Thyristors

Zheng Yang(ERF 3017, email: yangzhen@uic.edu)

ECE442 Power Semiconductor Devices and Integrated Circuits

BackgroundThe silicon controlled rectifier (SCR) or thyristor proposed by William Shockley in 1950 at Bell

Labs was developed in 1956 by power engineers at General Electric (GE) led by Gordon Hall and commercialized by GE's Frank Gutzwiller. The thyristor exhibits bi-stable characteristics allowing operation in either a blocking mode with low off-state current or a current conduction state with low on-state voltage drop. The power thyristor provides both forward and reverse voltage-blocking capability making it well suited for AC power circuit applications. The device can be triggered from the forward-blocking off-state to the on-state by using a relatively small gate control current. Once triggered into the on-state, the thyristor remains stable in the on-state even without the gate drive current. In addition, the device automatically switches to the reverse-blocking off-state upon reversal of the voltage in an AC circuit. These features greatly simplify the gate control circuit, relative to that required for the power transistor, reducing its cost and size. Furthermore, a structure, called the TRIAC, has been developed that enables triggering the device into the on-state even during operation in the third quadrant.

The thyristor contains two coupled bipolar transistors that provide an internal positive feedback mechanism that allows the device to sustain itself in the on-state. This internal feedback mechanism makes it difficult to turn off the structure by external means. To enable operation at elevated temperatures, it is necessary to short circuit the emitter and base regions of the thyristor. To solve this problem, a structure, called the gate turn-off thyristor (GTO), was also developed in the 1960s. In this device, the structure is modified to enable the switching of the device from the on-state to the off-state while operating in the first quadrant. This is performed by the application of a large reverse gate drive current, akin to that used for turning off the bipolar power transistor. In spite of the bulky and expensive gate control circuits required for the GTO, it was widely adopted for the control of motors in traction (electric streetcars and electric locomotives) applications until recently. The scaling of the power-handling capability of the insulated gate bipolar transistor (IGBT) to handle very high (megawatt) power levels in the twenty-first century has resulted in the replacement of these devices by the IGBT in traction applications.

Evolution of thyristor current rating

The ability to control operation between blocking states and on-states for a thyristor by using a third terminal was first reported in the 1950s. The extensive application potential for these devices to home appliances and power distribution systems generated strong interest in making improvements in the power ratings for the thyristors. The growth of the current-handling capability for the power thyristors is shown in the figure above. Starting with a modest current of 100 A in the 1950s, the current rating has been scaled to approach 5,000 A for a single device. These high-current levels are required for power distribution systems such as high-voltage DC transmission networks. From the figure, it can be observed that the most rapid increase in the current-handling capability took place at the end of the 1970s. This outcome can be traced to the development of the neutron transmutation doping (NTD) process in the mid-1970 time frame. Using the NTD process, it became possible to obtain larger diameter silicon wafers with uniform properties enabling the observed scaling of the current-handling capability of thyristors.

Evolution of thyristor voltage rating

Beginning with devices capable of operating up to a few hundred volts in the 1950s, the voltage rating for thyristors has been escalated to 8,000 V. The increase in the voltage rating had to be accomplished by the availability of higher resistivity silicon wafers. This was initially achieved by the development of the float-zone process. However, the resistivity variation produced by this process was inadequate for utilization in the large diameter wafers desired to increase the current ratings. The NTD process was instrumental in providing the breakthrough required to create large diameter silicon wafers with low N-type doping concentration and high uniformity in the resistivity across the wafers. Consequently, a substantial gain in the voltage rating occurred in the late 1970s after the commercial availability of NTD silicon as indicated in the figure above. Today, single thyristors are available with the capability to block over 8,000 V and conduct 5,000 A in the on-state. Consequently, a single thyristor device can control 40 MW of power. Such devices are attractive for power distribution networks.

Type of thyristor devices Convertional Thyristor, also called Silicon Controlled Rectifier (SCR) (covered

by textbook, to discuss this class)

Gate Turn-Off Thyristor (GTO) (covered by textbook, to discuss this class)

TRIAC (triode for alternating current) (covered by textbook, to discuss this class)

MOS-Controlled Thyristor (MCT) and MOS-gated thyristor (discussed in optional class)

Base Resistance-controlled Thyristor (BRT) (discussed in optional class)

Emitter Switched Thyristor (EST) or Emiter Turn-Off Thyristor (ETO) (discussed in optional class)

Integrated gate-commutated thyristor (IGCT) (not covered, not to discuss in class)

Light-Triggered Thyristor (LTT) or Light-Activated SCR (LASCR) (covered by textbook but not to discuss in class)

Reverse Conducting Thyristor (RCT) (not covered, not to discuss in class)

Structure of thyristor

The basic structure for an N+-P-N-P+ power thyristor is illustrated in the figure above. The structure is usually constructed by starting with a lightly doped N-type silicon wafer whose resistivity is chosen based upon the blocking voltage rating for the device. The anode P+ region is formed by the diffusion of dopants from the backside of the wafer. The P-base and N+ cathode regions are formed by the diffusion of dopants from the front of the wafer. Electrodes are formed on the front side of the wafer to contact the cathode and P-base regions, and on the backside of the wafer to contact the anode region. No contact electrode is usually attached to the N-drift (N-base) region.

