Can anyone give me a short, no nonsense explanation of MOSFETs and their data sheets?

Thread Starter

FuneralHomeJanitor

Joined Oct 12, 2019
22
I have been trying to learn alot about MOSFET circuits, and have read many articles, used 3 different books, and have watched several videos, some multiple times, and have even successfully built a couple basic circuits using some MOSFETs I bought, but am still just not fully grasping a few things and hope someone can clear them up. I had similar issues with the BJT and after people helped me on here I have really been doing great with BJTs. My books have less info on Mosfet but do have biasing circuits for JFETs. They don’t really have diagrams or explain important things. I will list the questions I have below with the hopes someone can clear these up. These are regarding enhancement mode mosfets, and I believe I used a power mosfet (IRF520N) for breadboarding and it worked pretty well.

My questions are:

1) I understand that Rds(on) is an important value from the datasheet. What does this actually mean, what region of operation does this apply to, and does this value with respect to any input or output values like VGS or ID?

2) I know in the BJT there is the hfe value that makes it easy to understand the relationship between base and collector currents, and I know the MOSFET used a voltage instead. What is the relationship between the VGS input voltage and the output voltage or current? Is there anything similar to the beta value but for mosfets?

3) Are the biasing configurations the same as for JFETs? For example, self-bias, gate bias, voltage divider bias?

4) What are some important things to consider when using MOSFETs for both switching and amplification purposes?


Hopefully someone can answer these for me, I am sorry if they are dumb questions, but I am definitely missing something in my books and videos, and it will clear alot of things up for me. Sorry for the wordiness. Thanks in advance!
 

MaxHeadRoom

Joined Jul 18, 2013
21,636
Do a search of International publications.
Hexfet Power Mosfet Designers manual Volumes 1 to 3.
They used to be available from IR in hard copy, now you may find some out there on the net.
The Vol 1 would give you what you are looking for I believe.
Vol 3 is all the data sheets.
Max.
 

bloguetronica

Joined Apr 27, 2007
1,423
I'll answer your questions point by point.
...
1) I understand that Rds(on) is an important value from the datasheet. What does this actually mean, what region of operation does this apply to, and does this value with respect to any input or output values like VGS or ID?
...
Rds(on) refers to the source to drain resistance when the device is fully on.

...
2) I know in the BJT there is the hfe value that makes it easy to understand the relationship between base and collector currents, and I know the MOSFET used a voltage instead. What is the relationship between the VGS input voltage and the output voltage or current? Is there anything similar to the beta value but for mosfets?
...
There are two ways of figuring this:
- The gate threshold voltage or Vgs(th) parameter of the datasheet tells you the required gate voltage, in relation to the source of the transistor, that is required to turn on the MOSFET, for a given current, or;
- The "transfer characteristics" graph, that tells you how much drain current you can get for a given Vgs, because the conduction area tends to saturate more as you increase the current. The conduction area increases with Vgs thus, the greater the Vgs, the more current it can pass before it saturates (pinch off effect).

The BSS138 datasheet provides a nice way to figure this out.

...
3) Are the biasing configurations the same as for JFETs? For example, self-bias, gate bias, voltage divider bias?
...
JFETs operate only in depletion mode, which means they resemple a triode, because they are fully on with no voltage applied to the grid, and you need to apply a negative voltage in relation to their source to shut them off. MOSFETs can either operate in depletion or in enhancement mode, depending on the MOSFET. However, the great majority of them operate in enhancement mode, which means you need to apply a positive voltage, in relation to their source, to turn them on.

...
4) What are some important things to consider when using MOSFETs for both switching and amplification purposes?
...
I can't talk in regards to amplification purposes, because I don't have that knowledge, and I suspect it requires a whole book just for that. As for switching purposes, simply apply any voltage greater than the threshold Vgs for the current you need to switch on. For instance, you can use 3.3V logic to directly turn on the gate of a BSS138 MOSFET, without any resistor in the middle. Consider the voltage drop that the MOSFET will create, though (typically, it is not much).
 
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dl324

Joined Mar 30, 2015
11,914
1) I understand that Rds(on) is an important value from the datasheet. What does this actually mean, what region of operation does this apply to, and does this value with respect to any input or output values like VGS or ID?
It's the on resistance and it isn't always of primary importance. It varies with gate-source voltage, drain-source voltage, and temperature, and other things.

From this On Semiconductor graph, you can calculate resistance for a typical BSS138 by calculating the slope of the curve at your operating point:
clipimage.jpg
2) I know in the BJT there is the hfe value that makes it easy to understand the relationship between base and collector currents, and I know the MOSFET used a voltage instead. What is the relationship between the VGS input voltage and the output voltage or current? Is there anything similar to the beta value but for mosfets?
Vgs affects on resistance, so you can visualize the MOSFET as a variable resistance.
3) Are the biasing configurations the same as for JFETs? For example, self-bias, gate bias, voltage divider bias?
They're similar.
4) What are some important things to consider when using MOSFETs for both switching and amplification purposes?
Every parameter can be important. You should read the datasheet and understand which are important for your application.

If you're using with a low voltage, threshold voltage would be important. If you're switching a large current, on resistance and maximum drain current would be important. If you're switching it at a high frequency, gate capacitance would be important.

One important consideration is that threshold voltage isn't well controlled during manufacturing. Die from the same wafer and in close proximity would have better matching. With discrete devices, you can't even be certain they came from the same wafer lot; though devices at both extremes can come from the same batch of wafers.

EDIT: Added "lot" to wafer description. Manufacturers process wafers in lots. This is important when a factory manufactures different types of devices or is doing experiments during process development.
 
