In PN diodes in reverse bias and for small forward voltage the capacitance is determined by the depletion capacitance, whereas for large forward biases the diffusion capacitance dominates over the depletion capacitance which increases exponentially with the applied voltage, in forward bias after a certain voltage the capacitance starts decreases. Why does this happen?
Dear Prashanth Kumar M,
As i said before , your question is interesting. The papers you presented are for organic not silicon pn junction diodes. I think investigations are needed to determine the dependence the the pn diode capacitance on the voltage at high current densities where the diode is deeply stressed in the forward biasing. However this problem is of theoretical importance because at high forward biasing the forward current is limited by the ratings of the diode. Also, the diode is operated as switch therefore one is interested in in its switching characteristics rather than its small signal performance. may be small signal performance is required for small signal diodes and for the junction characterization. One can also get information about the conduction mechanisms of charge carriers in the diode. This was to show the importance of such topic. For crystalline pn junction diode, The behavior of the the diode capacitance is well studied and well known. The depletion layer junction capacitance decreases with the increase of the revere voltage. I n case of forward biasing, there is also the storage diffusion capacitance. The CV behavior of the diode depends on how every one depends on the the voltage,
It is so that the junction capacitance increases with forward biasing till the electrostatic barrier at the pn junction interface is reduced to zero by this forward bias voltage. However this may need very large diode current density leading to the burning of the diode.It is an extreme stress on the diode. Once the barrier is eliminated, the junction capacitance is no longer existing. Under this condition, the diode current is no longer an injection diffusion current but it will be drift current. Here also the diffusion capacitance will be reduced to zero in the limit.
Which means under this extreme biasing conditions, the diode behaves as an ohmic resistance. The real situation may be more complicated than that. But i simplified the picture to show that the diode capacitance can pass through a maximum and then rapidly reduced because the conduction changes from diffusion controlled to drift field controlled with heavy forward biasing.
This may shed some light on this phenomena. More investigations are needed
to explore this object.
Wish you success.
Dear Prashath,
could you please present your C-V curve and tell us how did you measure it?
Sir, I didn't do any experiment, In some papers I saw but I didn't find the reasons why it is happening like that.
Well, can you give the link of such papers? I find it really an interesting question.
Actually we have done simulation work on this (saadoune et al Solid-State Electronics 50 (2006) 1178–1182). we have attributed this to defects. Some other refernces attribute this to accumulated charge at P-N interface. Some others even observed negative capacitance. It is also attributed to defects which change the semicondcutor from lifeteime regime to relaxation regime. See papers by B K Jones, M McPherson,
Sorry for the late reply Sir, I am going through the paper, though I asked for Si PN diode, but the trend was observed in other material devices. Here are some papers PRL100,086802 (2008),Applied Physics Letters 102, 253303 (2013); doi: 10.1063/1.4812487
Dear Prashanth Kumar M,
As i said before , your question is interesting. The papers you presented are for organic not silicon pn junction diodes. I think investigations are needed to determine the dependence the the pn diode capacitance on the voltage at high current densities where the diode is deeply stressed in the forward biasing. However this problem is of theoretical importance because at high forward biasing the forward current is limited by the ratings of the diode. Also, the diode is operated as switch therefore one is interested in in its switching characteristics rather than its small signal performance. may be small signal performance is required for small signal diodes and for the junction characterization. One can also get information about the conduction mechanisms of charge carriers in the diode. This was to show the importance of such topic. For crystalline pn junction diode, The behavior of the the diode capacitance is well studied and well known. The depletion layer junction capacitance decreases with the increase of the revere voltage. I n case of forward biasing, there is also the storage diffusion capacitance. The CV behavior of the diode depends on how every one depends on the the voltage,
It is so that the junction capacitance increases with forward biasing till the electrostatic barrier at the pn junction interface is reduced to zero by this forward bias voltage. However this may need very large diode current density leading to the burning of the diode.It is an extreme stress on the diode. Once the barrier is eliminated, the junction capacitance is no longer existing. Under this condition, the diode current is no longer an injection diffusion current but it will be drift current. Here also the diffusion capacitance will be reduced to zero in the limit.
Which means under this extreme biasing conditions, the diode behaves as an ohmic resistance. The real situation may be more complicated than that. But i simplified the picture to show that the diode capacitance can pass through a maximum and then rapidly reduced because the conduction changes from diffusion controlled to drift field controlled with heavy forward biasing.
This may shed some light on this phenomena. More investigations are needed
to explore this object.
Wish you success.
I find the answer from Abdelhalim Zekry very interesting. The C-V measurement usually assumes a parallel conduction model (Cp and Gp). Once the conduction becomes drift-limited, the capacitance disappears, and the model falls apart. The accuracy of the data can be checked using the quality factor (Q) of the measurement (XR/Xc). If the resistance is too small, the small signal 'probe' current through the capacitor (Cp) may be too small to give an accurate C-V. Usually a Q greater than 5 is expected.
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