Npn transistor how does it work

To put it another way, in the case of a lightly doped region, the width of the depletion region will be greater than in the case of a highly doped region. This is why the depletion width at the collector-base junction is wider than at the emitter-base junction. These two depletion regions serve as a potential stumbling block to any further majority carrier flow.

The width of the depletion region, also called PN Junction, narrows as a result of the forward applied voltage at the emitter-base junction. Similarly, the width of the collector-base junction is widened by the reverse applied voltage.

This is why, in comparison to the collector-base junction in the previous figure, the emitter-base junction has a thin depletion region.

Electrons begin to inject into the emitter region as a result of the forward applied voltage VBE. The electrons in this region have sufficient energy to overcome the emitter-base junction's barrier potential and reach the base region. Because the base region is very thin and doped lightly. As a result, only a few electrons combine with the holes once they reach their destination. Because of the strong electrostatic field, electrons begin to drift at the collector region due to the very thin base region and the reverse voltage at the collector-base junction.

As a result, these electrons are now collected at the transistor's collector terminal. The electrons begin to move towards the collector as recombined holes and electrons become separated from one another. A very small base current also flows through the device as a result of this movement. This is why the emitter current is equal to the sum of the base and collector currents.

Region of the emitter: It is the largest section of the structure, which is larger than the base region but smaller than the collector region.

It has a lot of doping in it. It is used to transfer majority carriers into the base region, which are electrons. It is a forward-biased region, which means it is always provided with the base region forward biased. Region of the base: The base region is located in the middle of the structure. In comparison to the transistor's emitter and collector regions, it has a small region.

It is lightly doped to ensure that there is minimal recombination and a high current at the collector. Region of the collector: It is the structure's rightmost section, and its function is summed up in its name: it collects the carriers transferred by the base region.

When compared to the base region, this region receives reverse biassing. What is NPN Transistor? Follow article. EmmaAshely 29 Apr A valve can be completely opened, allowing water to flow freely -- passing through as if the valve wasn't even present.

Likewise, under the right circumstances, a transistor can look like a short circuit between the collector and emitter pins. Current is free to flow through the collector, and out the emitter. In the same way, a transistor can be used to create an open circuit between the collector and emitter pins. With some precise tuning, a valve can be adjusted to finely control the flow rate to some point between fully open and closed.

A transistor can do the same thing -- linearly controlling the current through a circuit at some point between fully off an open circuit and fully on a short circuit. From our water analogy, the width of a pipe is similar to the resistance in a circuit.

If a valve can finely adjust the width of a pipe, then a transistor can finely adjust the resistance between collector and emitter. So, in a way, a transistor is like a variable, adjustable resistor. There's another analogy we can wrench into this. Imagine if, with the slight turn of a valve, you could control the flow rate of the Hoover Dam's flow gates.

The measly amount of force you might put into twisting that knob has the potential to create a force thousands of times stronger. We're stretching the analogy to its limits, but this idea carries over to transistors too. Transistors are special because they can amplify electrical signals, turning a low-power signal into a similar signal of much higher power.

Kind of. There's a lot more to it, but that's a good place to start! Check out the next section for a more detailed explanation of the operation of a transistor. Unlike resistors , which enforce a linear relationship between voltage and current, transistors are non-linear devices. They have four distinct modes of operation, which describe the current flowing through them. When we talk about current flow through a transistor, we usually mean current flowing from collector to emitter of an NPN.

To determine which mode a transistor is in, we need to look at the voltages on each of the three pins, and how they relate to each other. The simplified quadrant graph above shows how positive and negative voltages at those terminals affect the mode. In reality it's a bit more complicated than that.

Let's look at all four transistor modes individually; we'll investigate how to put the device into that mode, and what effect it has on current flow. Note: The majority of this page focuses on NPN transistors. Saturation is the on mode of a transistor. A transistor in saturation mode acts like a short circuit between collector and emitter. In saturation mode both of the "diodes" in the transistor are forward biased.

Because the junction from base to emitter looks just like a diode , in reality, V BE must be greater than a threshold voltage to enter saturation. For a lot of transistors at room temperature we can estimate this drop to be about 0. Another reality bummer: there won't be perfect conduction between emitter and collector.

A small voltage drop will form between those nodes. Transistor datasheets will define this voltage as CE saturation voltage V CE sat -- a voltage from collector to emitter required for saturation. This value is usually around 0. This value means that V C must be slightly greater than V E but both still less than V B to get the transistor in saturation mode. Cutoff mode is the opposite of saturation. A transistor in cutoff mode is off -- there is no collector current, and therefore no emitter current.