The output characteristics for the thyristor structure are illustrated in the figure above. The thyristor structure contains three P-N junctions that are in series. When a negative bias is applied to the anode terminal of thedevice, the P+ anode/N-drift junction (J1) and the N+ cathode/P-base junction (J3) become reverse biased while the P-base/N-drift junction (J2) becomes forward biased. Due to high doping concentrations on both sides of the N+-cathode/P-base junction (J3), it is capable of supporting less than 50 V. Consequently, most ofnegative bias applied to the anode terminal is supported by the P+ anode/N-drift junction (J1). The reverse-blocking voltage capability for the device is determined by the doping concentration and thickness of the N-drift region. The width of the N-drift region between these two junctions must be carefully optimized to maximize the blocking voltage capability and minimize the on-state voltage drop.

Output characteristics of thyristor

When a positive bias is applied to the anode terminal of the thyristor, the P+ anode/N-drift junction (J1) and the N+ cathode/P-base junction (J3) become forward biased while the junction (J2) between the P-base region and the N-drift region becomes reverse biased. The applied positive bias is mostly supported across the N-drift region. As in the case of reverse-blocking operation, the blocking voltage capability is determined by open-base transistor breakdown rather than avalanche breakdown. Although not perfectly symmetrical, the reverse- and forward-blocking capability for the thyristor structure are approximately equal making it suitable for use in AC power circuits.

Output characteristics of thyristor (cont’d)

The thyristor contains two coupled bipolar transistors that provide an internal positive feedback mechanism. Current flow through the thyristor can be induced by considering a current supplied through the gate terminal to trigger the device into its on-state. The gate current forward biases the N+-cathode/P-base junction (J3) to initiate the injection of electrons. The injected electrons trigger a positive feedback mechanism produced by the two coupled bipolar transistors within the thyristor structure. The first bipolar transistor is an N-P-N transistor formed between the N+-cathode/P-base/N-drift regions while the second bipolar is a P-N-P transistor formed between the P+-anode/N-drift/P-base regions in the equivalent circuit of thyristor above. Once current flow is initiated through the transistors, they are able to provide the base drive current for each other by a process referred to as the regenerative action. In this process, the collector current of the N-P-N transistor provides the base drive current for the P-N-P transistor and the collector current of the P-N-P transistor provides the basedrive current for the N-P-N transistor. The regenerative action inherent within the thyristor structure allows stable operation of the device in its on-state without any external gate drive current. This is one of its advantages when compared with the bipolar transistors.

Equivalent circuit of thyristor

Once the thyristor is operating in its on-state, the i-v characteristics can be shown to become similar to that for a P-i-N rectifier, resulting in the anode current increasing exponentially with the on-state voltage drop. Consequently, thyristors can be designed with very high-voltage blocking capability with low on-state voltage drops making them excellent power devices for circuits used in power distribution systems. The power thyristor can be switched from its on-state to the blocking state by reversing the bias applied to the anode electrode. The reverse bias applied to the anode electrode forces the thyristor to undergo a reverse recovery process similar to that observed in a P-i-N rectifier. Once the thyristor has entered the reverse-blocking mode, a positive voltage can once again be applied to the anode without turning on the device until a gate control signal is applied.

Equivalent circuit of thyristor (cont’d)

As discussed early the thyristor structure contains a set of coupled transistors that provide a regenerative action during the conduction of current in the on-state. These devices are designed for operation in AC circuits where the anode voltage cycles between positive and negative values. The regenerative action is disrupted whenever the anode voltage reverses from positive to negative. The turn-off of the device then occurs with a reverse recovery process to establish blocking voltage capability. Such device structures are not suitable for applications in DC circuits unless expensive commutation circuits are added to reverse the anode voltage polarity.

Until the development of the IGBT, power bipolar transistors were the device of choice for DC circuits. The current gain and on-state characteristics of bipolar power transistors degrade rapidly with increasing voltage rating. This has precluded the development of devices with voltage ratings above 2,000 V applications. The development of a thyristor structure that can be designed to turn on and turn off the current flow under control by a gate signal in a DC circuit was motivated by this need. Such thyristors have been named gate turn-off (GTO) thyristors. The GTO is turned on in the same manner as the thyristor structures described early. The turn-off for the GTO is accomplished by the application of a large reverse gate current. The gate current must be sufficient to remove the stored charge from the P-base region and disrupt the regenerative action of the internal coupled transistors.

