First, Introduction to Transistors
A transistor, also known as a semiconductor transistor, is a semiconductor device that controls current. Its function is to amplify weak signals into larger amplitude signals. It is also used as a contactless switch. Generally, a transistor amplifies current. Its structure consists of two closely spaced PN junctions on a semiconductor substrate. The two PN junctions divide the entire semiconductor into three parts: the base region in the center, and the emitter and collector regions on either side. They are usually arranged in two different configurations: PNP and NPN.

The two PN junctions of a transistor are similar to two common cathode or common anode diodes.

Second, Three Transistor Operating States
Off-State: The voltage at the transistor's emitter junction is less than the conduction voltage of the PN junction. The base current is zero, and both the collector and emitter currents are zero. The transistor loses its current amplification function. The region between the collector and emitter is equivalent to the off state of a switch. We call the transistor in the off state.

Amplification State: When the voltage applied to a transistor's emitter junction is greater than the forward voltage of the PN junction, the emitter junction is forward biased and the collector junction is reverse biased. At this point, the base current controls the collector current, causing the transistor to amplify current.

Saturation On State: When the voltage applied to a transistor's emitter junction is greater than the forward voltage of the PN junction, and when the base current increases to a certain level, the collector current no longer increases with the base current, but remains constant around a certain value. At this point, the transistor loses its current amplification function, and the voltage between the collector and emitter is very low, effectively acting as a switch in the on-state. This state of the transistor is called the saturation on state.

Third, The Function of the Transistor
Amplifying Weak Signals: A small signal input to the base causes a large current change in the collector. This is one of the most important and core components of electronic circuits.

Used as a contactless switch: Utilizes the cutoff and conduction characteristics of transistors to control or drive loads; examples include gate circuits and switching power supplies composed of transistors.

Fourth, Types of Transistors
There are many types of transistors, classified by:
① Based on semiconductor material: silicon and germanium transistors;
② Based on operating frequency: low-frequency and high-frequency transistors;
③ Based on power: low-power, medium-power, and high-power transistors;
④ Based on function: switching transistors, power transistors, Darlington transistors, phototransistors, etc.

Common NPN-type transistors include: 8050, 9013, 9014, 9018, C1815, D882, TIP41, 3DD15, 2N3055, 2N3773, etc.
Common PNP-type transistors include: 8550, 9012, 9015, B772, TIP42, 2N5884, etc.
Among them, TO-92-packaged transistors like the 8050, 8550, 9013, and 9012 are low-power transistors.

Medium-power transistors include the D882, B772, TIP41, and TIP42.
Gold-sealed iron-capped transistors like the 3DD15, 2N3055, 2N3773, and 2N5884 are high-power transistors.
All of the above are through-hole transistors. SOT-23 packaged surface-mount transistors are also common.

Fifth, Important Transistor Parameters
1. β Value:
β is the most important parameter of a transistor, as it describes its ability to amplify current signals. The higher the β value, the greater the amplification capability for small signals, and vice versa. However, the value should not be too large, as it can lead to unstable transistor performance. A value between 30 and 80 is generally appropriate. Generally, the β value of a transistor is not a fixed value; it generally changes slightly with the device's operating conditions.

2. Interelectrode Reverse Current:
The smaller the interelectrode reverse current, the greater the transistor's stability.

3. Transistor Reverse Breakdown Characteristics:
A transistor consists of two PN junctions. If the reverse voltage exceeds the rated value, it will breakdown like a diode, causing performance degradation or permanent damage.

4. Operating Frequency:
The β value of a transistor remains constant only within a certain operating frequency range. Beyond this frequency range, it will drop sharply as the frequency increases.