I'm a little confused about whether transistors use voltage or current to control the amount of current though the drain/source. I'm reading a book that gave the formula: beta = Ic/Ib That seem to me that current is controlling it, but when I tried with a n-channel MOSFET(2n7000). First I used two D batteries in series to get 3V and about 25mA. I used a direct connection to the gate and it wouldn't drive a LED. So I tried a 9V batter(but really at 7.9V) with about 18mA and it would drive the LED.

This makes me believe that the voltage is what controls it. Can someone clear this up for me.

 

The control mechanism for a BJT is generally considered to be base current. The control mechanism for an enhancement-type MOSFET is voltage applied to the gate relative to the source.

hgmjr

 

Hello,

In an enhancement gate mosfet the gate voltage creates a channel for the current.

This image is from the attached PDF fron fairchild.

Bertus

 

Thank you, that makes sense.

voltage applied to the gate relative to the source

So if I want the 3V battery to work I should drop the voltage to the drain and give the gate a higher voltage than that?

 

Thank you, that makes sense.

So if I want the 3V battery to work I should drop the voltage to the drain and give the gate a higher voltage than that?

First we need you to tell us whether you want to use an n-channel or p-channel enhancement-type MOSFET or NPN or PNP bipolar junction transistor.

hgmjr

 

A 2N7000 is an N-ch enhanced MOSFET, not a bjt (bipolar junction transistor).

bjt's are current controlled devices; a small current through the base controls a relatively large current through the collector.

Enhanced MOSFETs are voltage controlled devices. In the datasheet you can find specifications for the threshold voltage given as Vgs (voltage on the gate relative to the source terminal), but that's just where they begin to conduct. You really need to look at the Rds(on) (resistance from the drain to the source when the MOSFET is considered turned on) specification to determine how high you need to get Vgs in order to get the specified Rds(on).

 

I'm confused about what the data sheet is saying. When it says this:

Rds: Vgs = 10V, Id = 500mA

After that it gave the resistance it would give. So does that mean I need to give the gate 10V and .5A? That seems kind of high to me, especially the current. I was getting it to work with a 9V battery and it's current isn't anywhere close to that high(but its hard for me to tell because my multimeter is very cheap and the current functions do not work very well.).

 

The 10 volts is between the gate and source (Vgs). The current is at the drain, meaning 500 ma current through the device. Rds is the Resistance between the Drain and the Source (Resistance, Drain to Source) when the gate is 10 volts positive with respect to the source, and conduction current is 500 ma.

 

Oh, I see, those name(like Vgs) make a lot more sense now. Thanks

 

Very General (basic) Rule Of Thumb: Use MOSFETs (voltage controlled current source) for switching On/Off.

Use BJT Transistors for linear small signal response (current controlled current source).

The 2N7000 is a Logic Level (and small signal) Switching MOSFET, compared to Power MOSFETs.

The Datasheet has all the information you need.

Vth is the voltage where the MOSFET will begin conducting
Rds ON is the most important aspect when used in a switching application, always pay attention to Vgs and Vds and Ids when deciding on a device.

Ciss is input capacitance, which determines switching time. With Power MOSFETs, this is typically a coulomb rating rather than a farad rating.

Hope this helps.

 

Yes, that definitely helps. I even wrote that in my notes so I don't forget.