Chapter 10: Thyristors

Chapter 10: Thyristors
Thyristors
Thyristors are a class of semiconductor devices characterized by 4-layers of alternating p- and n-material. Four-layer devices act as either open or closed switches; for this reason, they are most frequently used in control applications such as lamp dimmers, motor speed controls, ignition systems, charging circuits, etc. Thyristors include Shockley diode, silicon-controlled rectifier (SCR), diac and triac. They stay on once they are triggered, and will go off only if current is too low or when triggered off. Some thyristors and their symbols are in figure 1.


Figure 1

Shockley Diode
The 4-layer diode (or Shockley diode) is a type of thyristor that acts something like an ordinary diode but conducts in the forward direction only after a certain anode to cathode voltage called the forward-breakover voltage is reached. The basic construction of a 4-layer diode and its schematic symbol are shown in Figure 2.

Figure 2: The 4-layer diode.

The 4-layer diode has two leads, labeled the anode (A) and the cathode (K). The symbol reminds you that it acts like a diode. It does not conduct when it is reverse-biased.
The concept of 4-layer devices is usually shown as an equivalent circuit of a pnp and an npn transistor. Ideally, these devices would not conduct, but when forward biased, if there is sufficient leakage current in the upper pnp device, it can act as base current to the lower npn device causing it to conduct and bringing both transistors into saturation

Figure 3: A 4-layer diode equivalent circuit.

Shockley Diode Characteristic Curve
The characteristic curve for a 4-layer diode shows the forward blocking region. When the anode-to-cathode voltage exceeds VBR, conduction occurs. The switching current at this point is IS. Once conduction begins, anode current (IA) increases rapidly and will continue until IA is reduced to less than the holding current (IH). This is the only way to stop conduction.

Figure 4: A 4-layer diode characteristic curve.

The Silicon-Controlled Rectifier
An SCR (silicon-controlled rectifier) is a 4-layer pnpn device similar to the 4-layer
diode except with three terminals: anode, cathode, and gate. The basic structure and schematic symbol of SCR are shown in Figure 5.