be blocked as a result of the rectifier action. Thyristors are able to switch high voltages and withstand reverse voltages making them ideal for switching applications. AC power switching. . Within photographic flash lights where they act as the switch to discharge a stored voltage through the flash lamp. The thyristor device has three terminals: Anode. It requires a certain level of current to flow in the gate to "fire" the thyristor. it will only conduct for a maximum of half the cycle. reflecting thermionic valve / vacuum tube technology. Thyristor symbol The thyristor symbol used for circuit diagrams or circuit seeks to emphasis its rectifier characteristics while also showing the control gate. Thyristor basics The thyristor is a device that has a number of unusual characteristics. Therefore when the device is used with AC.this obviously happens at the end of the half cycle over which the thyristor conducts. the thyristor will not conduct initially. the device is a "one way device" giving rise to the GE name for it the silicon controlled rectifier. there is a form of thyristor called a reverse conducting thyristor. In addition to reducing the anode cathode voltage. except that the .these are variants of the basic thyristor component. The reverse conducting thyristor can be used where a reverse or freewheel diode would otherwise be needed. The structure of the GATT is similar to that of the standard thyristor. To assist in this process a negative gate voltage can sometimes be applied. This has an integrated reverse diode to provide conduction in the reverse direction. although there is no control in this direction. Gate Assisted Turn-Off Thyristor. RCT: Although thyristors normally block current in the reverse direction. the thyristor itself and the diode do not conduct at the same time. This means that they do not produce heat simultaneously. Within a reverse conducting thyristor. This reverse gate voltage helps in draining the minority carriers stored on the n-type base region and it ensures that the gate-cathode junction is not forward biased. but they offer different capabilities that can be used in various instances and may be useful for certain circuits. Reverse conducting thyristors are often used in frequency changers and inverters. As a result they can be integrated and cooled together.Thyristor symbol for circuit diagrams and schematics Other types of thyristor There is a number of different types thyristor . Reverse conducting thyristor. GATT: The GATT is used in circumstances where a fast turn-off is needed. The three junctions are normally denoted as J1. and two within transistors. and J3. Basic thyristor structure The thyristor consists of a four layer p-n-p-n structure with the outer layers are referred to as the anode (p-type) and cathode (n-type). See further page in this series more fully describing the GTO. J2. Asymmetric Thyristor: The asymmetric thyristor is used in circuits where the thyristor does not see a reverse voltage and therefore the rectifier capability is not needed. Gate Turn-Off Thyristor. The control terminal of the thyristor is named the gate and it is connected to the p-type layer located next to the cathode. As a result it is possible to make the second junction.there is no requirement to remove the anode cathode voltage. They are numbered serially with J1being nearest to the anode. GTO: The gate turn-off thyristor is sometimes also referred to as the gate turn off switch. The resulting n-base region provides a reduced Von as well as improved turn on time and turn off time. This device is unusual in the thyristor family because it can be turned off by simply applying a negative voltage to the gate . often referred to as J2 (see page on Thyristor structure) can be made much thinner. SCR As a result the thyristor has three junctions rather than the one junction of a diode.narrow cathode strips are often used to enable the gate to have more control because it is closer to the centre of the cathode. Thyristor materials . Structure of a thyristor or silicon controlled rectifier. Thyristor semiconductor structure and fabrication The level of doping varies between the different layers of the thyristor. The gate and anode are the next heavily doped. C. other materials including silicon carbide. . The trade name for this type of device . Another advantage is that the processes for silicon are more mature. This is also thicker than the other layers and these two factors enable a large blocking voltage to be supported. high temperature and high frequency.silicon controlled rectifier also indicates that silicon is the most popular material. and hence cheaper to run. Nevertheless silicon still remains the most popular substance. and semi-wide-gap semiconductor material gallium arsenide. The lowest doping level is within the central n type layer. silicon is the most popular. than those for other materials. Silicon provides good thermal conductivity as well as a high voltage and current capability. Thinner layers would mean that the device would break down at lower voltages. gallium nitride. diamond. Although it is possible to use a variety of different materials for thyristors. The cathode is the most heavily doped. GaAs as well. However. GaN. SiC. have been investigated and according to the research they demonstrated promising properties under extreme conditions of high power. This has a variety . It can be seen from the diagram that both the cathode and anode connections connect to n+ and the p regions in the case of the cathode and the p+ and n regions on the case of the anode. The "short" between the p and n regions has the effect of adding a resistor between the junctions. This is accomplished in such a way that heat is removed from the silicon to the package. thermal considerations are of paramount importance.e. The anode of the SCR or silicon controlled rectifier is usually bonded to the package since the gate terminal is near the cathode and needs to be connected separately. i. the external heat-sinking considerations for the thyristor must be carefully implemented otherwise the device may overheat and fail. Thyristor structure at the semiconductor level In view of the very high currents and power levels that some thyristors are used to switch. Apart from the internal considerations. Asymmetric thyristor structure The asymmetric thyristor is characterised by what is termed a cathode short and an anode short. cathode to gate in the case of the cathode connection. " The thyristor consists of four semiconductor regions . It will remain conducting until the forward current drops below a threshold value known as the "holding current.of effects including reducing carrier lifetime and improving the transient response time. .p-n-p-n. Asymmetric thyristor structure at the semiconductor level Thyristor theory and operation basics The thyristor has three basic states: Reverse blocking: In this mode or state the thyristor