Join Barron Stone for an in-depth discussion in this video Field effect transistors, part of Electronics Foundations: Semiconductor Devices.
- [Instructor] Unlike Bipolar Junction Transistors which work by biasing a pair of P-N junctions, Field-Effect Transistors turn on and off by using an electric field to control the behavior of a semiconductor material, making it more or less conductive. There are two main types of Field-Effect Transistors, Junction Field-Effect Transistors referred to as JFETs and Metal-Oxide Semiconductor Field-Effect Transistors or MOSFETs. There are a lot of similarities between JFETs and MOSFETs, but MOSFETs are more widely used, because they have a higher input impedance than JFETs so for this course, I'll be focusing primarily on MOSFETs.
The physical structure of a MOSFET begins with a slab of doped semiconductor material called the body. And for this example, I'll use P-Type Semiconductor material for the body. A small section of oppositely doped material, called a well, is deposited near each end of the body and in this case those wells are N-Type material. One of those wells is connected to a terminal called the source and the other well is connected to terminal called the drain.
Between the two wells a layer of insulative material called silicon dioxide, which is similar to glass is laid on top of the body. And then on top of that insulator is a conductive metal plate connected to a third terminal called the gate. These three layers down the center are what give the Metal-Oxide Semiconductor Field-Effect Transistor it's name. To use this type of MOSFET, I'll connect the source terminal to ground and connect the drain terminal to a positive voltage source that will be supplying current through the transistor.
The voltage between the gate and source terminal, labeled here as VGS, is what controls whether this transistor is turned on or off, whether it is operating in the cutoff region or the saturation region. If the source and gate are at the same voltage, so the difference between them is zero, this type of transistor will be in the cutoff region. The P-Type body between the two N-Type wells puts up a really high resistance that prevents current from flowing between the source and drain terminals.
To turn this transistor on, I'll raise the voltage at the gate until it's above a minium threshold voltage. Raising the voltage at the gate terminal means the metal plate attached to the gate will accumulate more positive charge. And since oppositely charged particles attract each other the extra positive charge at the gate will attract some of the negative charge from the body causing it to accumulate at the top near the insulator layer. This increased amount of negative charge between the two N-Type wells creates a low resistance path that allows current to flow from the drain to the source.
Now if I only increase the gate voltage up to the minimum threshold, the transistor will only be partially on and allow some current to pass through. If I increase the gate voltage to be even higher, that will attract more negative charges towards the gate creating an even larger channel which makes the drain to source resistance even smaller to allow more current through. The resistance between the source and the drain when this MOSFET is completely turned on is one of the key characteristics of a MOSFET, labeled here as rDS(ON) and it's often less than 100 milliohms.
It's important to point out that the insulating layer of silicon dioxide between the metal gate and the P-Type body prevents current from flowing in or out of the gate terminal. And this highlights one of the key differences between Bipolar Junction Transistors and Field-Effect Transistors. BJTs are current controlled devices. A small amount of input current to the base terminal will produce a much larger current between the emitter and collector. MOSFETs on the other hand are voltage controlled devices.
The amount of voltage between the source and gate terminal controls the amount of current that'll flow through the transistor. That gives the MOSFET a really high input impedance at it's gate terminal, which means it will hardly draw any current from the device that's generating the control signal. Now just how Bipolar Junction Transistors come in two opposite flavors, in P-N and P-N-P MOSFETs also come in two varieties that behave in opposite ways, N-Channel and P-Channel.
The N-Channel MOSFET shown here on the left is the same transistor I demonstrated earlier in this video. It's called N-Channel, because when a positive voltage is applied to the gate it attracts negatively charged electrons to the top of the body, which create a channel for current to pass through, between the two N-Type wells. In the P-Channel MOSFET on the right, the body is made from an N-Type semiconductor material and the two wells are both P-Type material. As it's name implies, the P-Channel transistor uses positive charge in the body to create the channel for current to pass through between the two P-Type wells.
Turning the P channel transistor on requires the gate terminal to have a negative voltage relative to the source to attract those positively charged holes within the body material. Now, to confuse things just a little bit more, the two transistors shown here are called Enhancement Mode MOSFETs. Enhancement Mode MOSFETs get that name because when the correct gate to source voltage is applied, it enhances the channel between the source and drain which turns the transistor on and allows current to flow.
