Join Barron Stone for an in-depth discussion in this video NPN bipolar junction transistors, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] A bipolar junction transistor or BJT consists of three alternating layers of doped N-Type and P-Type semiconductor material joined together. And there are two ways that those three layers of material can be stacked. The BJT in the left is called an NPN Transistor because it contains a section of P-type material, sandwiched between two N-Type sections. And its counterpart on the right is a PNP Transistor. Because it has two P-Type sections with a region of N-Type material in the middle.
For now, I'll just focus on how an NPN Transistor works. From a physical standpoint, the three alternating layers of material create two PN junctions. At first glance, those back to back PN Junctions might look somewhat like a pair of diodes. With their cathodes connected together pointing away from each other. This is a common simple vocation that many people like to use when explaining BJTs. But it's important to recognize that this is a major simplification.
If you try to understand how a transistor can behave like two diodes, you'll just end up confusing yourself because it doesn't. You cannot replicate a transistor on your breadboard by connect two diodes together, they are physically different things. There are several important aspects of the N and the P layers that give the BJT its unique behavior. For one, the middle layer of a BJT between the two junctions is designed to be as thin as possible which affects how the holes in electrons shift around amongst the BJT layers as it operates in different modes.
Additionally, the top and bottom layers will have different amounts of doping concentrations. So each of the two PN junctions will behave slightly differently. A bipolar junction transistor is not symmetrical. Each of the N and P type regions is connected to one of the three BJT terminals. Called the collector, the base and the emitter. You can think of the base terminal like the handle of a valve. I'll be applying a signal to the base terminal to control the transistor.
Turning it on or off. The amount of voltage and current that I provide at the base will control the amount of current that can pass between the other two terminals. The emitter and collector were given their respective names because in an NPN transistor when it's turned on, electrons begin their journey through the transistor at the emitter. They flow up thought the base and then pass through the collector on their way out. However, since by convention, we say that electrical current is the flow of the holes, which represent the absence of electrons and those holes move in the opposite direction of the electrons, the conventional current through an NPN transistor flows from the collector terminal to the emitter terminal.
The schematic symbol for an NPN has an arrow that indicates the direction of conventional current flow through the transistor. On the symbol shown here, the top terminal is the collector, the bottom terminal with the arrow is the emitter and the base terminal is connected to the flat bar on the left. Before diving into how the BJT operates, it's important to go over some common notation that's used to describe the various voltages involved. The voltage at each of the transistor terminals relative to ground is labeled using a single subscript.
VC is the collector voltage, VB is the base voltage and VE is the emitter voltage. A double subscript is used to indicate the voltage between two transistor terminals. So V BE is the voltage drop between the base and the emitter and V CE is the voltage drop from the collector to the emitter. When using an NPN transistor, the voltage at the collector has to be higher than the emitter.
So VCE will always be positive. That's a requirement because the current through an NPN always flows from the collector to the emitter. I'll use a similar set of subscripts to refer to the current flowing in and out of each of the terminals. IB indicates the current entering the base terminal, IC is the current flowing into the collector and IE is the current exiting the emitter. When the same letter is repeated in the subscript for a voltage, that indicates a power supply voltage.
VCC is the positive power supply voltage associated with the collector and VEE is the negative supply voltage associated with the emitter. It's the relationship between all of those voltages and currents at the transistor's terminals that determines its operating mode. And it's the responsibility of the circuit designer to build up an appropriate network of other components surrounding the transistor to control those voltages and currents to meet their needs. Now that's a task easier said than done.
In designing transistor circuits really is its own art form. For simplicity, as I dive into the transistor's operating modes, I'll focus on just the transistor and the voltages and currents directly at its terminals. But keep in mind that those input voltages and currents are coming from a network of other components surrounding the transistor that I'm just not showing. When a transistor is operating in the cutoff mode, it's considered to be fully off. It acts like a closed valve on a faucet and prevents any current from passing through.
My NPN will be in the cutoff mode when the voltage drop from the base to the emitter is less than a certain threshold voltage. Referred to here as VTH. To understand where that threshold voltage comes from, remember, that inside of an NPN transistor, there are two PN junctions which kind of looks like diodes. Although as a whole, the BJT doesn't behave like two connected diodes. If I focus on just the bottom PN junction between the base and the emitter terminals, I'll see that it does behave similar to a diode which means it will require a certain amount of forward voltage to turn on.
For most common BJT transistors, that threshold to turn on the base emitter junction is usually somewhere around 0.7 volts. Which corresponds to the forward voltage that's necessary to turn on the bottom diode light junction. When the base to emitter voltage is less than 0.7 volts, the bottom diode will be in the cutoff region. Which means no current flows into the base and therefore no current flows between the collector and the emitter. The transistor is basically acting like an open circuit between its three terminals.
If I raise that base voltage up to the 0.7 volt threshold, the base to emitter junction will become forward biased and allow current to flow into the base. The voltage between the base and emitter will not rise above that threshold for the same reason that the voltage across a forward biased diode doesn't increase above its forward voltage threshold. At this point, it's acting like a short circuit. Under this condition, the transistor is said to be biased, or turned on and it will not limit the amount of current that flows into its base.
