The behavior of (I-V) curve of some Cds sensors takes Ohmic (linear) behavior and schottky behavior, I need to explain when the Ohmic and schottky behavior of sensors appears?
Whenever 2 different materials with different bandgaps are in contact (such as metal and semiconductor), it creates a 'potential barrier between the two materials which prevents the most charges (both electrons or holes) from traveling through the barrier (only a small number of carriers get enough energy to get through the barrier and can cross over to the other material). Making a potential difference between the junction lowers the barrier (from the semiconductor side). The ohmic characteristics are dependent upon the work function and electron affinity of the metal and the semiconductor respectively.
In the case of the ohmic behavior of the metal-semiconductor interface, (when a small potential is applied) charge carriers are free to move in or out of the semiconductor due to nominal resistance across the contact interface (however, due to biasing the barrier height does not change from the metal side).
Moreover, the 'Ohmic type contact' is a low resistance junction (non-rectifying) that provides current conduction from metal to semiconductor and vice versa. Theoretically speaking the current should increase/ decrease linearly with the applied voltage. With an immediate response for any small voltage.
If a metal work function is larger than a semiconductor electron affinity, therefore contacting metal to the semiconductor leads to electron flow from the semiconductor to the metal as a prerequisite of Fermi levels alignment and thermal equilibrium. The remaining positively charged atoms in the semiconductor part create a space charge region. Thus electron flows from semiconductor to metal see a barrier of height known as the Schottky barrier.
Schottky contact has a rectification capability, with a large current in the forward bias and very low in the reverse bias, which can be seen from the I-V of a diode. The I-V curve for the Schottky junction in the metal-semiconductor connection shows nonlinear I-V characteristics compared to the ohmic contact.
Hi Khawla Ahmad Aloueedat,
I totally agree with the explanations given by Tanzila Nurjahan above.
However if you want to obtain ohmic behavir of your contacts, you have to reduce your contact resistance.
In this purpose, you may first proceed with a local n-type doping with chlorine of your CdS sample under your contacts positions as explained in the reference below.
This might be done before processing metallic contacts.
This local n-type doping of CdS prior to metallic contacts processing is usually helping to obtain good ohmic (non Schottky-like) behavior of your CdS sample I-V characteristics.
Reference :
Synthesis of high quality n -type CdS nanobelts and their applications in nanodevices R. M. Ma, L. Dai, H. B. Huo, W. Q. Yang, G. G. Qin, P. H. Tan, C. H. Huang, and J. Zheng Citation: Applied Physics Letters 89, 203120 (2006); doi: 10.1063/1.2387982 View online: http://dx.doi.org/10.1063/1.2387982
Good luck
Abderrahmane Kadri
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