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Marc Sugrue

Joined Jan 19, 2018
167
I have been trying to learn alot about MOSFET circuits, and have read many articles, used 3 different books, and have watched several videos, some multiple times, and have even successfully built a couple basic circuits using some MOSFETs I bought, but am still just not fully grasping a few things and hope someone can clear them up. I had similar issues with the BJT and after people helped me on here I have really been doing great with BJTs. My books have less info on Mosfet but do have biasing circuits for JFETs. They don’t really have diagrams or explain important things. I will list the questions I have below with the hopes someone can clear these up. These are regarding enhancement mode mosfets, and I believe I used a power mosfet (IRF520N) for breadboarding and it worked pretty well.

My questions are:

1) I understand that Rds(on) is an important value from the datasheet. What does this actually mean, what region of operation does this apply to, and does this value with respect to any input or output values like VGS or ID?

2) I know in the BJT there is the hfe value that makes it easy to understand the relationship between base and collector currents, and I know the MOSFET used a voltage instead. What is the relationship between the VGS input voltage and the output voltage or current? Is there anything similar to the beta value but for mosfets?

3) Are the biasing configurations the same as for JFETs? For example, self-bias, gate bias, voltage divider bias?

4) What are some important things to consider when using MOSFETs for both switching and amplification purposes?


Hopefully someone can answer these for me, I am sorry if they are dumb questions, but I am definitely missing something in my books and videos, and it will clear alot of things up for me. Sorry for the wordiness. Thanks in advance!
1) I understand that Rds(on) is an important value from the datasheet. What does this actually mean, what region of operation does this apply to, and does this value with respect to any input or output values like VGS or ID?

Rds(On) is only as important as it is for your application, its not always desirable to have a perfect switch. Rds(On) is normally specified for particular conditions of Gate-Source Voltage and Drain Source Current. Mosfets have a linear region where the Drain-Source Resistance is dependent on the Gate-Source voltage - again this depends on current. A mosfet can only pass so much Drain-Source current given a particular Gate-Source voltage. At saturation it can pass maximum current.

2) I know in the BJT there is the hfe value that makes it easy to understand the relationship between base and collector currents, and I know the MOSFET used a voltage instead. What is the relationship between the VGS input voltage and the output voltage or current? Is there anything similar to the beta value but for mosfets?

Mosfets provide VGS vs drain current characteristics. For NMOS the larger the positive drain source voltage the lower the RDS(On). For PMOS its negative volts.

3) Are the biasing configurations the same as for JFETs? For example, self-bias, gate bias, voltage divider bias?

You can bias a Mosfet to hold in partial conduction in a similar manner to JFETS.

4) What are some important things to consider when using MOSFETs for both switching and amplification purposes?

For switching, gate capcitance is a consideration depending on speed of operation. Also inductance and capacitance between the drain and source of the MOSFET can lead to parasitic resonances which can lead to EMI issues if not damped in power conversion applications. Mosfets also contain a body diode accross the drain-source which will conduct if reversed biased. Mosfets also have a particular feature called avalanche rating, this rating effectively clamps when overvolted and a maximum amount of energy can be disipated into the device.

In amplification (not my particular forte) temperature drift would be a consideration as gains would become less stable over hot and cold cycling as the characteristic of the mosfet changes. I think JFETs are used in some applications instead of mosfets as they are more linear and stable (don't quote me on that though). Also Temperature in general will be a major consideration as typically in amplication you are almost always operating in the linear region with higher Rds(On) this means you are potentially disipating Watts in the switch so it must be thermally bonded or cooled by an heat sink
 
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TechWise

Joined Aug 24, 2018
137
One thing to note is that if you're using the MOSFET as a hard switch, as in most power electronics applications, you're generally not interested in 90% of the datasheet which describes all the interesting things that happen while the switch is neither on nor off. The headline specifications are what will generally draw you to the device, namely:
  • What is its voltage rating? This is probably your first search criteria when selecting a device.
  • What is its Rdson? This is a so-called "figure of merit". The lower the better. This will give you an idea of the device's current handling capability and will be an important factor in designing the heatsink.
  • What are it's gate drive requirements? If it's 0-5V, great, we can drive it from a microcontroller. If it's 0-12V, great, we can drive it with a simple gate driver. if it's -3-12V, hmmm, we'll need a split power supply which has implications.
  • What package is it in? For high power, you're probably going to find most MOSFETs in TO-247 packages, maybe the odd TO-220.
These are the things you'll expect to see front and centre on the datasheet of a power MOSFET. Once you've chosen a device, you'll start to look further into the datasheet:
  • What do the switching transitions look like? If it gives you a turn-on/off energy in Joules, great. That makes it easy to calculate the switching losses. If it gives you t_on, t_off, t_delay etc, things will be more difficult.
  • What's the thermal resistance from junction to case? Very important for heatsinking but generally fairly standard for a given package size.
  • What's the gate capacitance? This will be important when designing the gate driver.
  • Does it come in a four-legged package with a Kelvin connection to the source? This is becoming commonplace for Silicone Carbide devices where stray inductance is a killer.
 

crutschow

Joined Mar 14, 2008
25,680
2) I know in the BJT there is the hfe value that makes it easy to understand the relationship between base and collector currents, and I know the MOSFET used a voltage instead. What is the relationship between the VGS input voltage and the output voltage or current?
If operated in the linear region as an AC amplifier, the relationship is the forward transconductance, gfs (Ids/Vgs).
An example of that is that is shown below for the 2N7000 N-MOSFET.

Note that it applies only when the MOSFET is in the linear region (Vgs above its threshold voltage).

1606010699904.png

Otherwise to determine the nominal Vgs voltage required for a particular source-drain current, you can look at the data sheet characetristic curves.
 
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