Transistor Working Regions and Switch Circuit Characteristics


semiconductor transistor is used to amplify, control, and generate electrical signals and power. It is a variable current switch capable of controlling the output current based on the input voltage. Unlike ordinary mechanical switches (such as relay, switch), transistors use electrical signals to control their own opening and closing, so the switching speed can be very fast.

Transistors Basic Explained



Ⅰ Transistor Electrons and Holes

Ⅱ Transistor Characteristics

Ⅲ Three Transistor Regions

Ⅳ Input and Output Characteristics

4.1 Input Characteristics

4.2 Output Characteristics

Ⅴ Saturation and Cutoff Distortions

5.1 Waveform Analysis of Basic Common Emitter Amplifier Circuit

5.2 Feasibility and Necessity of Transistor Switch Circuit Design

Ⅵ Transistors Switch Circuits

6.1 Basic Switching Circuit of NPN Transistors

6.2 Basic Switching Circuit of PNP Transistors


Ⅰ Transistor Electrons and Holes

The transistor is a current-type control device, similar in function to the field effect transistor (FET), and can be used as signal amplification, oscillation and modulation.
A transistor has three poles, three regions, and two PN junctions (observing the structure of it helps to understand how it works.

Transistor Structure

Figure 1. Transistor Structure

Movement of Charge Carriers

Movement of Charge Carriers

Figure 2. Movement of Charge Carriers

(1) The hollow in the figure is positively charged for holes, and the solid ones are negatively charged for electrons. Some people say how can holes move? That is due to the recombination of electron movement and hole, which is easy to understand from the macroscopic point of view as the hole is moving.

(2) The emitter process is characterized by high concentration, so when the emitter junction is forward biased, a large amount of electrons will be emitted, so it is called an emitter.

(3) The concentration of many electrons in the P region of the base is holes, the concentration is low and very thin. When the electrons from the emitter junction come over, there is little recombination with the base here, resulting in the base current IB, and most of the electrons are received. The electric field force will drift to the collector.

(4) The collector has a large area, which helps to collect the electrons emitted by the emitter, so it is called the collector. The purpose is to collect electrons to generate a large current, which forms an amplification effect on the base current.

Diffusion motion forms emitter current IE, recombination motion forms base current IB, and drift motion forms collector current IC, where IE=IC+IB.

Ⅱ Transistor Characteristics

(1) IB controls IC.

The direction of the transistor arrow is the current direction.

The NPN type current direction is B->E, the PNP type current is E->B, that is, the current controls the current, and the small current IB controls the large current IC.

(2) The transistor has an amplifying function.

The current flowing on IB is very small. If the current flowing on IB is 1mA, the current on IC is twice that of IB, and the current on IC is 90~100 times that of IB, that is 100mA.

(3) For the NPN tube, when the e pole is grounded, when IB>=1mA, Rce≈0, Uce≈0.3v, that is, the transistor is saturated and turned on. When the PNP is used as a switch, the e pole should be connected to the power supply, and the c pole should be connected to the load.

(4) When IB>1mA, Vbe=0.7V, the triode is completely turned on and plays a switching role, Vbe>=0.7V => IC tends to infinity. If Vbe<0.7v, the transistor is not fully turned on, and Vce has a large voltage drop. That is, Rce is large.

Note: When designing switching circuits with transistors, there are two design methods:

PNP type can be selected, when the power supply is connected to the e pole, and the load is connected to the c pole.

NPN type can be selected, when the power supply is connected to the c pole, and the e pole is connected to GND.

However, the PNP type method requires another NPN type transistor to cooperate, which increases the cost. Generally, the NPN type is recommended, the power supply is connected to the c pole, and the e pole is connected to the GND.

Ⅲ Three Transistor Regions

Cut-off Region, Active Region, Saturation Region

Transistor Circuit And Operational Regions

Figure 3. Transistor Circuit And Operational Regions

The transistor is too common in hardware design, but it is not so simple to understand its characteristics. The cut-off region and the active region in the curve below are easier to understand, while the saturation region cannot be understood by looking at the graph, otherwise it will be very confusing.

