Pin Diodes

       The p type and n type semiconductors are extrinsic semiconductors while intrinsic semiconductor is denoted as I. the device obtained by sandwiching pure intrinsic semiconductor I between p type and n type material is called PIN diode.
       The pin diode consists of a p and n type heavily doped materials alongwith intrinsic material. The intrinsic region is sandwiched between n type and p type. This is shown in the Fig. 1(a). the symbol is shown in the Fig. 1(b).

       When reverse biased, its transition capacitance is predominant and constant. Hence in reversed biased condition it acts as a constant capacitance, as shown in the Fig. 2(a)
       The characteristics of intrinsic semiconductor is such that it has low forward resistance RF. it further decreases with increasing current. Hence pin diode acts as current controlled variable resistance, when forward biased. This is shown in the Fig. 2(b).

1. PIN Diode Characteristics
       The pin diode has two characteristics which are
i) The forward series resistance characteristics showing the graph of forward series resistance against forward current. This is shown in the Fig. 3(a).
ii) The reverse capacitance characteristics showing the graph of diode capacitance CT against the reverse voltage VR. this is shown in the Fig. 3(b). it shows that CT.

2. Applications of PIN Diode
       The pin diode is used in attenuator applications where resistance is required to be controlled by the current. The pin diode is used as a d.c. controlled microwave switch. The variable forward resistance characteristics make it suitable to be used as a modulating device. Some pin diodes are used as photodetectors in fiber optic systems.
3 .Comparison
The Table 1 gives the comparison of pin diode and p-n junction general purpose diode


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