What is the voltage required to turn on the transistor?

1, Definition of turn-on voltage for transistors
The turn-on voltage of a transistor, in short, refers to the minimum voltage value required to transition the transistor from the off state to the on state. The definition and measurement method of the turn-on voltage for different types of transistors, such as bipolar junction transistors (BJTs) and field-effect transistors (FETs), are slightly different.
Bipolar junction transistor (BJT): In BJT, the turn-on voltage usually refers to the moment when the base emitter voltage (Vbe) reaches a certain critical value, at which the transistor begins to conduct. This critical value depends on the material and manufacturing process of the transistor, generally between 0.6V and 0.7V (for silicon-based BJTs), but may also vary depending on the specific model.
Field Effect Transistor (FET): The turn-on voltage of an FET refers to a specific value that the gate source voltage (Vgs) needs to reach in order for the channel to begin forming or strengthening, thereby causing the FET to transition from the off state to the on state. This value is commonly referred to as the threshold voltage (Vth), and its magnitude is also influenced by the FET type (such as N-channel or P-channel), material (such as silicon or gallium arsenide), and manufacturing process.
2, Factors affecting the turn-on voltage of transistors
The turn-on voltage of a transistor is not fixed and is influenced by various factors
Temperature: As the temperature increases, the intrinsic carrier concentration of semiconductor materials increases, causing a change in the turn-on voltage of transistors. Generally speaking, the turn-on voltage of BJT will slightly decrease with increasing temperature, while the threshold voltage of FET may rise or fall, depending on the type and manufacturing process of FET.
Manufacturing process: Different manufacturing processes can cause changes in the geometric dimensions, doping concentration, and other parameters of transistors, thereby affecting their turn-on voltage. With the continuous advancement of semiconductor technology, the turn-on voltage of transistors is also constantly decreasing to meet the requirements of high performance and low power consumption.
Materials: In addition to silicon, other materials such as gallium arsenide, silicon carbide, etc. are used to manufacture transistors. These materials have different physical and chemical properties, which can also affect the turn-on voltage of transistors.
3, Method for measuring the turn-on voltage of transistors
Measuring the turn-on voltage of transistors requires the use of professional testing equipment, such as semiconductor parameter analyzers or oscilloscopes. Here is a simplified measurement step example (using N-channel MOSFET as an example):
Connect the drain (D) of the MOSFET to the positive pole of the power supply and the source (S) to the negative pole of the power supply to form a drain source channel.
Apply a gradually increasing voltage (Vgs) to the gate (G) using a signal generator or voltage source.
Meanwhile, use an ammeter to monitor the changes in drain current (Id). When Id starts to significantly increase (usually reaching a preset threshold current), the corresponding Vgs is the threshold voltage (Vth) of the MOSFET.
It should be noted that due to various errors and uncertainties in the measurement process (such as contact resistance, temperature drift, etc.), the actual measured opening voltage may have some deviation from the theoretical value or the nominal value in the data manual.
4, The importance of transistor turn-on voltage in practical applications
The turn-on voltage of transistors has a significant impact on circuit design and performance optimization. For example, in digital circuits, in order to ensure the correct switching of logic gates and reduce power consumption, it is necessary to precisely control the turn-on voltage of transistors. In addition, in analog circuits, the turn-on voltage of transistors also determines key parameters such as circuit gain and bandwidth. Therefore, when designing and manufacturing transistors, it is necessary to fully consider the characteristics and requirements of their turn-on voltage.
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