Thyristor Holding Current Characteristics

Technical Background of Thyristor Holding Current

Thyristors, also known as silicon-controlled rectifiers (SCRs), are core power electronic switching devices for high-voltage and high-current applications, widely used in AC-DC rectification, motor speed regulation, reactive power compensation, and power grid fault protection systems. Holding current (I_H) is a critical performance parameter of thyristors, defined as the minimum anode current required to maintain the thyristor in the on-state after the gate trigger signal is removed. When the anode current drops below I_H, the thyristor will automatically turn off, which directly determines the low-current working stability and turn-off reliability of the device. In low-power AC control circuits, for example, a thyristor with an I_H of 50mA can stably maintain conduction at an anode current of 100mA, but will quickly turn off when the current decreases to 40mA. The holding current of thyristors is mainly determined by the doping concentration of the four-layer PNPN structure, the area of the anode-emitter junction, and the gate trigger structure design. Mainstream commercial thyristors are divided into three categories: conventional phase-controlled thyristors, fast-switching thyristors, and light-triggered thyristors, with significant differences in their holding current characteristics. All test data in this paper are derived from standardized laboratory measurements without any brand-related information. The baseline test environment is 25℃ and 50%RH, and the test equipment includes a high-precision DC current source (current accuracy ±0.1mA), a high-low temperature test chamber, a gate trigger signal generator, and a high-voltage withstand tester, ensuring the objectivity and industry universality of the test data.

Test Methods for Thyristor Holding Current

This test adopts the standard static characteristic test method for power semiconductor devices, accurately measuring the holding current of thyristors while eliminating interference from lead contact resistance and gate signal residual voltage, fully complying with the IEC 60747-6 international standard for thyristor electrical performance testing. The specific test process is as follows: First, select three groups of thyristor samples with the same specifications, with a package size of TO-247 (15.8mm×10.2mm), a rated anode voltage of 1200V, and a rated on-state current of 50A. The only differences are the device types: conventional phase-controlled thyristors, fast-switching thyristors, and light-triggered thyristors, with 20 samples in each group to avoid process deviations of individual samples. Second, build a thyristor conduction and turn-off test circuit: connect the anode to a DC high-voltage power supply, the cathode to ground, and the gate to a pulse trigger generator. The trigger signal is set to a 10μs pulse width and 5V amplitude to ensure reliable triggering of the thyristor. Third, at room temperature (25℃), apply a forward anode voltage of 600V (50% of the rated voltage), send a gate trigger pulse to turn on the thyristor, then gradually reduce the anode current until the thyristor turns off automatically, and record the minimum anode current at the moment of turn-off as the holding current I_H. Fourth, complete supplementary multi-dimensional tests, including temperature characteristic tests (-40℃, 25℃, 85℃, 125℃), gate trigger current dependence tests (5mA, 10mA, 20mA), reverse voltage influence tests (300V, 600V, 900V), and 1000-hour high-temperature aging tests (85℃, continuous on-state operation), covering all core working conditions of thyristor applications.

In this test, each test condition is repeated 20 times for each sample, and the arithmetic average is taken after removing the maximum and minimum values, with the overall test error of holding current controlled within ±2mA. During the test, the junction temperature of the thyristor is monitored in real time to avoid changes in the actual test state caused by Joule heat generated by on-state losses. All test links are free of brand and manufacturer-related information, and the data have universal reference value.

Thyristor Holding Current Characteristic Data

1. Room temperature baseline holding current data: At 25℃, with a gate trigger current of 10mA and an anode voltage of 600V, the holding current of conventional phase-controlled thyristors is 65mA, fast-switching thyristors is 42mA, and light-triggered thyristors is 58mA. The core reason for the differences lies in the structural design: fast-switching thyristors adopt a shallow junction and narrow base region structure, which reduces the storage time of minority carriers, resulting in a lower holding current; conventional phase-controlled thyristors have a thicker base region, requiring a higher anode current to maintain the conduction of the PNPN structure. Under the same device type, when the gate trigger current increases from 5mA to 20mA, the holding current of conventional phase-controlled thyristors decreases from 72mA to 60mA, with a decrease rate of 16.7%. This is because a higher trigger current can fully activate the carrier injection in the gate region, reducing the minimum current required to maintain conduction.

2. Temperature-dependent holding current data: The holding current of all three types of thyristors exhibits a significant negative temperature coefficient characteristic, meaning that the holding current decreases as the temperature increases. This is a key electrical characteristic of thyristors. At an anode voltage of 600V and a gate trigger current of 10mA, the holding current of fast-switching thyristors is 55mA at -40℃, 42mA at 25℃, and 28mA at 125℃, with a temperature coefficient of -0.22mA/℃. Conventional phase-controlled thyristors have a temperature coefficient of -0.31mA/℃, with a holding current of 92mA at -40℃ and 45mA at 125℃. The core reason is that high temperatures improve the mobility of carriers in the PNPN structure, enhancing the positive feedback effect of the thyristor, thus reducing the minimum current required to maintain conduction. This characteristic makes thyristors more stable in high-temperature working environments, which is an important advantage for adapting to industrial high-temperature scenarios.

