My Hands-On Look: Enhancement vs. Depletion MOSFETs
Alright, let’s talk about MOSFETs. Specifically, the enhancement and depletion types. I remember messing around with these a while back because, honestly, the names sounded similar and I wanted to really see the difference myself, not just read about it.
So, I dug through my component drawers. Found a couple of standard n-channel enhancement-mode MOSFETs, the kind you use everywhere, like the 2N7000 or similar. Then I had to specifically hunt down a depletion-mode one. They aren’t as common in the hobbyist stuff I usually do, but I managed to find one, I think it was a BF-something JFET-like depletion MOSFET, doesn’t matter the exact part number for this.
First thing, I got my trusty breadboard out. Didn’t need anything fancy. I set up a simple circuit.
- A low voltage power supply, maybe 5 or 9 volts.
- An LED with a current-limiting resistor connected to the drain.
- The source pin went straight to ground.
- Another resistor connected to the gate, just to pull it down to ground initially or to limit current if I connected it elsewhere.
- My multimeter ready to measure voltages and check continuity.
Testing the Enhancement Mode First
I popped the enhancement-mode MOSFET into the breadboard. Connected the power. The LED stayed off. Exactly what I expected. The gate was essentially at zero volts (pulled to ground). No current flowing from drain to source, except maybe a tiny leakage.
Then, I took a wire and connected the gate to the positive supply voltage (through a resistor, always safe!). Bam! The LED lit up. Brightly. Disconnected the gate from positive voltage, connected it back to ground. LED off. It was clear: this type needs a positive voltage on the gate (relative to the source for n-channel) to create a path, to ‘enhance’ the channel so current can flow. No gate voltage, no conducting channel, it’s basically ‘off’.
Now for the Depletion Mode
Okay, swapped it out for the depletion-mode MOSFET. Same circuit. Powered it up with the gate connected to ground through the resistor. Guess what? The LED turned ON immediately! That was the key difference, right there. This thing conducts drain-to-source even with zero volts on the gate. It’s naturally ‘on’.
So, how do you turn it off? I tried applying a positive voltage to the gate, like with the enhancement mode. The LED stayed on, maybe got a tiny bit brighter or dimmer depending on the specific part, but it didn’t turn off.
Then I remembered how these are supposed to work. You need a negative voltage on the gate (relative to the source for n-channel) to turn it off. I didn’t have a negative supply handy on my bench at that moment, so I got creative. I put the gate resistor between the gate and source (so Vgs = 0V, LED is ON), then I tried pulling the gate slightly below the source potential using a voltage divider from ground, simulating a negative Vgs. It was fiddly, but yeah, as I made the gate voltage negative relative to the source, the LED dimmed and eventually turned off. Applying that negative voltage ‘depletes’ the channel, pinching it off and stopping the current.
The Main Takeaway For Me
So after fiddling around:
- Enhancement Mode: Think ‘normally off’. You need to apply the right gate voltage (positive for n-channel) to turn it ON. Like a standard switch that’s open until you press it.
- Depletion Mode: Think ‘normally on’. It conducts with zero gate voltage. You need to apply the opposite gate voltage (negative for n-channel) to turn it OFF. Like a switch that’s closed until you press it to open.
It seems simple on paper, but actually wiring them up and seeing one turn on when you expect it to be off, and vice versa, really sticks the concept in your head. Knowing which one you have is pretty important when you’re designing or troubleshooting something! Grabbing the wrong type can lead to some confusing behavior in your circuit. That little hands-on session cleared it up for me way better than just datasheets.