P-N Junction Diode - Charge question

In summary: N4148 junction diode, but got weird results. He was able to find the values for I0 and q from the data sheet, but is still confused about why the charge isn't 1.6x10^-19 C.
  • #1
bsr25
7
0
We have been looking at P-N junction diodes in our lab session this week and investigating the current through the circuit and how this relates to the Voltage.
We are using the approximation:

I = (I0)*exp(qV/kT)

We measured the voltage and the current for a few different voltages and this could be plotted into a graph. A regression analysis gave the reverse saturation current as I0 = 3.2 x 10^-6 A, and the effective charge carrier charge as q = 4.78 x 10^-21 C.

The diode being used is the 1N4148 junction diode.

Im trying to understand why the carrier charge is not equal to 1.6*10^-19 C. We were told that it has something to do with the data-sheet for this diode. But so far I am not sure what on the sheet I should be looking for and even how this would related to solving this problem.

Any help would be appreciated,

Thanks Ben.
 
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  • #2
the reverse saturation current as I0 = 3.2 x 10^-6 A

Is that a bit high?
 
  • #3
bsr25 said:
Im trying to understand why the carrier charge is not equal to 1.6*10^-19 C. Any help would be appreciated,

Thanks Ben.

That makes no sense. q = charge of an electron. If you came up with any other value then you have operated the diode outside the region where i = i0 exp(qV/kT) applies. A 1N4148's 'good' region is around i = 0.01 mA to 10 mA.

I agree the value of i0 is totally unrealistic. It should be in the pico to femtoampere region. Are you running the diode at or near room temperature?
 
  • #4
Yeah. This is my problem too. I expected the q value we got to at least be on the order of magnitude of the elementery charge. And from research I had done, expected I0 to be less.

However, the lab manual states: "Finally, if all goes well your value for the electron charge, q, will probabbly not be what you expect. At this point you should go back and look carefully at the datasheet for a PN-junction diode, and the values for 1N4148 in particular and try and reconcile the results".

This has me baffled.
 
  • #5
Oh right so I just looked at my data, and the currents we calculated were within the range:

0.08 mA to 14 mA.

And the diode is just at room temperature.
 
  • #6
Will get back later on this. The problem seems to lie with i0 which does not behave like a constant over voltage.
Obviously q = 1.6e-19 C, you would not want to come up with a model that says anything else ...
 
  • #7
I guess you measured on the forward-biased diode. The original equation for the junction diode is I=Io(exp(qV/kT)-1), and Io is usually determined by measuring on reverse-biased diode (negative V).

Even that equation is quite bad approximation for the relation between the measured voltage and current at high currents, when the current is limited by the parasitic resistances and internal resistances of the meters.

ehild
 
  • #8
Was the voltmeter (or whatever you used to measure the voltage) connected when you were measuring the reverse current?
 
  • #9
Data sheet here..
http://www.nxp.com/documents/data_sheet/1N4148_1N4448.pdf

Figure 5 suggests the reverse current should be < 100nA ish. Thats probably what they mean by checking the data sheet.
 
  • #10
A square wave generator was connected to a diode and resistor that were in series. Leads going to an oscilloscope were placed over the diode to measure that voltage drop. Leads were than placed over both the diode and resistor to get the voltage of the source. The oscilloscope measured both peak voltages and from that we calculated the resistance over the resistor (i.e. the source voltage minus the diode voltage). From this we got the current in the circuit. For a number of different source voltages we did this and got the current.

This was plotted as a graph (I against V across diode). The fit gave us values for I0 and q.
Is there an error in this setup? Perhaps where we are measuring something extra or something?

I don't know really. Shooting blanks here.

Thanks for the help so far!
Ben
 

1. What is a P-N junction diode?

A P-N junction diode is a type of semiconductor device that allows current to flow in one direction, while blocking it in the opposite direction. It is made up of a P-type semiconductor, which has an excess of positively charged holes, and an N-type semiconductor, which has an excess of negatively charged electrons. The boundary between these two types of semiconductors is called the P-N junction, and it is where the diode's unique properties come from.

2. How does a P-N junction diode work?

When a P-N junction diode is forward biased, meaning the positive terminal of a battery is connected to the P-type semiconductor and the negative terminal is connected to the N-type semiconductor, the diode allows current to flow. This is because the positive terminal attracts the negatively charged electrons in the N-type semiconductor, while the negative terminal attracts the positively charged holes in the P-type semiconductor. This creates a flow of current across the P-N junction. However, when the diode is reverse biased, meaning the positive terminal is connected to the N-type semiconductor and the negative terminal is connected to the P-type semiconductor, it acts as an insulator and blocks current flow.

3. What is the role of charge carriers in a P-N junction diode?

Charge carriers, such as electrons and holes, play a crucial role in the functioning of a P-N junction diode. In the P-type semiconductor, there is an excess of positively charged holes, which act as charge carriers. In the N-type semiconductor, there is an excess of negatively charged electrons, which also act as charge carriers. When the diode is forward biased, these charge carriers move towards the P-N junction and allow current to flow. When the diode is reverse biased, the charge carriers are unable to cross the junction and thus, no current can flow.

4. What is the purpose of a P-N junction diode in electronic circuits?

P-N junction diodes have many important applications in electronic circuits. They are commonly used as rectifiers, which convert alternating current (AC) into direct current (DC). They are also used as voltage regulators, as they have a constant voltage drop across the P-N junction. Diodes are also used in signal processing, switching circuits, and as protection devices against voltage spikes.

5. How does temperature affect the performance of a P-N junction diode?

Temperature can have a significant impact on the performance of a P-N junction diode. As temperature increases, the number of charge carriers also increases, causing an increase in current flow. However, at very high temperatures, the diode can become damaged or even destroyed. Additionally, the reverse current in the diode also increases with temperature, making it less effective as a current-blocking device. Special precautions, such as heat sinks, are often used to prevent overheating in diodes.

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