PIN DIODE: In a PIN(p-intrinsic-n) diode, the semiconductor wafer has heavily doped narrow layer of p type material separated from an equally heavily doped narrow layer of n type material by a thicker layer of high resistivity material that is intrinsic(or very lightly doped material).
silicon is widely used because of its power handling capacity and high resistivity in the intrinsic region and easy fabrication(although GaAs can also be used but without these advantages).
Electrical contacts are taken from the two heavily doped regions.
PIN diode in forward bias mode, offers a variable resistance-decreasing with the increasing in forward current. For larger dc current it will appear like a short. In reverse bias mode, it offers infinite resistance.
when there is no bias applied to the diode, there will be diffusion of charge carrier due to concentration gradient across the junction. Let us consider NI junction. Diffusion of holes and electrons across NI junction form a very thin depletion layer in N-region and a relatively thicker depletion layer in I-region containing equal but opposite fixed charges. when the reverse bias is applied and gradually increased, the thickness of the depletion layers increases until the entire I-region is swept free of mobile carriers. the reverse bias required to sweep out mobile carriers from I-region is known as the swept-out voltage.in most microwave applications, sweeping-out of I-region is necessary, otherwise the current flow due to mobile charge carriers in the high resistivity I-region will be a source of signal loss. so PIN diodes, when used as microwave switches, are reverse biased. the bias is well beyond the swept out voltage(usually-2V).For larger reverse bias, a thin depletion layer appears in the P-region. At zero bias, most of the I layer has mobile charge carrier and diode has high resistance. more and more charge carrier inject into I-region from P- and N-regions and forward resistance is reduced.
thus when a PIN diode is forward biased it acts like a variable resistance.
PIN diodes are widely used for microwave power switching, limiting and modulation.
The PIN diode acts as a low frequency rectifier that could rectify more power than an ordinary p-n junction diode. upto about 100MHz, the operation is similar to an ordinary p-n junction diode.
at higher frequencies the PIN diode acts like a variable resistance.Rectification ceases at higher frequencies due to carrier storage junction and the transit time across the large intrinsic region.
APPLICATIONS OF PIN DIODE:
1.PIN diode as a switch :
It can be used either in series or in shunt.In the first case when the diode is reverse biased,switch is 'off' or open and when it is forward biased it is closed.the bias is changed by a suitable control system.
In the second case when the diode is forward biased,it offers a shot circuit.Hence energy is fully reflected back and no power flows to the load.Hence switch is open.If reverse biased the diode is open.Hence the load receives the power i.e.,,switch is closed.
2.PIN diode as an Amplitude Modulator:
the diode is kept at low reverse bias and in series with the low frequency modulating signal. the modulating signal amplitude is kept smaller than the RF carrier signal.the modulating signal changes the RF resistance of diode so that varying amount of mismatch results. hence the amount of carrier power reflected back and hence the amount of carrier passed beyond diode circuit towards the output varies as the input value of modulating signal.
3.PIN diode as a phase shifter.
The circular coupled phase shifter the input signal travels a distance equal to L or L+l depending on whether S is closed or open respectively.Therefore between the two conditions we have a phase shift of 2 θ (the signal has to travel to permanent short and return). By adjusting 'l' phase shift can be adjusted.'L' will be far greater in magnitude than 'l' which will be fraction of a wavelength. It finds applictions in phased array radars.
4.PIN Diode as a limiter. when the input power is moderate,output follows inut power and for larger input,the diode absorbs power and output will be limited i.e., will not vary much with the input power.
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