Thyristor / SCR

Thyristor (often called as a Silicon Controlled Rectifier, SCR) – a semiconductor, bistate (on, off) electronic component. Its principle of operation resembles a semiconductor diode or BJT transistor in a switch configuration. However, SCRs are not fully controllable switches, because they can’t be turned off at will. Once they’ve been turned on by a current trigger from a gate terminal, they can’t be turned off by the gate, even after removing its signal. Thyristor will stay in the conducting state until a turn-off condition occurs (reverse bias of the thyristor or its forward current decrease under certain threshold level called ”holding current”). However, there is also thyristor called GTO Thyristor (Gate Turn Off), which can be turned on and off (reverse current) by a gate signal. This component can therefore be called as a  semiconductor diode, that can be turned on.

The first commercially available SCR thyristors appeared on the market in 1956, while the GTO thyristor was invented in 1962 by General Electric.

Fig. 1. Thyristor (SCR) symbol (same as GTO).

Thyristor – Division

The thyristor can be divided mainly due to its properties and then by the number of its terminals. A distinction is made between the following four types of thyristors (Fig. 3.):

• Dynistor – unidirectional two-terminal element,
• Triode thyristor (SCR) – unidirectional three-terminal element,
• DIAC (Diode for Alternating Current) – bidirectional two-terminal element,
• TRIAC (Triode for Alternating Current) – bidirectional three-terminal element.

From aforementioned thyristors, a triode thyristor, widely known as the SCR thyristor, is most commonly used.

Fig. 2. Example of how real SCR component in typical enclosure can look in reality. Yellow lead from the gate terminal makes it easier to recognize.

The ‘thyristor’ name has traditionally been preserved in a narrow sense, including only a three-terminal switching element operating in only one-way. As you can see in Figure 3, the name of the thyristor has a wider meaning.

Fig. 3. There are many types of thyristors, depending on the number of terminals and the shape of the current-voltage characteristics in the 3rd quarter of the I(V) characteristics.

Thyristor – Internal construction

SCR thyristor consists of three (or more) junctions, hence the same with four (or more) semiconductor layers differently doped, in the “p-n-p-n” system. It has 3 electrodes attached to it, two of them are connected to the external layers of the component (anode and cathode), and the third electrode to one of the middle layers – the “p-type” layer (gate).

Fig. 4. Internal structure of the Thyristor (SCR).

In the Figure 5 below, there is a division of thyristors due to the direction of operation:

Fig. 5. Classification of the thyristors (SCR).

Thyristor – Principle of operation

Thyristor can simultaneously work only in one of the three following operating states:

• Forward blocking mode – thyristor is blocking forward current conduction that would normally be carried by a forward biased diode. Junctions J₁, J₃ are forward-biased and the J₂ junction is reverse-biased,
• Forward conducting mode – thyristor has been triggered by the gate signal into conducting state. It will remain conducting until the forward current drops below a threshold value known as the “holding current.”
• Reverse blocking mode – thyristor blocks the current in the same way as in the reverse biased diode. Current flowing through the thyristor in this operating state is a very small reverse current of J₁, J₃ junctions.
• As the voltage increases, at a certain value it will exceed a breakdown voltage level of the J₁ junction, then the J₃ junction will also be broken down. Thyristor characteristics in the reverse-bias range do not differ from those of the semiconductor diode.

SCR thyristor conducts current from the anode to the cathode, similar to the semiconductor diode. If anode is at higher energetic state (positive) than the cathode (negative), the external “p-n” junctions are forward-biased and the “n-p” middle junction is reverse-biased.

As long as there is no positive voltage applied to the SCR’s gate, it won’t conduct the current. Supplying positive voltage to the gate will cause the ‘gate current’ to flow and the thyristor will be triggered. This is also called as the “firing” of the thyristor.

In contrast to the BJT transistor, the triggered thyristor still conduct current after cutting off the gate current, which is its unquestionable advantage in some applications, which reduces the total current consumption in the circuit. It loses these properties only after the loss of the load current (below the conduction current value, minimum holding current value) or with the reverse-bias of the electrodes. Then it is necessary to fire the thyristor again.

Fig. 6. A two- BJT transistor model as an example of the thyristor’s operation principle.

Thyristor – Current-voltage characteristics

Figure 7 at them bottom presents a Current-Voltage characteristic of the SCR:

Fig . 7. Current-Voltage characteristics IA(VAC) of the SCR thyristor for different values of the gate current IG. In the figure SCR’s threshold voltage VT and its holding current IH are also marked.

Thyristor – Applications

Due to their principle of operation, thyristors are generally used in power systems to control high power with smaller ones. More applications are listed down below:

• AC power controlling (light dimmers, adjusting electric motor speed etc.),
• Overvoltage protection for power supplies,
• HVDC (High Voltage Direct Current) power transmission,
• AC power switching circuits (SCRs can withstand reverse voltages well),
• Oscillators,
• Inverters.

Source: W. Marciniak: “Przyrządy półprzewodnikowe i układy scalone”, WNT, Warszawa 1984