(1) Cut-off Region: Simply speaking, the transistor is not turned on, Ube< turn-on voltage, generally less than 0.5V or 0.7V, at this time IB=0, IC=Iceo≈0.

(2) Active Region: When VBB gradually increases, VBB>Uon and VBB>=UBE, the emitter junction is forward biased and the collector junction is reverse biased. From a microscopic point of view, the emitter junction is turned on, resulting in the diffusion of a large number of electrons, a small part of the base is recombined, and most of the electrons are collected by the collector junction under the action of the electric field force (the collector junction is reverse biased), forming a β times larger current of the IB. (The emitter concentration is high and the collector junction area is large, so the process is determined), that is, the current amplification effect is produced. At this time, the transistor is working in an enlarged state, because the collector's ability to collect electrons is very abundant, and IB increases, then more electrons can be collected, resulting in IC being β times that of IB.

(3) Saturation Region: when VBB continues to increase to a certain extent, since VCC remains unchanged, IC increases, and the voltage on RC increases, UCE will naturally decrease. When UCE<UBE, the collector junction is forward biased, and the collector collects electrons. The ability of the electric field will decrease (some people are very puzzled here, it is obviously on the wrong side, but it is sure that UCE>0, in this way, the electrons will still move from the emitter to the collector. In addition, the movement is mainly diffusion movement and drift movement. The drift movement is suppressed, but the electron concentration from the emitter is high, and the diffusion movement will continue). When IB continues to increase normally, IC cannot increase by β-fold, and it appears that IC has reached "saturation", that is, it cannot continue to increase.
For the collector voltage, it needs to be calculated according to the formula Uce=VCC-βIb. As Ib increases, Uce will decrease, so that Ube>Uce, that is, the collector junction is forward biased.

The cut-off region and saturation region are commonly used in embedded to realize the function of "switch". When the transistor is in the cut-off region, the "switch" is turned on, Uce≈VCC, and when the transistor is in the saturation region, the "switch" is closed, Uce≈ 0V. The drive transistor switches between the cut-off area and the saturation area, requiring the CPU to output a pulse signal, only high and low levels, not analog signals.

This is a simplified diagram of a typical common emitter connection.

Voltage Characteristic

Figure 4. Voltage Characteristic

1. When the UI is relatively small and the turn-on voltage is not reached, then IC=0, RC does not divide the voltage, and UO=VCC.

2. When UI continues to increase and reaches the turn-on voltage, IB will increase uniformly, IC=β*IB will also increase uniformly, the partial pressure on RC will increase uniformly, and UO=VCC-URC, UO will decrease uniformly.

3. When UI continues to increase, IB continues to increase, IC also increases, and the partial pressure of RC also increases, causing UC to decrease until it approaches 0, the ability of the collector to collect electrons is insufficient, and the transistor reaches saturation. In the analog circuit, try to make the circuit work in the magnified area, the linear area in the above figure.

Ⅳ Input and Output Characteristics

4.1 Input Characteristics

Input Characteristic

Figure 5. Input Characteristic

When UCE=0, it is equivalent to the direct connection between the collector and the emitter (equivalent to two PN junctions in parallel), so the input characteristic of iB is the volt-ampere characteristic of the PN junction.

When the UCE increases, it is equivalent to the enhancement of the ability of the collector C to collect electrons, that is, the IC increases, and because the IE remains unchanged (as mentioned above, the IE is formed due to the diffusion of the high concentration of polytrons), IE=IC+IB, Therefore, IB decreases. Therefore, when the UBE is the same, the larger the UCE, the smaller the IB, which is reflected in the graph as the curve shifts to the right.

When UCE increases to a certain extent, it can already collect most of the electrons. Increasing UCE will not greatly increase the number of collected electrons, so IC does not increase, IB does not decrease, and the curve does not shift to the right. Therefore, for low-power tubes, an input characteristic curve with UCE greater than 1V can be tested as a representative.