So, it's very important that whatever circuit is connected to the base terminal, is designed to control and limit the current into the base. That usually involves having a resistor in series with the base terminal. Now that the transistor is biased, the amount of current that flows into the base will control whether the transistor is operating in the active mode or the saturation mode. When the amount of current into the base is relatively small the transistor will be in the active mode where it acts like a partially opened valve to control the current flowing into the collector terminal.
The active mode is often called the linear region because when the transistor is operating in the active mode, the current into the base and collector terminals have a linear relationship. With the collector current being equal to the base current times a factor called the current gain. The transistor's current gain or amplification factor is a unitless value that describes the ratio between the collector and the base currents. When the transistor is operating in the active mode.
Current gain is usually represented with a capital letter beta, but you'll also see it represented as a lower case H with a subscript FE. This beta value is dependent on the transistor's physical characteristics and it will differ from transistor to transistor due to variations in the manufacturing process. It's rare to encounter two transistors that have the exact same beta characteristics. To make things even more confusing, the beta ratio doesn't remain stable for all operating conditions.
It can change based on the amount of collector current, the transistor's temperature, the frequency of the input signal and other factors. Because of all that variability, good transistor circuits will be designed so that their behavior is independent of that transistor's unique beta value. For simplicity in my examples, I'll assume that my transistors have a beta of 100. This is a common rule of thumb to use when designing circuits, because most basic small signal transistors have beta values around 100.
If the transistor is operating in the active mode with one milliamp of current flowing into its base, the current flowing into the collector terminal will be 100 times that. Which is 100 milliamps. Now according to Kirchhoff's Current Law, the amount of current entering any circuit node has to be equal to the amount of circuit exiting that node because electrons are neither created nor destroyed. Therefore, all of the current that's entering the transistor through the base and collector terminals must be exiting through the emitter terminal.
The emitter current will be equal to the sum of the base and collector currents. So, if the base current is one milliamp and the collector current is 100 milliamps, the emitter current will be 101 milliamps out of this NPN transistor. As the current into the base terminal is increased or decreased, the current through the collector and emitter will change accordingly. If the base current increases to two milliamps, the collector current will be 100 times that or 200 milliamps and the emitter current will be 202 milliamps.
In the other direction, if the base current was lowered down to half a milliamp, the collector current will decrease to 50 milliamps so the emitter current would only be 50.5 milliamps. One of my favorite ways to understand the transistor's behavior is using a simple model known as the transistor man which was made famous by the book The Art of Electronics. Imagine that inside of every transistor, there's a tiny little man. His job is to look at the amount of current flowing into the base terminal which behaves like a forward biased diode connected to the emitter.
And then, based on what he sees, he adjusts the resistance of a rheostat on the other side between the collector and emitter to control the amount of current flowing into the collector. The transistor man's entire goal in life is to maintain that beta relationship between the base and collector current. When he sees the base current decrease, he'll raise the rheostat's resistance to reduce the collector current. And when he sees the base current increase, he'll lower the rheostat's resistance to let in more collector current.
If the base current increases too much, the transistor man will lower the rheostat as much as he can until it basically acts like a short circuit between the collector and the emitter. At this point, the transistor man runs into a problem. Because he can no longer maintain the relationship between the base and the collector. He's letting as much current as possible into the collector. If the base current continues to rise, there's nothing he can do to increase the collector current more. When that happens, the transistor is operating in the saturation mode.
The transistor is considered to be fully on and acts like an open valve on a faucet or a short circuit. Letting current flow freely from the emitter to the collector. Since the transistor is acting like a short circuit, ideally, the voltage drop between the collector and emitter should be zero. In reality, there will be a small voltage drop between the saturated transistor's collector and emitter. That drop is usually around 0.2 volts or less and it's special in the value that it gets its own subscript.
Indicating that it's the saturated collector emitter voltage. It's important to note that in this condition, the base voltage VB will actually be higher than the collector voltage. And both the base and collector will be higher than the emitter. Since the saturated transistor lets current flow freely between the collector and emitter, it'll be up to whatever circuits are connected to those two terminals to limit the current through the transistor. For example, if the circuit connected to the collector terminal only provides 200 milliamps of current in the saturated state, if the base current was five milliamps, then the output from the emitter will be 205 milliamps.
Even though the transistor is saturated, it can never violate Kirchhoff's Current Law. The relationship that the emitter current equals the base plus collector current will always hold true. If the base current increased from five to 10 milliamps, the collector current would remain the same at 200 milliamps because the circuit's saturated. But the emitter current would increase to 210 milliamps in accordance to Kirchhoff's Law.
- Semiconductor materials
- Diode applications
- Rectifying a signal
- Detecting the signal peak
- Protecting against large signals, reverse current, and flyback voltage
- Special purpose zener diodes, Schottky diodes, and photodiodes
- NPN and PNP bipolar junction transistors
- Using a BJT as a switch
- Field effect transistors
- Differences between BJTs and MOSFETs
- Operational amplifiers
- Op-amp applications
- Comparing signals
- Buffering signals
- Amplifying signals
- Filtering signals
- Combining signals