# MOSFET off state at certain voltage?

I'm new to electronics and today I've discovered MOSFET transistors. It lets the current flow between source and drain terminals, when the voltage, applied to gate terminal, reaches a certain level. But I want a MOSFET (if it's still a MOSFET) that is normally letting the current flow between S and D, and is cutting the flow when the voltage applied to the gate terminal reaches a certain level.

So what kind of a device do I need?

• You can use a depletion mode MOSFET, but you'll have to apply a negative voltage wrt the source to turn it off (and they're only available commercially (AFAIK) in the N-channel persuasion). – Spehro Pefhany Jun 7 '14 at 20:42
• Useful in certain cases: Connect gate to a +ve bias source so that Vgs turns the FET on. Rais source voltage from zero and FET will turn off when VGS falls below critical Vth voltge. Source driver must carry load current. Not as useless as it may sound for a range of reasons. – Russell McMahon Jun 8 '14 at 11:43

You can invert the input of the mosfet with 2 resistors and a regular NPN transistor. R1 will pull the Gate of the mosfet high when the input is Low. When you put a High on the input the NPN transistor will pull the voltage on the Gate back to ground

simulate this circuit – Schematic created using CircuitLab

• I still don't fully understand how this works. Can you give more detail about what's happening here? I'm new to this stuff, so it would be really nice if you would draw/explain how is the current flowing, and how the current/voltage change in components. – Qualphey Jun 10 '14 at 9:42
• The left side of the circuit is just a basic inverter. See electronicsclub.info/transistorcircuits.htm#inverter and youtube.com/watch?v=pTVek7v0_R8 . If it's still unclear after that, just let me know. – Gerben Jun 10 '14 at 15:50
• So if the voltage on NPNs base is high, then the collector attracts more current or something? And the MOSFETs gate gets lower voltage, and if the NPNs base voltage is low, the gate current is not attracted by the NPNs transistor? Do I get the idea? – Qualphey Jun 13 '14 at 11:37
• Yes, that is basically it. Try imagining the NPN is a push button. If it's not pressed, VCC will go via R1 to the Gate of the mosfet. If you press the button the gate is connected, via the button, to ground. The 10k resistor is "overruled" by the 0Ohm (direct connection) going to ground. – Gerben Jun 13 '14 at 13:19

"...I want a MOSFET that is normally letting the current flow between S and D, and is cutting the flow when the voltage applied to the gate terminal reaches a certain level...."

I suppose you mean that the control voltage will vary in analog way. Thus, the OFF - ON (or ON - OFF) transition fot the drain current will not be exactly "digital". In other words, reach $Vgs_{th}$ just causes the beginning of the mosfet conduction - enlarging the channel width. Maybe you need an analog comparator instead.

You can go for any of the following:

1. Depletion channel MOSFET ($De$-MOSFET), eg: LN150
2. Junction Field Transistor (JFET), eg: 2N5457

These devices (n-channel) are normally ON ans you need a negative voltage at gate with respect to source to turn it OFF.