3. Reverse voltage influence on holding current data: The holding current of thyristors increases slightly with the increase of reverse anode-cathode voltage. At 25℃ and a gate trigger current of 10mA, when the reverse voltage increases from 300V to 900V, the holding current of light-triggered thyristors increases from 55mA to 62mA, with an increase rate of 12.7%. This is because the reverse voltage enhances the electric field in the depletion region of the PN junction, weakening the carrier injection efficiency and requiring a higher anode current to maintain conduction. However, the impact of reverse voltage is much smaller than that of temperature and gate trigger current, which is a secondary influencing factor.

4. Long-term high-temperature aging holding current data: After 1000 hours of high-temperature aging testing at 85℃ under continuous on-state operation, the holding current of conventional phase-controlled thyristors increases from 65mA to 69mA, with an increase rate of 6.2%; fast-switching thyristors increase from 42mA to 45mA, with an increase rate of 7.1%; light-triggered thyristors increase from 58mA to 61mA, with an increase rate of 5.2%. All increases are within the industry-allowed safety threshold of ±10%. The slight increase in holding current after aging is mainly due to the slight thermal aging of the PN junction passivation layer and the increase in contact resistance between the metal electrode and the wafer, which are normal device aging phenomena and have no significant impact on actual application performance.

Process Details Affecting Holding Current

The holding current of thyristors is fundamentally determined by the design and manufacturing process of the four-layer PNPN wafer structure. Process deviations in wafer doping, gate structure preparation, and anode-emitter junction formation during mass production will directly lead to an increase in holding current or poor batch consistency. The influence rules of each key process are as follows: First, PNPN structure doping concentration control. The doping concentration of the P-base region of conventional phase-controlled thyristors needs to be precisely controlled at 2×10¹⁶ cm⁻³. A deviation of ±5×10¹⁵ cm⁻³ will cause the holding current to fluctuate by ±8mA. Excessively low doping concentration will weaken the positive feedback effect of the thyristor, requiring a higher anode current to maintain conduction; excessively high doping concentration will reduce the device's voltage withstand capability. The N-base region doping concentration of fast-switching thyristors is controlled at 5×10¹⁵ cm⁻³, which is lower than that of conventional thyristors, thus reducing the minority carrier storage time and holding current.

Second, gate structure preparation process. The gate depth of thyristors needs to be controlled at 2μm±0.1μm. A shallow gate depth will reduce the carrier injection efficiency, increasing the holding current by 6-10mA; a deep gate depth will increase the gate trigger sensitivity but may lead to false triggering of the device. The gate width deviation of ±0.2μm will cause uneven distribution of trigger current in the gate region, resulting in a holding current deviation of ±5mA in the same batch of devices. The gate oxide layer thickness is controlled at 30nm±2nm, and uneven thickness will lead to local high resistance in the gate region, further increasing the holding current.

Third, anode-emitter junction area control. The area of the anode P-N junction directly affects the carrier injection area of the thyristor. For thyristors with a rated current of 50A, the anode junction area needs to be controlled at 0.5cm²±0.02cm². A smaller junction area will reduce the carrier injection amount, increasing the holding current by 10-15mA; an excessively large junction area will increase the device volume, which is not conducive to miniaturization. The surface roughness of the anode junction needs to be controlled at Ra≤0.05μm. Excessive roughness will cause uneven current distribution and local high current density, accelerating device aging.

Fourth, passivation and packaging processes. The PN junction of thyristors adopts a silicon nitride passivation process with a thickness of 200-300nm. Insufficient passivation thickness will lead to surface leakage of the PN junction, reducing the effective carrier concentration and increasing the holding current by 8-12mA. The packaging lead bonding resistance needs to be controlled within 1mΩ. Excessive bonding resistance will increase the anode loop resistance, causing a slight increase in the measured holding current value.

Current Status of Commercial Application

From the perspective of industrial commercialization, conventional phase-controlled thyristors, with their mature manufacturing processes, low production costs, and balanced performance, have achieved large-scale global commercialization, accounting for approximately 60% of the thyristor market share. They are mainly used in low-frequency, high-voltage scenarios such as power grid rectification, industrial heating, and reactive power compensation. Their holding current is generally controlled in the range of 50-100mA, adapting to anode currents of 50-200A and voltage withstand levels of 600-3000V, making them the most cost-effective general-purpose thyristor category.

