# P-N junction

Discussion in 'Homework Help' started by Hello, Dec 18, 2008.

1. ### Hello Thread Starter Active Member

Dec 18, 2008
82
0
A certain P-N junction diode of germanium has a leakage current of 10^-8 A at 127 Deg C. The diode is forward-biased with a constant-current source of 1 mA at room temperature. If current is assumed to remain constant calculate the junction barrier voltage at 127 Deg C.

- Not sure if i'm doing the right method.

Any help is greatly appreciated.

2. ### veritas Active Member

Feb 7, 2008
167
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Which method are you using? Can you show work you've done thus far?

3. ### Hello Thread Starter Active Member

Dec 18, 2008
82
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I used the Boltzman diode Eqn. for Germanium, took value of T as 400.15 K. Not sure if i have to include the room temperature for the net current flow through the diode, Is.

I only included value of T=400.15K and found V = 0.397 volts.

Not sure if its right!

4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
The answer depends on the ideality factor (n) in the diode equation. What value did you use?

5. ### Hello Thread Starter Active Member

Dec 18, 2008
82
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I used n=1 for germanium.

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I also got 397mV when I used N=1, but I don't think it's correct. Silicon would give you the same answer, and I'm pretty sure that there are bigger differences between Silicon and Germanium diodes than the leakage current.
I'm wondering if you either missed a class, or you are being flim-flammed by your instructor.
If you calculate it using the bandgap voltage for Germanium, you might get a different (better?) answer, except that the paper I referenced says that the equation does not work at 127C.
Bottom line - I don't have a clue.

7. ### Hello Thread Starter Active Member

Dec 18, 2008
82
0
My lecturer sucks, I only have handouts with the theory no examples nor any relevant ones shown in lectures.

Anyways thanks for trying.

8. ### steveb Senior Member

Jul 3, 2008
2,433
469
It's hard to be sure, but I think you have done the problem correctly based on the information you were given. By the way, that diode equations is typically referred to as the Shockley Diode Law, named after one of the inventors of the transistor.

Also, under the assumptions of the Shockley Diode Law, the reverse saturation current is the main parameter that differentiates silicon and germanium (or any other material or diode type, such as Schottky). However, this is an idealized equation and real diodes do not obey it perfectly.

Last edited: Dec 19, 2008