4.2 Output Characteristics

Output Characteristic

Figure 6. Output Characteristic

(1) When the control IB is unchanged, a curve is obtained, and multiple IBs have multiple curves.

(2) Look at IB, when the UCE increases, it means that the collector's ability to collect electrons increases, and the IC must increase. The picture shows the saturation area. Many people here do not understand why the saturation is first, and then the amplification. This picture is not a chronological picture, do not use chronological order to understand. If you want to look at time, when the input voltage UBE is increased, the output voltage UCE is the normal time sequence from large to small, so the graph should look at the horizontal axis in reverse, and you can see that the amplification area is first, and then the saturation is experienced.

Ⅴ Saturation and Cutoff Distortions

5.1 Waveform Analysis of Basic Common Emitter Amplifier Circuit

UCE is the output voltage UO, and the change of UCE changes up and down at the static operating point UCEQ (when IB changes dynamically).

Waveform Analysis of Common-emitter Amplifier Circuit

Figure 7. Waveform Analysis of Common-emitter Amplifier Circuit

(1) When saturating distortion, it is due to the increase of IB, the increase of RC partial pressure and the decrease of UCE. According to the output characteristics, UCE is small, and excessive transmission but insufficient collection, so it enters saturation distortion, reflecting bottom distortion, IB increases, and UCE is too small and close to 0.

(2) When the cut-off distortion occurs, IB decreases, the RC partial pressure decreases and UCE increases. If IB is close to 0, the transmitter junction is not conducting, and IC is also close to 0, then UCE is close to VCC, and cut-off distortion occurs.

5.2 Feasibility and Necessity of Transistor Switch Circuit Design

Feasibility: Anyone who has used a transistor knows that it has a characteristic, that is, it has a saturated state and a cut-off state. It is precisely because of these two states that it is possible to apply it to a switching circuit.

Necessity: Suppose we are designing a system circuit, and some voltages, signals, etc. need to be cut off during system operation, but they cannot be cut off mechanically. At this time, they can only be processed by software, which requires a switching circuit as a basis.

Ⅵ Transistors Switch Circuits

6.1 Basic Switching Circuit of NPN Transistors

Transistor Switch Circuit

Figure 8. Transistor Switch Circuit

The following figure is a basic triode switch circuit. The base of NPN needs to be connected to a resistor (R2) and the collector to a load resistor (R1).

First of all, we need to know that when the base has no current, there is no current in the collector, and the tube is in the off state. When there is current in the base, it will cause the collector to flow a larger current, that is, it enters the saturation state, which is equivalent to off state. Of course, the base must have a voltage input that meets the requirements to ensure that the tube enters the cut-off region and the saturation region.

6.2 Basic Switching Circuit of PNP Transistors

Basic Switching Circuit of PNP Transistor

Figure 9. Basic Switching Circuit of PNP Transistor

The most commonly used transistors are switching circuits. The PNP and NPN transistors are introduced below. Let's talk about the PNP type triode first, the commonly used models are 9012, 8550 and so on. How to use it, as shown below:

FM is a buzzer, 8550 is a PNP type triode, the C terminal is grounded, the B terminal is controlled by the microcontroller, and the E terminal is connected to VCC through FM. According to the direction of the arrow, when the E terminal is high voltage, when the B terminal is also high voltage, then E and C are disconnected, and when the B terminal is low voltage, then E and C are directly turned on to realize the function of the switch. Pay attention to: the diode in the direction of the arrow on the triode, as long as the diode is conducting forward, then the triode can be turned on up and down.


1. How does a semiconductor transistor work?

A transistor works when the electrons and the holes start moving across the two junctions between the n-type and p-type silicon. The small current that we turn on at the base makes a big current flow between the emitter and the collector.

2. How transistor can be used as a switch?
The transistor operates as a Single Pole Single Throw (SPST) solid state switch. When a zero input signal applied to the base of the transistor, it acts as an open switch. If a positive signal applied at the input terminal then it acts like a closed switch.