It almost looks like an open circuit. To get a transistor into cutoff mode, the base voltage must be less than both the emitter and collector voltages. Thus, the base voltage must be less than the collector, but greater than the emitter. That also means the collector must be greater than the emitter. Usually this voltage is usually around 0. Active mode is the most powerful mode of the transistor because it turns the device into an amplifier.

Current going into the base pin amplifies current going into the collector and out the emitter. It's usually around , but can range from 50 to Active mode model. What about the emitter current, I E? In active mode, the collector and base currents go into the device, and the I E comes out. That means I C is very close to, but less than I E in active mode.

Just as saturation is the opposite of cutoff, reverse active mode is the opposite of active mode. A transistor in reverse active mode conducts, even amplifies, but current flows in the opposite direction, from emitter to collector. Reverse active mode isn't usually a state in which you want to drive a transistor. It's good to know it's there, but it's rarely designed into an application.

After everything we've talked about on this page, we've still only covered half of the BJT spectrum. What about PNP transistors? You pull the base low to turn the PNP on, and make it higher than the collector and emitter to turn it off. In active and saturation modes, current in a PNP flows from emitter to collector.

This means the emitter must generally be at a higher voltage than the collector. If you're burnt out on conceptual stuff, take a trip to the next section.

The best way to learn how a transistor works is to examine it in real-life circuits. Let's look at some applications! One of the most fundamental applications of a transistor is using it to control the flow of power to another part of the circuit -- using it as an electric switch.

Transistor switches are critical circuit-building blocks; they're used to make logic gates , which go on to create microcontrollers, microprocessors, and other integrated circuits. Below are a few example circuits. Let's look at the most fundamental transistor-switch circuit: an NPN switch. Our control input flows into the base, the output is tied to the collector, and the emitter is kept at a fixed voltage.

While a normal switch would require an actuator to be physically flipped, this switch is controlled by the voltage at the base pin.

When the voltage at the base is greater than 0. When the voltage at the base is less than 0. The circuit above is called a low-side switch , because the switch -- our transistor -- is on the low ground side of the circuit.

Alternatively, we can use a PNP transistor to create a high-side switch:. Similar to the NPN circuit, the base is our input, and the emitter is tied to a constant voltage. This time however, the emitter is tied high, and the load is connected to the transistor on the ground side. This circuit works just as well as the NPN-based switch, but there's one huge difference: to turn the load "on", the base must be low. This can cause complications, especially if the load's high voltage V CC being 12V connecting to the emitter V E in this picture is higher than our control input's high voltage.

For example, this circuit wouldn't work if you were trying to use a 5V-operating Arduino to switch off a 12V motor. In that case, it'd be impossible to turn the switch off because V B connecting to the control pin would always be less than V E. You'll notice that each of those circuits uses a series resistor between the control input and the base of the transistor. Don't forget to add this resistor! A transistor without a resistor on the base is like an LED with no current-limiting resistor.

Recall that, in a way, a transistor is just a pair of interconnected diodes. We're forward-biasing the base-emitter diode to turn the load on. The diode only needs 0. Some transistors may only be rated for a maximum of mA of current to flow through them.

If you supply a current over the maximum rating, the transistor might blow up. The series resistor between our control source and the base limits current into the base. The base-emitter node can get its happy voltage drop of 0.

The value of the resistor, and voltage across it, will set the current. The resistor needs to be large enough to effectively limit the current, but small enough to feed the base enough current. Here a high voltage into the base will turn the transistor on, which will effectively connect the collector to the emitter. If the input is low, on the other hand, the transistor looks like an open circuit, and the output is pulled up to VCC.

This is actually a fundamental transistor configuration called common emitter. More on that later. If either transistor is turned off, then the output at the second transistor's collector will be pulled low. If both transistors are "on" bases both high , then the output of the circuit is also high. In this circuit, if either or both A or B are high, that respective transistor will turn on, and pull the output high. If both transistors are off, then the output is pulled low through the resistor.

An H-bridge is a transistor-based circuit capable of driving motors both clockwise and counter-clockwise. When the forward bias is applied across the emitter, the majority charge carriers move towards the base. This causes the emitter current I E. The electrons enter into the P-type material and combine with the holes. The base of the NPN transistor is lightly doped. Due to which only a few electrons are combined and remaining constitutes the base current I B.

This base current enters into the collector region. The reversed bias potential of the collector region applies the high attractive force on the electrons reaching collector junction. Thus attract or collect the electrons at the collector. The whole of the emitter current is entered into the base.