Enhancement Mode MOSFETs are equivalent to a normally open switch, because by default they're turned off and block current. You have to apply the correct voltage to the them to turn them on. Depletion Mode MOSFETs on the other hand act like a normally closed switch, because by default they're turn on and let current through. When a certain gate to source voltage is applied to the Depletion Mode MOSFET, it depletes the channels ability to carry current, which turns the transistor off.
So between these different types of channels and modes, there are four main types of MOSFETs. N-Channel Enhancement Mode, P-Channel Enhancement Mode, N-Channel Depletion Mode, and P-Channel Depletion Mode. The Enhancement Mode MOSFETs are by far more commonly used than their Depletion Mode counterparts, so I'll focus on Enhancement Mode MOSFETs for the rest of this video. It's important to be aware of the different MOSFET types that exist, to avoid using the wrong one.
When using N and P-Channel MOSFETs in a circuit the orientation of their source and drain terminal will be flipped. On an N-Channel MOSFET, the drain should always be connected to a higher voltage than the source terminal, so the voltage drop from the drain to the source, or VDS, will be positive. And on a P-Channel MOSFET, the drain voltage should always be lower than the source, so the drain to source voltage drop will be negative. In practice, N-Channel MOSFETs are usually drawn in circuit diagrams with their source terminal on the bottom 'cause it'll often be tied to ground, sometimes through a resistor and for P-Channel MOSFETs, the source terminal is usually oriented on top, because it's connected to a positive supply voltage.
In both cases, the drain terminal is typically connected to the electrical load, that the transistor will be regulating current through. In this configuration, to turn the N-Channel MOSFET on, I need to raise the gate voltage until the voltage drop from the gate to the source is greater than a minimum threshold. To turn on the P-Channel MOSFET, on the other hand, I'll need to do the opposite, by lowering the gate voltage until the voltage drop from gate to the source is more negative than the threshold.
Another way to look at that relationship between the gate and the source is that in either type of Enhancement Mode MOSFET, if the gate and source have roughly the same voltage the transistor will be turned off. Raising the gate voltage high enough above the source will cause an N-Channel MOSFET to turn on and lowering the gate voltage far enough below the source will cause the P-Channel MOSFET to turn on. The schematic symbols used to represent Enhancement Mode MOSFETs have quite a few variations, which can get a bit confusing.
The simplest symbol, shown here with three terminals is commonly used to represent MOSFETs when they're used in digital circuits. There's a tiny circle on the P-Channel MOSFET that indicate that it's control signal has the opposite polarity of the N-Channel MOSFET. Another three terminal symbol used to represent MOSFETs looks similar to the symbols used for Bipolar Junction Transistors. In both of these symbols, the arrow is connected to the source terminal, similar to how the arrow on a BJT symbol is connected to the emitter terminal.
And in both cases, the arrow points in the direction of the conventional current flow through the transistor. Now, the third set of transistor symbols is where things start to get confusing, because it adds an extra terminal to the middle, representing the body of the transistor. Behind the scenes, MOSFETs actually have four terminals and the fourth terminal is connected to the body or bulk of the transistor. The reason for having a body terminal is beyond the scope of this video, but you usually don't even need to think about it, because the body will be directly connected to the source terminal inside of the transistor package.
This four terminal MOSFET symbol indicates that by connecting the middle body terminal to the source leg. Additionally, notice that the arrows have moved from the source terminal to the body terminal, and more importantly it's flipped directions. So pay close attention when you're reading or drawing schematics to make sure you use the right type of MOSFET. The broken bar between the source, body, and drain terminals indicates that this is an Enhancement Mode Transistor, which behaves like a normally open switch, hence the breaks.
In the symbol for a Depletion Mode Transistor, the bar between those three terminals in continuous, to indicate that it acts like a normally closed switch. Finally, one more accessory you might encounter on this symbol is a little upward facing diode between the source and the drain. MOSFETs are especially sensitive to a electrostatic discharge, so many MOSFET devices will include an internal diode to protect itself. Although there are other variations of the MOSFET symbol, these are the primary ones you'll encounter in the wild.
And if you're looking at older circuit diagrams or hand drawn schematics, you may also see a circle drawn around the transistor symbol, which makes it easier to read and busy drawings. You can usually find MOSFET components in similar packages as Bipolar Junction Transistors. The TO-02 is a popular three terminal package for low powered MOSFETs that works well for breadboarding circuits. And the TO-220 is its counterpart for high powered transistors that need to dissipate more heat.
- 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