Friday, 4 January 2013

PIN DIODE

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PIN diode (switch)

PIN stands for P-type, intrinsic, and N-type semiconductor material layers and it can be made using either GaAs  or Silicon as the semiconductor substrate. Microwave switches are often made using PIN diode, where P is the anode, and N is the cathode. Conventionally the current injection side (under forward bias) is called the anode. In the schematic symbol the anode is the side with the arrow, the cathode is the side with the plate.



                                                              

Classification of PIN diodes:
Following figure shows a horizontal PIN diode also called as H-PIN. Here the P and N layers are formed on top of the I layer.

However in the vertical PIN diode or V-PIN, the diode has a stack of the three materials, P layer at the top, N-layer at the bottom (on substrate) and I layer sandwiched between them.

          
If we reverse the order of stacking the above semiconducting layers, we make a NIP (a PIN reversed upside down) that looks as: 

Characteristics of PIN diode: Following diagram shows the variation in the resistance of PIN diode as a function of dc current flowing through it. The higher the current injected through its middle layer (I region), the lower is its RF resistance. Thus it acts like a current controlled resistor. Its ideal resistance (R) – current (I) relationship is R = K / I, where K is a constant. If plotted on a log-log scale it looks like a straight line.

One can obtain a resistance ranging from 0.1 Ω to 10 KΩ for the PIN diode, which covers the entire horizontal axis of the Smith chart. And the fact that the resistance value of 50 Ω appears almost in the middle of the response, makes it such a versatile device. It offers itself as an open circuit, a short circuit, or provides any reflection coefficient between these extremes. Thus it can be used to make switch, phase shifter and variable attenuator.



Limitations:
Conventional diode has non-linear I-V characteristics and it rectifies a signal, irrespective of the frequency of the input signal. A forward biased diode allows very-very high current to flow through it for a signal voltage of ~1 volt or more. And a reverse biased diode allows no current for finite values of signal voltage until the breakdown occurs. An example is Schottky diode, which is used as a detector taking RF as input and giving dc as output.

A PIN diode also behaves like a rectifier at low frequencies. At µwave frequencies, its I–V curve undergoes a change, and it starts behaving like a resistor, whose resistance is determined by the amount of DC current flowing through the I-region. Thus a PIN diode is essentially a DC-controlled high-frequency resistor. If no DC current is flowing through the I-region, PIN diode behaves like an open circuit.

PIN diode has a low frequency limitation due to carrier lifetime. The frequency at which the PIN diode makes a transition from behaving like a diode to a current controlled resistor is a function of the thickness of the I-region. Thicker diode can be used as switch at lower frequencies. By choosing its fabrication parameters, a PIN diode can be made to work as switch operating at a frequency as low as 1 MHz. For a DC control current as small as a few mA, a PIN diode can work as switch to carry µwave signals of very large amplitude, for instance like 1000 mA. Thus PIN diode is a wonderfully efficient device for designing RF switch handling substantial power.

Limiters:
A limiter is a device which offers low attenuation to small signals and its attenuation increases as the power level rises. PIN diodes can also be used to create such a nonlinear device viz. limiter. For this purpose, PIN diode is put as shunt element across a transmission line and put at a gap of quarter wave to improve small signal response. Some PIN diode limiters are passive and the PIN diode creates the nonlinear response by itself. On the other hand, an active limiter includes a Schottky diode detector that applies DC current to the PIN diode to turn it on at lower power.

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