3. What is the pn junction of a transistor?
In a transistor, the middle layer (here n-region) is called the base, the forward biased p-n junction is called the emitter junction and the reverse biased p-n junction is called collector junction. Due to the positive potential at the emitter junction, the holes in the p-region cross into the n-region (the base).

4. How many PN junctions are there in a transistor?
2 PN Junctions
Hence a transistor has 2 PN Junctions.

5. What are two basic types of transistors?
Transistors typically fall into two main types depending on their construction. These two types are bipolar junction transistors (BJT) and Field Effect Transistors (FET).

6. What are the terminals of a transistor called?
A transistor is an electronic device that contains three terminals named emitter, base, and collector. The small current at one terminal is used to generate a large current at the remaining terminals. Transistors are mainly employed for switching and amplification purposes.

7. What are the 3 leads of a transistor?
There are typically three electrical leads in a transistor, called the emitter, the collector, and the base—or, in modern switching applications, the source, the drain, and the gate.

8. Can a transistor be used as an amplifier?
One of the key characteristics of a transistor is that it can be used as an amplifier. Transistors can act as amplifiers while they are functioning in the active region or when it is correctly biased.

9. What is a PNP and NPN transistor?
The main difference between the NPN and PNP transistor is, an NPN transistor turns on when the current flows through the base of the transistor. In this type of transistor, the current flows from the collector (C) to the emitter (E). A PNP transistor turns ON, when there is no current at the base of the transistor.

10. What is NPN transistor explain?
NPN transistors are a type of bipolar transistor with three layers that are used for signal amplification. It is a device that is controlled by the current. A negative-positive-negative transistor is denoted by the abbreviation NPN.

11. What does a NPN transistor made of?
A bipolar junction transistor is made up of three pieces of silicon. Depending on what is added to the silicon, it will be either N-type or P-type. An NPN transistor has a piece of P-type silicon (the base) sandwiched between two pieces of N-type (the collector and emitter).

12. What are the three regions of a transistor?
A BJT consists of three differently doped semiconductor regions: the emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP transistor, and n type, p type and n type in an NPN transistor.

13. In which region a transistor acts as a open switch?
saturation region
Transistor acts as a switch in the saturation region and cutoff region. The emitter-base junction and the collector-base junction is reverse biased in the cutoff region. Both the junctions are forward biased in the saturation region.

14. How a transistor behaves as a switch?
One of the most common uses for transistors in an electronic circuit is as simple switches. In short, a transistor conducts current across the collector-emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is off. When base voltage is present, the switch is on.

15. Can we remove distortion from a signal?
When the output of the amplifier swings negative near ground the output stage may introduce some distortion. One possible solution is to bias the voltage on the VREF pin to a more positive voltage. ... This gives more headroom for the amplifier on the negative signal swing and can reduce the distortion.

16. How do you connect a transistor as a switch in a circuit?
To connect the transistor as a switch in a circuit, we connect the output of the device that will switch on the transistor to the base of the transistor. The emitter will connect to ground of the circuit. And the collector will connect to the load that the transistor will turn on and the supply voltage of the circuit.

17. Why do transistors need a resistor?
The resistors are used as a means of generating voltage drops and thereby pulling the transistor into the desired operating region. because you need to limit the current to semiconductor terminals. otherwise, the transistors will be damaged.

18. What is the most popular transistor use for which circuits?
Usage of MOSFETs and BJTs
The MOSFET is by far the most widely used transistor for both digital circuits as well as analog circuits, accounting for 99.9% of all transistors in the world. The bipolar junction transistor (BJT) was previously the most commonly used transistor during the 1950s to 1960s.

19. Which type of signal is used when the transistor is used as a switch?
positive signal
With a positive signal applied to the Base of the transistor it turns “ON” acting like a closed switch and maximum circuit current flows through the device.

20. Which operating modes are required for transistor switch?
Operating Modes of Transistors
Depending on the biasing conditions like forward or reverse, transistors have three major modes of operation namely cutoff, active and saturation regions.

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