How does an NPN transistor work
A transistor is a semiconductor component, which is usually referred to as the bipolar transistor. There are also unipolar transistors, also known as field effect transistors.
Bipolar transistors are typically made of silicon. Or from germanium or mixed crystals, which are, however, not very common.
The term transistor is derived from its function. If there is a change in resistance in one semiconductor layer, the resistance in the other layer is also influenced. "Transfer resistor" became the name of transistor.
Transistors are mainly used as switches or amplifiers.
Structure of the bipolar transistor
Each bipolar transistor consists of three thin semiconductor layers that are laid one on top of the other. A distinction is made between an npn or pnp layer sequence. The middle layer is very thin compared to the other two layers. The layers are provided with metallic connections that lead out of the housing. The outer layers of the bipolar transistor are called the collector (C) and emitter (E). The middle layer has the designation base (B) and is the control electrode or the control input of the transistor.
|NPN transistor||PNP transistor|
|The NPN transistor consists of two n-conductive layers. In between there is a thin p-conductive layer.||The PNP transistor consists of two p-conducting layers. In between there is a thin n-conductive layer.|
Note: The circuit symbol with the two diodes connected against each other is often used to represent the basic structure of the transistor. In reality, however, the functioning of a transistor cannot be reproduced in this way. The reason lies in the changed behavior due to the very thin middle layer of the transistor.
Voltage and current distribution
|Current and voltage on the NPN transistor||Current and voltage on the PNP transistor|
This circuit is only intended to show the current and voltage curves and their relationship to one another. Basically, in the IB.- and in the IC.-Circuit a current-limiting resistor must be used.
Please note: The technical direction of current from plus to minus applies here.
With the PNP transistor, the polarity of the voltage and current distribution is exactly the other way around. In practice you only have to pay attention to the polarity of the operating voltage. NPN transistors are used for positive voltages. PNP transistors are used for negative voltages.
UCE = Collector-emitter voltage
UBE = Base-emitter voltage (threshold value)
I.C. = Collector current
I.B. = Base current
How a transistor (NPN) works
The direction of the current must be taken into account when the transistor works. If one wants to explain the physical principle, one speaks of the electron flow or the physical flow direction (from minus to plus). It is used in the following implementation. The technical direction of current (from plus to minus) is used in circuits and mathematical calculations.
By applying a voltage UBE of about 0.7 V, the lower diode (principle) is switched in the forward direction. The electrons get into the p-layer and are drawn from the positive pole of the voltage UBE dressed.
Since the p-layer is very small, only a small part of the electrons is attracted.
Most of the electrons move further into the upper boundary layer. This makes it conductive and the positive pole of the voltage UCE attracts the electrons. A collector current I flowsC..
With conventional transistors, around 99% of the electrons slip from the emitter to the collector. About 1% of the electrons remain in the base layer and flow away there.
Characteristics of the bipolar NPN transistor
- The collector current IC. only flows if there is also a base current I.B. flows. If the base current IB. changes, then the collector current I also changesC.. Within the transistor, the change in the base current acts like a change in resistance. When the base current changes, the transistor acts like an electrically controlled resistor.
- The collector current IC. is a multiple of 20 to 10,000 times greater than the base current I.B.. This difference in size comes from the division of the electron flow from collector (C) and base (B). This difference in size is called the current gain B. It can be calculated from the ratio I.C. to IB. to calculate.
- The base current IB. flows only when the threshold voltage UBE is reached at the base-emitter path. The threshold value depends on the semiconductor material. Usually one uses silicon transistors with a threshold value of 0.6 to 0.7 V. There are also germanium transistors with a threshold value of 0.3 V.
By means of an auxiliary voltage UBE the threshold value can be set in advance. This procedure is referred to as setting the operating point. The base current can now control the collector current around this set voltage.
- If there is no base current IB. flows, then the transistor blocks. Its resistance in the collector-emitter path is infinite. The voltage at the collector-emitter is very high. If a base current flows, the transistor becomes conductive. His resistance has diminished. This means that the voltage at the collector-emitter is also lower. More precisely, an increase at the input (base) leads to a decrease at the output (collector-emitter). This is also called inverting behavior. This property is the switching behavior of the bipolar transistor and is used very often in electronics (transistor as a switch).
- When the voltage UCE is smaller than the voltage UBE, then the bipolar transistor is in saturation or in saturation mode. This happens when the transistor is flooded by the base current. The base current is then so large that the maximum current gain has long been reached and the collector current no longer increases.
In general, this has no negative effects as long as the maximum base current is not exceeded. If it does, the transistor will be destroyed.
However, the saturation mode has negative effects on the switching behavior of a transistor. With a fast switching process, when the collector-emitter voltage UCE has to change quickly. Then the transistor must first be cleared of the charge carrier flooding. This takes longer than if only a few charge carriers flow off via the base. This delay has a negative effect at high switching frequencies. Then the saturation mode should be avoided.
- The bipolar transistor combines two circuits. The circuit with the voltage UBE is called the control circuit. The circuit with the voltage UCE is referred to as the working or load circuit.
Bipolar transistors / standard types
|Type||NPN / PNP||casing||P.dead/ W||UCE/ V||I.C./ A||B (ß)||fG/ MHz|
|BC 107 B||NPN||TO-18||0,3||45||0,1||200-450||300|
|BC 547 A||NPN||SOT-54||0,5||45||0,1||110-220||300|
|BC 547 B||NPN||SOT-54||0,5||45||0,1||200-450||300|
|BC 547 C||NPN||SOT-54||0,5||45||0,1||420-800||300|
|BC 557 A||PNP||SOT-54||0,5||45||0,1||125-250||150|
|2 N 3055||NPN||TO-3||115||60||15||20-70||0,8|
Basic circuits of the transistor
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