A transistor is a semiconductor device consisting of three semiconductor regions and their corresponding electrodes. It is classified into NPN type and PNP type. Its operation is based on the movement of carriers and current control. When the base and the emitter are forward-biased, most of the carriers in the emitter are injected into the base region, and then part of them diffuse to the collector region to form the collector current. Moreover, a small change in the base current can lead to a significant change in the collector current. The important parameters include current amplification factor, parameters related to cut-off and saturation characteristics, and frequency characteristic parameters, etc. It is applied in fields such as amplification circuits, switching circuits, and signal modulation and demodulation.

A thyristor is a semiconductor device with four layers and three PN junctions, having three electrodes. Its operation is based on the internal PN junctions and positive feedback. When the anode potential is higher than that of the cathode and there is no gate trigger signal, it is in the forward blocking state. Once a trigger signal is applied, it will conduct. After conduction, as long as the anode current is greater than the holding current, it will remain conducting even if the trigger signal disappears. Its characteristics include controllability, high voltage and current withstand capabilities, and specific turn-off characteristics. It is applied in fields such as power control, voltage regulation and stabilization in electronic circuits, and light control.

Surge protection is mainly achieved by using surge protection devices (SPD). Common SPDs include varistors, gas discharge tubes, and transient voltage suppressor diodes, etc. The resistance value of a varistor changes with the change of voltage. It exhibits a high resistance state under normal voltage. When the voltage exceeds its threshold value, the resistance drops rapidly, diverting the surge current to the ground, thus protecting the subsequent circuits. The gas discharge tube is insulated under normal circumstances. When the voltage reaches its breakdown voltage, the gas inside the gas discharge tube is ionized, forming a conductive path to release the surge energy to the ground. The transient voltage suppressor diode utilizes its reverse breakdown characteristic. When the surge voltage arrives, the diode breaks down in the reverse direction, clamping the surge voltage within a safe range.

The operation of circuits is easily threatened by various abnormal conditions. For overvoltage (transient overvoltage caused by lightning, etc., which can be dealt with by using transient voltage suppressors (TVS) and varistors; continuous overvoltage caused by power grid fluctuations, etc., relying on zener diodes), overcurrent (relying on fuses and circuit breakers), electrostatic discharge (using ESD diodes and setting up anti-static areas), and electromagnetic interference (relying on shielding enclosures externally and proper wiring and filtering internally), protective measures are of vital importance to ensure the normal operation of circuit components and equipment. 

A diode is a semiconductor with unidirectional conductivity, which is composed of a PN junction and two electrodes. When it is forward-biased, the majority carriers diffuse and conduct electricity with a voltage drop. When it is reverse-biased, the minority carriers conduct electricity with a small leakage current. If the reverse voltage exceeds the breakdown voltage, the current will increase sharply and the diode is prone to damage. It has parameters such as forward voltage drop, reverse breakdown voltage and maximum forward current. Rectifier diodes are used to convert alternating current into direct current. Light-emitting diodes (LEDs) emit light when powered on and are applied in lighting, display and other fields. Zener diodes can stabilize specific voltages and are often used in voltage regulator circuits. 

A capacitor is an electronic component that stores electric charges. It is composed of two conductive electrodes and an insulating medium in the middle. The working principle is that it gets charged when a voltage is applied and discharges when connected to a load. Its characteristic parameters include capacitance value (which measures the ability to store electric charges), voltage withstand value, and dissipation factor. There are types such as ceramic capacitors, electrolytic capacitors, and film capacitors, which are used in high-frequency circuits, power filtering, motor starting, coupling and bypassing, etc., respectively.