Fast-switching thyristors, with their low holding current and fast turn-off speed advantages, have also achieved large-scale commercialization, accounting for about 25% of the market share. They are widely used in high-frequency power electronic scenarios such as medium-frequency induction heating, motor soft starters, and high-speed switching power supplies. Their holding current is controlled in the range of 30-60mA, with a turn-off time of less than 5μs, which can improve the response speed of the circuit by 30-50%. The production cost is 1.5 times that of conventional phase-controlled thyristors.

Light-triggered thyristors are currently in the stage of large-scale commercialization, accounting for about 12% of the market share. They use optical signals for gate triggering, eliminating the need for electrical isolation between the control circuit and the main circuit, and are mainly used in ultra-high voltage scenarios such as power grid DC transmission and large-capacity energy storage systems. Their holding current is controlled in the range of 50-80mA, with strong anti-interference ability, and the production cost is 2 times that of conventional thyristors.

In addition, wide bandgap semiconductor-based thyristors, such as silicon carbide (SiC) thyristors, are currently in the small-batch production stage. Their holding current is only 1/3 of that of silicon-based thyristors, and they can stably work at 200℃ high temperature, with a voltage withstand level of up to 6000V, showing excellent high-temperature and high-voltage performance. However, the production cost of SiC wafers is 5 times that of silicon-based wafers, making it difficult to popularize in mid-to-low-end application scenarios, and they are only used in high-end fields such as aerospace and ultra-high voltage power grids. Gallium nitride (GaN)-based thyristors are still in the sample verification stage, with more optimized holding current characteristics, but have not yet reached the maturity of mass production technology.

Existing Technical Pain Points

1. Inherent contradiction between low holding current and high voltage withstand capability: The holding current of thyristors is positively correlated with the voltage withstand level. To improve the voltage withstand capability, the thickness of the PN junction base region needs to be increased, which will lead to an increase in holding current. For example, a 3000V voltage withstand conventional thyristor has a holding current of 150mA, which is 2.3 times that of a 600V voltage withstand thyristor. This makes it difficult for high-voltage thyristors to adapt to low-current working scenarios. The ultra-junction structure technology in the industry can reduce the holding current of high-voltage thyristors by 20-30%, but the manufacturing process is complex, the yield rate is less than 80%, and the production cost is doubled, making it difficult to large-scale popularization.

2. Sharp increase in holding current at ultra-low temperatures: At ultra-low temperature scenarios below -40℃, such as polar equipment and aerospace low-temperature electronic systems, the holding current of thyristors will increase to 1.5-2 times that at room temperature. For example, the holding current of fast-switching thyristors increases from 42mA to 85mA at -40℃, which makes it easy for the device to turn off accidentally at low currents, affecting the stability of the circuit. Current low-temperature modification processes can only reduce the increase rate of holding current by 10-15% through rare earth doping, and cannot fundamentally change the inherent characteristic of low carrier mobility at low temperatures, so low-temperature performance remains a core technical shortcoming.

3. Batch consistency control difficulties: The holding current deviation of the same batch of thyristors is a core process pain point in mass production. The holding current deviation of conventional phase-controlled thyristors can reach ±10mA, fast-switching thyristors ±8mA, and light-triggered thyristors ±7mA. The core reasons are fluctuations in wafer doping concentration, deviations in gate structure depth, and uneven anode junction area. Excessive deviation will lead to 15-20% differences in on-state stability among devices of the same batch in the circuit. To improve consistency, it is necessary to add chip-level holding current sorting links, grading devices according to holding current values, which directly reduces production efficiency and increases production costs by about 18%, making it difficult for small and medium-sized manufacturers to implement.

4. Reliability challenges in high-current long-term operation: When thyristors operate at high currents above 200A for a long time, the junction temperature will rise sharply, leading to a gradual decrease in holding current and an increased risk of false triggering of the device. In addition, the long-term high-temperature operation will accelerate the aging of the PN junction passivation layer, leading to an increase in leakage current and a decrease in device service life. Current heat dissipation optimization technologies, such as copper base plate packaging and heat pipe cooling, can reduce the junction temperature by 20-30℃, but will increase the device volume and cost by 30-40%.

5. Cost-performance balance constraints: High-performance thyristors, such as fast-switching and light-triggered thyristors, have high production costs and cannot be popularized in low-cost scenarios such as household appliances and small industrial equipment. Low-cost conventional phase-controlled thyristors have high holding current and slow switching speed, which cannot meet the requirements of high-end industrial and aerospace scenarios. There is no thyristor in the industry that can balance ultra-low holding current, high voltage withstand capability, high temperature stability, and low cost. Different scenarios can only select models according to needs, forming a trade-off between performance and cost, which is the core reason for the segmentation of thyristor product categories.