Gate Turn-Off (GTO) Thyristor

Although the basic structure for the GTO thyristor similar to the conventional thyristor structure, it is important to note that the GTO structure does not contain cathode shorts. In addition, the width of the cathode region for the GTO structure is made much shorter than for the conventional thyristor to facilitate turning off the anode current. The electric field profile for this structure is shown on the right-hand side of the figure during the forward-blocking mode of operation. The voltage is supported across the P-base/N-base junction (J2). The forward-blocking capability is determined by the open-base breakdown voltage of the N-P-N transistor as described earlier for the conventional thyristor. The above structure has approximately the same reverse-blocking voltage capability when the voltage is supported by the P+-anode/N-base junction (J1).

Symmetric GTO thyristor structure

Since the GTO structure is intended for use in DC circuits, its reverse-blocking capability does not have to match its forward-blocking capability. An asymmetric GTO structure that takes advantage of this is illustrated in the figure above. Here, an N-buffer layer is added in the N-base region adjacent to the P+ anode region. The N-buffer layer has a much larger doping concentration than the lightly doped portion of the N-base region. These changes result in a trapezoidal shape for the electric field profile as illustrated on the right-hand side of the figure above. The same forward-blocking capability can be achieved for the asymmetric GTO structure with a smaller net thickness for the N-base region than necessary for the symmetric structure. This enables reduction of the on-state voltage drop. The presence of the N-buffer layer also reduces the current gain of the P-N-P transistor.

Asymmetric GTO thyristor structure

Asymmetric GTO Structure

better forward-blocking capability (i.e., smaller net thickness for the N-base region than necessary for the symmetric structure; or say, larger blocking voltage when at the same thickness);

poor reverse-blocking capability (i.e., applicable for DC circuits only)

GTO: Symmetry vs Asymmetric

Symmetric GTO Structure

not as good forward-blocking capability as asymmetric GTO (due to the “non-punch-through” structure);

better reverse-blocking capability

[cited from “B. J. Baliga, “Trends in Power Semiconductor Devices”, IEEE Transactions on Electronic Devices 43(10), 1717-1731 (1996)]

Evolution of GTO thyristor structures

Power is most commonly delivered to applications using AC sinusoidal sources as in the case of residences. The thyristor can conduct current during the positive half-cycle of the AC voltage source after it is triggered at an appropriate time during the half-cycle. It supports the AC voltage during the negative half-cycle preventing power delivery to the load. More efficient operation is achieved by delivering power to loads during both half-cycles of the AC voltage source, with the current conduction time during both half-cycles controlled by the power circuit. This capability can be achieved by connecting two thyristors in a back-to-back configuration as shown as illustrated in the figure above. A thyristor-based device structure that has been developed with these features is referred to as the triode AC switch or TRIAC.

Motivation of TRIAC

The TRIAC structure illustrated in the figure above can be considered to be the monolithic combination of two thyristors in a back-to-back configuration. However, to create this structure, it is necessary to form two P-base regions on opposite surfaces of the structure. This introduces a lightly doped portion to the anode region for both the thyristors. The gate region is formed on only one side of the device. The gate electrode makes contact to the P-base region as in the case of conventional thyristor structures. Cathode shorts are incorporated for both thyristors within the triac to obtain a good blocking voltage capability.

TRIAC Structure and Output Characteristics

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Summary

The physics of operation of the thyristor structure has been described in this chapter. The thyristor structure contains two coupled transistors that create a regenerative action within the device that can sustain on-state current flow with no external stimulus. Very high-voltage devices can be synthesized with low on-state voltage drop and high-current handling capability. Thyristors are extensively used in HVDC transmission systems. They are also employed for driving very high power motors.

A thyristor structure (GTO) capable of turning off current in DC power circuits has been developed for motor drives used in traction (electric locomotives) applications. These devices found extensive use in high speed rail transport systems until recently. The development of the IGBT in the early 1980s followed by scaling its voltage- and current-handling capability in 1990s has resulted in the replacement of the GTO in traction applications. However, the physics of operation for the thyristor continues to have relevance because a four-layer (parasitic thyristor) device is formed within the IGBT structure. To learn and understand IGBT requires a good understanding of thyristor physics.

Sections discussed: 8.1, 8.7.1, 8.8.1 and 8.9

Sections not discussed: 8.2-8.6, 8.72-8.79, 8.82-8.84