Multijunction devices: thyristors and IGBTs
A microsecond pulse on the gate and the thyristor keeps conducting by itself until the next zero crossing. It is the same mechanism that, when no one wants it, burns out a CMOS circuit in latch-up.
01Four layers, three junctions
A thyristor (or SCR, silicon-controlled rectifier) is a PNPN structure: a P anode, a wide, lightly doped N layer that withstands the voltage, a P layer that carries the gate terminal, and an N cathode. It has three junctions in series, and with the anode positive the middle one is reverse biased and blocks. So far it looks like a diode that does not conduct in either direction. The trick is that, once it starts to conduct, it keeps conducting by itself, even if the gate signal is removed.
It is a power device: there are ones of a few amperes for controlling a lamp and ones of several kilovolts and thousands of amperes, made on an entire silicon wafer, for high-voltage direct-current transmission stations.
02The two-transistor model
The PNPN structure can be cut into two interlocked transistors: a PNP formed by the first three layers and an NPN by the last three. The collector of each one feeds the base of the other. With common-base gains \( \alpha_1 \) and \( \alpha_2 \), the anode current becomes
That is positive feedback: a gate pulse makes the NPN conduct, its collector feeds the base of the PNP, the PNP returns more base current to the NPN, and within microseconds both are saturated. From that moment the gate no longer has control. The thyristor turns off by itself only when the anode current drops below the holding current \( I_H \), which in alternating current happens by itself at every zero crossing.
| Triggering method | What happens |
|---|---|
| Gate pulse | The normal, controlled way |
| Forward breakover voltage | The leakage current grows until \( \alpha_1 + \alpha_2 = 1 \) |
| Excessive \( dv/dt \) | The current that charges the capacitance of the central junction acts as gate current: an unwanted trigger |
| Temperature | Leakages and gains rise |
| Light | Pairs generated in the central junction; the thyristors of high-voltage valves are triggered by optical fiber |
03Phase control
On an AC mains, firing the thyristor with a delay \( \alpha \) relative to the zero crossing lets through only the final part of each half-cycle. By varying the angle, the power delivered to the load is regulated while dissipating almost nothing in the control. With a resistive load and full-wave control:
A 100 Ω resistive load on the 220 V, 50 Hz mains, controlled by a thyristor (half-wave) or by a TRIAC (full-wave). You choose the firing angle: the plot shows the mains, the voltage across the load and the gate pulses over one cycle.
04The four-layer family
| Device | Structure | Use |
|---|---|---|
| TRIAC | Two thyristors in antiparallel with a common gate | Dimmers, speed control of universal motors, solid-state relays |
| DIAC | Triggers in both directions when about 30 V is exceeded, with no gate | Generates the TRIAC trigger pulse in a dimmer |
| GTO | Thyristor that can be turned off with a large negative gate current | Electric traction and large converters, now almost replaced |
| TRIAC optocoupler | An LED that triggers a TRIAC by light, isolated | Drives a power TRIAC from a microcontroller |
The same four layers appear unintentionally in CMOS circuits: the latch-up of the CMOS inverter is a parasitic thyristor that triggers and short-circuits the supply, and the two-transistor model explains why it is avoided by placing substrate contacts close to every transistor: they lower the parasitic gains.
05The IGBT
The insulated-gate bipolar transistor (IGBT) combines a MOSFET at the input with a PNP bipolar at the output: the gate is driven by voltage, with almost no current, like a MOS, and the main current is conducted by a bipolar structure. It also has four layers, but it is designed so that the parasitic thyristor never triggers.
Its advantage is conductivity modulation: when conducting, the P collector layer injects holes into the lightly doped N region that withstands the voltage and fills it with carriers, greatly lowering its resistance. A 1200 V MOSFET has to have that lightly doped region thick and it conducts poorly; the IGBT, with the same region flooded with carriers, drops only about 1.5 to 2 V. The cost is the tail current at turn-off, while those carriers recombine, which limits the frequency to tens of kilohertz.
| Power MOSFET | IGBT | Thyristor | |
|---|---|---|---|
| Control | Voltage | Voltage | Current pulse; it cannot be turned off through the gate |
| Typical range | Up to a few hundred volts | 600 V to 6.5 kV | Up to more than 8 kV |
| Frequency | Hundreds of kilohertz | Up to tens of kilohertz | That of the mains |
| Use | Switching power supplies | Motor inverters, welders, induction cooktops | Phase control, high-voltage transmission |
06In the lab
Activities with 220 V are carried out with an isolation transformer, with the circuit mounted in an enclosure and under the supervision of the instructor. The oscilloscope is not connected directly to the mains: differential probes are used.
With a thyristor, a 12 V DC source and a 12 V lamp, trigger it with a pushbutton on the gate and verify that the lamp stays on when the button is released. Lower the voltage until it goes off and estimate the holding current.
Build a thyristor with a BC548 and a BC558 connected according to the model, and verify that it latches just like the integrated device.
With a commercial TRIAC and DIAC dimmer powered from a low-voltage isolation transformer, use the oscilloscope to observe the voltage across the load and measure the firing angle for three potentiometer positions.
07Common mistakes
- Expecting the thyristor to turn off when the gate is removed. It stays latched until the current drops below \( I_H \).
- Using an ordinary thyristor in direct current without planning how to turn it off. In DC the current never passes through zero.
- Forgetting the snubber network. Without an RC in parallel, the \( dv/dt \) of the mains or of the inductive load triggers the device unintentionally.
- Measuring with the oscilloscope referenced to earth on the mains. The probe ground short-circuits the circuit.
- Confusing the IGBT with a large MOSFET. It conducts with a nearly fixed drop, not with a resistance, and it has tail current at turn-off.
08Self-assessment
Why does the thyristor keep conducting without gate current?
Because each of the two transistors of the model supplies base current to the other; with \( \alpha_1 + \alpha_2 = 1 \) the feedback sustains itself.
How is a thyristor turned off in an AC circuit?
By itself: at every zero crossing the current drops below the holding current.
With a TRIAC and \( \alpha = 90° \), what fraction of the maximum power does a resistive load receive?
Half: \( (\pi - \pi/2 + 0)/\pi = 0.5 \).
What is conductivity modulation in the IGBT?
The flooding of the drift region with carriers injected by the P collector, which lowers its resistance when conducting.
What is the relationship between the thyristor and latch-up?
Latch-up is the triggering of a parasitic thyristor formed by the P and N regions of a CMOS circuit.
09Further reading
- B. Jayant Baliga. Fundamentals of Power Semiconductor Devices. 2nd ed., Springer, 2019. The reference on thyristors, IGBTs and power MOSFETs, written by the inventor of the IGBT.
- Muhammad H. Rashid. Power Electronics: Devices, Circuits, and Applications. 4th ed., Pearson, 2014. Phase control and trigger circuits, with many worked examples.
- Ned Mohan, Tore M. Undeland and William P. Robbins. Power Electronics: Converters, Applications, and Design. 3rd ed., Wiley, 2003. The devices seen from the converter: what each application demands.