Explain PIN diode and give its application.
The p-i-n diode is a stack of a very highly doped p+- layer, and nearly intrinsic layer i and a very highly doped n+ layer. The variable element is resistances are Ri, that is related to the i- layer thickness. This layer has very high resistivity that can be increased further by reverse biasing the diode but that decreases rapidly when the diode is forward biased as mobile carrier from the p and n regions are injected into the i layer. In practice however the idealized ith layer is approximated by either high resistivity p layer or a high resistivity n layer. The structure, impurity distribution and electric field distribution of a p – i – n diode are as shown in Fig. 24(a), (b) and (c) respectively. Although gallium arsenide can be used in the construction of PIN diode but silicon tends to be the main material, because of its high power handling capacity and resistivity and at the same time ease of fabrication. The equivalent circuit of a packaged p-i-n diode is shown in Fig. 24(d).
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Fig. 24 (a) Structure of print, (b) Doping profile, (c) Electric field distribution
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Fig. 24(d) Equivalent circuit of PIN diode.
Ls=total series inductance (normally due to lead)
Cp=stray capacitance of the package
Cf=fringing capacitance
Cj=junction capacitance due to the store charge at the boundaries of the depletion layer
Rj=resistance, i.e., the reciprocal of the conductance caused by carriers generated with in the region
Cd=diffusion capacitance due to the charge storage or current flows through the depletion region
Ri, Ci=are the resistance and capacitance of the portion of the i.layer, exclusive of the swept out region
Rs=some of the resistance of the p+, n+ layers and metal semiconductor contact resistance
Now, basic principle of operation of the diode is the variation of resistance by varying the biasing voltage as mentioned. Under reverse biased condition resistance changes from 5 – 10 KΩ and under forward bias condition the resistance varies between 1 and 10Ω. Thus when the diode is mounted across a co – axial a waveguide, by varying the reverse biasing voltage the diode impedance can be matched with the impedance of the transmission line and as a result maximum power can be transmitted. Whereas, under forward bias condition the diode offers very low impedance (nearly equal to zero) across the waveguide as a result most of the power will be reflected back and hardly any is transmitted. Thus the p – i - n diode acts as a switch. Also, a continuous variation of bias voltage from negative to positive value makes the use of a diode as avertable attenuator.
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Fig. 15(a).png)
Fig. 14(a)