# Thermal Velocity of an Electron?

Discussion in 'Homework Help' started by Lee_831, May 22, 2011.

1. ### Lee_831 Thread Starter New Member

Nov 17, 2009
6
0
Hi,
Apologies if this has been asked before, I performed a search but didn't find much!
Working on a textbook of semiconductor device physics, and have come across a formula for the thermal velocity of a hole/electron.

It's listed as Vth = √3kT/m*
Where m* is the effective mass.

Now entering the data for a piece of n-type silicon as follows:
k = 1.38x10^-23 JK-1
T = 300K
m* = 1.18Mo

I work this out to be 1.02x10^-10, however the textbook answer is 1.08x10^5, which is nowhere near.
Am I doing something wrong with the units or is something else off? I appreciate any help.
Thanks,
Lee

2. ### jegues Well-Known Member

Sep 13, 2010
735
43
You are doing your algebra incorrectly, I was able to obtain the correct answer listed.

$V_{t} =\sqrt{\frac{3kT}{1.18m_{o}}} = 1.08 \times 10^{5}$

Last edited: May 23, 2011
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3. ### Lee_831 Thread Starter New Member

Nov 17, 2009
6
0
Hi,
Thanks for your quick reply, can you possibly suggest where I am going wrong? I am simply entering 3x300x1.38x10^-23, dividing that by 1.18 and then taking the square root.
Lee

4. ### jegues Well-Known Member

Sep 13, 2010
735
43
You have to divide by,

$1.18 \times m_{o}=1.18 \times(9.11\times10^{-31})$

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5. ### Lee_831 Thread Starter New Member

Nov 17, 2009
6
0
Sorry for the late reply.. Thanks everyone! Thought I was going insane.