physics geeking revisited -- quantum circuits! woohoo!

[danaeris]

A quantum dot is a thing that is basically a finite potential well. So, any electrons caught in it will have quantized energy levels.

A resonance tunneling device is this thing which has a quantum dot in the middle. Applying a voltage across the entire device increases the energy in the quantum dot and the right hand side of the device so that the quantum dot's energy levels line up with those outside of it, and the electrons are able to tunnel (quantum tunneling) due to the resonance (aka, the energy levels lining up). If the electrons can tunnel through, they can basically just travel through the entire device without any difficulty, so it's sort of like a voltage activated switch.

A resonance tunneling transistor is supposed to be a resonance tunneling device with a gate electrode added in on top of the quantum dot, and that addition changes its function to that of amplifying the varying voltage applied to the gate electrode. So supposedly, you put a voltage across the gate electrode in addition to a small voltage across the entire thing, and combined these cause the energy levels to line up, resonance occurs, and the device conducts again all the way across.

OK, so I think I now understand what the gate electrode does... all it is is a method for applying a voltage JUST across the quantum dot, as opposed to the entire device. What I don't understand is why this amplifies anything. With the device, you put a potential difference across the whole thing and if it is one of the right amounts (ie. causes one of the quantom dots' energy levels to line up with the other regions' energy levels), the device conducts. With the transistor, you've got a small voltage that isn't the right voltage applied across the whole device (so energy levels don't quite line up and the transistor is not conducting yet). When another voltageis applied to the gate/quantum dot, it adds enough energy to cause the energy levels to line up after all, and the transistor conducts. How is the end result different, and why does the transistor amplify the potential applied to the gate electrode? And where does this amplified output exit the transistor?

Thanks to bk2w for getting me a little further along in understanding this. Any more help from anyone would be greatly appreciated.

I have one more question on this chapter which I wanted to write on this topic, and then I'll be done with this chapter. Which will leave me with five more! woohoo!

Comments

[qedrakmar.livejournal.com]

It's more of a trigger than a direct amplification. In any transistor, your gate voltage is typically much less than the voltage across the source/drain, but it's based on the cross voltage. If you have a 2eV gate voltage, but 20eV across the transistor, then that's effectively an amplification. It's just amplificiation one step removed. Sort of...

[bk2w.livejournal.com]

Okay, let me see if I can describe what I think is happening better. Also without getting lines completely garbled.

So, looking at a pnp-FET, where the emitter and collector are p-doped (and have a shortage of electrons, or a surplus of 'holes', depending on how you want to look at it), and the well (including the channel directly below the gate) is n-doped, and has surplus of electrons.

In typical use, the emitter is connected to a high voltage source, and the collector to a low-voltage sink. If the gate has zero volts on it, then the channel-collector interface looks like a reverse-biased diode, and no current flows. If you start applying a low voltage to the gate, you start attracting electrons from the well into the channel. This can overcome the collector's repelling of electroncs in the channel, and sharply decreases the energy required to make an electron and hole meet at that interface. The higher the gate voltage, the more electrons are pulled up into the channel, and the lower the energy required to flow current from emitter to collector, and thus the more current that can flow.

The quantum dot adds an interesting side effect. As you noted, if you put the right voltage across the emitter and collector, you can get resonance in the dot, and current can flow. You can apparently adjust the potential energy level of the dot by putting a voltage across the gate.