Thyristor Structure & Fabrication

- details of the structure and fabrication of a thyristor detailing how this affects its operation and circuit design

The thyristor structure is consists of four layers rather than the basic three layers used for a standard transistor.

The thyristor structure is relatively straightforward and normally relies on processes that are well established. As a result thyristors are plentiful and generally low cost.

Basic thyristor structure

The thyristor consists of a four layer p-n-p-n structure with the outer layers are referred to as the anode (p-type) and cathode (n-type). The control terminal of the thyristor is named the gate and it is connected to the p-type layer located next to the cathode.

Structure of a thyristor or silicon controlled rectifier, SCR
Structure of a thyristor or silicon controlled rectifier, SCR

As a result the thyristor has three junctions rather than the one junction of a diode, and two within transistors.

The three junctions are normally denoted as J1, J2, and J3. They are numbered serially with J1 being nearest to the anode.

Thyristor materials

Although it is possible to use a variety of different materials for thyristors, silicon is the most popular. The trade name for this type of device - silicon controlled rectifier - also indicates that silicon is the most popular material.

Silicon provides good thermal conductivity as well as a high voltage and current capability. Another advantage is that the processes for silicon are more mature, and hence cheaper to run, than those for other materials.

However, other materials including silicon carbide, SiC; gallium nitride, GaN; diamond, C; and semi-wide-gap semiconductor material gallium arsenide, GaAs as well, have been investigated and according to the research they demonstrated promising properties under extreme conditions of high power, high temperature and high frequency. Nevertheless silicon still remains the most popular substance.

Thyristor semiconductor structure and fabrication

The level of doping varies between the different layers of the thyristor. The cathode is the most heavily doped. The gate and anode are the next heavily doped. The lowest doping level is within the central n type layer. This is also thicker than the other layers and these two factors enable a large blocking voltage to be supported. Thinner layers would mean that the device would break down at lower voltages.

Thyristor structure at the semiconductor level
Thyristor structure at the semiconductor level

In view of the very high currents and power levels that some thyristors are used to switch, thermal considerations are of paramount importance. The anode of the SCR or silicon controlled rectifier is usually bonded to the package since the gate terminal is near the cathode and needs to be connected separately. This is accomplished in such a way that heat is removed from the silicon to the package. Apart from the internal considerations, the external heat-sinking considerations for the thyristor must be carefully implemented otherwise the device may overheat and fail.

Asymmetric thyristor structure

The asymmetric thyristor is characterised by what is termed a cathode short and an anode short. It can be seen from the diagram that both the cathode and anode connections connect to n+ and the p regions in the case of the cathode and the p+ and n regions on the case of the anode.

The "short" between the p and n regions has the effect of adding a resistor between the junctions, i.e. cathode to gate in the case of the cathode connection. This has a variety of effects including reducing carrier lifetime and improving the transient response time.

Asymmetric thyristor structure at the semiconductor level
Asymmetric thyristor structure at the semiconductor level

By Ian Poole

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