# Homework Help: Total E in Quantum Well

1. Dec 15, 2012

### D__grant

1. The problem statement, all variables and given/known data

-This is a problem on my practice final that I haven't been able to solve. Hoping someone out there can take a crack & clarify it for me.

Quantum wells are devices which can be used to trap electrons in semiconductors. If the electron is in the well it has a lower energy than if it is outside, so it tends to stay in the well. Suppose we have a quantum well which has a width of DeltaX and a depth of 1.0 eV , i.e. if the electron is in the well it has a potential energy of -1.0 eV and if it is outside it has a potential energy of 0 eV. Use the uncertainty principle to find the value of DeltaX for which total energy kinetic & potential of an electron in the well is zero.
Note: This is the smallest size well we can have because if deltaX is any smaller, the total energy of the electron in the well will be bigger than zero, and escape.

2. Relevant equations
1. E=KE+PE
2. Vo= -1 eV
3. Total Energy > 1/2m x (h/2piDeltaX)^2 - Vo

3. The attempt at a solution

I set the Total Energy=0 and attempted to solve for deltaX. My first solution was the the order of 10^-11 but I doubt I answered it correctly. Also, the mass of an electron was not given on the exam so I'm wondering if there's a different path to take. Thank you

2. Dec 15, 2012

### BruceW

I think you have the right idea. If the mass of the electron was not given, then what constants were given?

3. Dec 15, 2012

### D__grant

Charge of an electron in joules? I noticed somewhere in my notes he used the mass of an electron in eV.
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I am just completely unsure as to whether this is "the way to do it."

Thank you though, Bruce

4. Dec 15, 2012

### BruceW

Yeah, I'm pretty sure that's the right way. This is one of those examples that are used over and over again to get students used to making "order-of-magnitude" calculations.