blocks the current in the same way as that of a reverse biased diode. The outer p region forming the anode. and the outer n region forming the cathode as shown below. Forward conducting: In this mode the thyristor has been triggered into conduction. Forward blocking: In this mode or state the thyristor operation is such that it blocks forward current conduction that would normally be carried by a forward biased diode. the cathode of . If a small current is passed through the gate electrode. i. The transistor with its emitter connected to the cathode of the thyristor is a n-p-n device whereas the transistor with its emitter connected to the anode of the SCR is a p-n-p variety.e. When this occurs it will cause the collector of TR2 to fall towards the voltage on the emitter. this will turn "on" the transistor TR2. This means that when a current starts to flow. The gate is connected to the base of the n-p-n transistor. Equivalent circuit of a thyristor or silicon controlled rectifier (SCR) This arrangement forms a positive feedback loop within the thyristor.Thyristor theoretical structure For the thyristor operation.and looking at the simplified block structure it can be seen that the device may be considered as two back to back transistors. As a result there is no complete path across the device. it quickly builds up until both transistors are fully turned on or saturated. When a voltage is applied across a thyristor no current flows because neither transistor is conducting. As a result it can be seen that the total current gain of the device exceeds one. In turn the output of the second transistor is fed back to the input of the first. The output of one transistor fed to the input of the second. Once switched on. This will cause current to flow in the emitter of TR2.typically 20. When this occurs it will cause current to flow through the base of TR1 and turn this transistor "on". This has the drawback that it renders the junction nearest to the cathode (normally referred to as J3) with a low breakdown voltage . while providing a good turn off characteristic for which a high doping level is needed. . Again the layers are p-n-p-n with the outside p layer providing the anode connection. . . . . not as reliable as that of a standard thyristor and small positive gate current must be maintained even after turn on to improve reliability.To attain high emitter efficiency. . and the outside n layer providing he cathode connection.the whole device. Gate turn-off thyristor structure Like the standard thyristor. there is a relatively small voltage between the terminals. As the gate-cathode behaves like PN junction. . Gate turn-off thyristor basics The gate turn off thyristor is behaves somewhat differently to a standard thyristor which can only be turned on and cannot be turned off via the gate. The gate turn off thyristor. In this way it only requires a small trigger pulse on the gate to turn the thyristor on. The turn on phenomenon in GTO is however.40 volts. This is to provide good emitter efficiency for which the doping level should be low. and it can also be turned-off by a gate signal of negative polarity. the cathode layer is highly doped to give an n+ region. the gate turn-off thyristor is a four layer device having three junctions. The doping level of the p region for the gate is graded. GTO can be turned-on by a gate signal. The device turn on is accomplished by a "positive current" pulse between the gate and cathode terminals. the thyristor can only be turned off by removing the supply voltage. Again this will now try to pull the voltage on the collector of TR1 towards its emitter voltage. causing its "on" state to be maintained. . As many devices may need to block voltages of several kilovolts. Another key parameter for a gate turn-off thyristor is the maximum forward blocking voltage. It can be thought of as being one PNP and one NPN transistor being connected in a regenerative configuration whereby once turned on the system maintains itself in this state.The gate electrode is often interdigitated to optimise the current turn=off capability. . This is determined by the doping level and thickness of the n type base region. the doping level of this region needs to be kept relatively low.e. High current devices. GTO are very similar to that of the ordinary thyristor. Gate turn-off thyristor structure Gate turn off thyristor operation Many aspects of the Gate turnoff thyristor. 1000A and above may have several thousand segments which are all connected to the common gate contact. i. but this mode would not be wanted for normal operation. The key capability of the gate turn-off thyristor is its ability to be turned off by the use of the gate electrode on the device. it is found that during the turn off phase of the GTO. and can cause device failure if the current is not extinguished quickly. current is crowded into higher and higher density current filaments in areas that are most remote from the gate region. The device turn off is achieved by applying a negative bias to the gate with respect to the cathode. To turn the device on it is necessary to inject current into gate circuit of the device. This pulls the collector of this transistor down towards the emitter voltage and in turn this turns on the other transistor . In this non-conducting state the gate turn-off thyristor is said to be in its forward blocking mode. and thus this feedback process ensures that once the gate turn-off thyristor like any other thyristor is turned on it remains on.TR2. Current would only flow if the voltage exceeded the breakdown voltage and current would flow as a result of avalanche action.TR1. no current will flow because neither device is turned on. This then stops the injection into the base region of TR1 and this prevents current flow in this transistor. When this is done.Equivalent circuit of a gate turn off thyristor When a potential is applied across the gate turn-off thyristor between the anode and cathode. The resulting voltage drop in the base starts to reverse bias the junction and thereby stopping the current flow in this transistor . The fact that TR1 is now switched on ensures current flows into the base of TR2. In terms of the physics of the turn off phase. . This extracts current from the base region of TR2. These high current density areas become hot. it turns on TR2 in the diagram. the gate turn-off thyristor enters its forward blocking state again. Accordingly the gate turn off thyristor is a useful tool for many applications. The gate turn off thyristor is similar to the ordinary thyristor in many ways. .When the current filaments are extinguished. but its capability of being able to be turned off by voltages on the gate provide more capability for the device and enable the gate turn off thyristor to be used in areas where the standard thyristor cannot be used. the overall current flow stops and the depletion layers around the junctions grow .