Small-Signal MOSFET gate capacitance - Page 1
Forum section: electronics / beginners

author: Laertes
author_rank: Regular Contributor
timestamp: February 15, 2014, 01:33:44 am
content: Hi there, people,I'm not exactly an electronics engineer, but I always thought I had the very basics covered. Now I was trying to build a little circuit with a microcontroller driving a few LEDs and thought to use a MOSFET as a little driver transistor for the LEDs. And doing that on my breadboard, I came across an interesting little effect I'm not quite sure how to explain.So I got some 2N7000 n-channel MOSFETs(in a TO-92 package) I still had around here and started by testing if I had calculated the current-limiting resistor correctly, so I just build the circuit up, connected the source to ground, drain to the LEDs and hooked the gate directly to my 5V supply. And my LEDs lit up nicely, no overcurrent, nothing getting hot, fine so far.However, when I disconnected the gate voltage, I noticed that the LEDs didn't go off directly, they slowly faded out over like three seconds.With a 10kOhm pull-down resistor on the gate, the transistor shuts off immediately.Now I think that the only thing that could cause this kind of behaviour would be the gate capacitance, keeping the gate charged for a moment and thus leaving the transistor conductive.So here's the question: Is that true? Because the length of time the transistor stayed on kind of baffled me, I mean it must be discharging via gate-source leakage, right?What's a typical small signal MOSFET gate capacitance going to be? I couldn't find any info on that in the datasheet of the 2n7000, but assuming a gate-source resistance of 1.5GOhms(10nA leakage @ 15V), a discharge time of 3s and a charged voltage of 5V, I get some 500pF gate capacitance. That sounds like quite a lot to me, or am I calculating this wrong here?GreetingsLaertes

author: kony
author_rank: Regular Contributor
timestamp: February 15, 2014, 02:18:16 am
content: Also parasitic capacitance of breadboard (and leakage) applies. Considering this - you have two capacitors connected in parallel and hence the longer time .

author: Marco
author_rank: Super Contributor
timestamp: February 15, 2014, 02:40:59 am
content: 1.5 GOhms? I think that's about 5 orders of magnitude off (for the MOSFET itself at least). I assume your breadboard is leaking like crazy, otherwise that LED should just stay on.PS. Well at room temperature. The leakage current given in the datasheet is in the right neighbourhood ... if you operate it at it's maximum operating temperature of 150 °C.

author: Laertes
author_rank: Regular Contributor
timestamp: February 15, 2014, 03:45:09 am
content: thanks for the answers. The capacitance of the breadboard should be(according to one of dave's recent videos) somewhere in the 2pF range.Quote from: Marco on February 15, 2014, 02:40:59 amAccording to the fairchild datasheet(seehttp://www.produktinfo.conrad.com/datenblaetter/150000-174999/151034-da-01-en-TRANSISTOR_2N7000.pdf) the gate-body leakage is 10nA at room temperature(granted, it may only be 20°C in here, not 25°C, but that's not gonna matter too much, is it?). 10nA at 15V, thats 1.5GOhms, or does the gate resistance change with gate-source voltage? Or maybe with drain-source voltage(because it's specified at 0V drain-source...)?

author: Marco
author_rank: Super Contributor
timestamp: February 15, 2014, 03:53:01 am
content: It's a max value ... I guess if you repeatedly shuffle around with wool socks and grab the device you could get leakage up there too. Leakage should be fA range though.

author: Laertes
author_rank: Regular Contributor
timestamp: February 15, 2014, 03:18:45 pm
content: Quote from: Dawgie on February 15, 2014, 04:33:52 amYeah, basically it is, I only used little discrete BJTs before and didn't expect that gate capacitance thing.So, practical leakage is way lower than the specified value, and don't float a FET gate. Good to know.Okay, that's it I guess, thank you all.