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.

MICROWAVE DETECTOR

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Microwave Detector

Detector is a two-terminal device that rectifies an RF signal (like a diode rectifies an ac signal for a power supply). Detectors are used as the receiving element for amplitude modulated signals.

Detectors are nonlinear semiconductor diodes that generate, mix, detect, and switch microwave signals. Detector was used for the first time as receiver of crystal radio to rectify amplitude modulated signal. It had a galena (PbS) crystal and a metal pin called "cat's whisker" that exhibited Schottky effect on this metal/semiconductor junction.

                                                      

Marconi used the detector “Coherer” in 1902 to receive Morse coded electrical signal across the Atlantic. The detected weak RF signal did something like micro-welding on metal filings in Coehrer, which became electrically conductive. 

Most diodes used in the microwave industry are made on Silicon, but for some applications gallium arsenide is a better choice.

Applications: Detectors convert amplitude-modulated μwave signals to baseband and so they are used for μwave power measurements. They are also used for scalar network analyzer to evaluate circuit gain as well as port impedance match. Detecor diodes are used as amplifiers, oscillators at µwave freq, for devices like airborne collision avoidance radar, anti-lock brakes, motion and traffic detectors, traffic signal controllers, car radar detectors, distance traveled recorders, slow-speed (22m/sec) sensors, automatic door openers, process control equipment to monitor throughput, burglar alarms, sensors to avoid derailment of trains, remote vibration detectors, rotational speed tachometers, moisture content monitors.

In general, diodes will conduct when the anode voltage is higher (more positive) than the cathode voltage. An exhaustive list of the important diodes is:

IMPATT diodes                                    

Gunn diodes                             

PIN diodes

Schottky diodes

Zener diodes

Tunnel diodes

Varactor diodes                        

Step recovery diodes                

Noise diodes

Schematic of a detector circuit: The heart of the circuit is the detector diode, whose non-linear characteristics facilitate the process of detection.

Operation: By rectifying the incident power, the diode produces a signal of single polarity whose amplitude is proportional to the input power level (square-law) and which gets applied to the bypass capacitor. This detector circuit gives a +ve voltage and if a -ve voltage is desired, the diode has to be reversed.

To obtain a dc voltage from the detector, a dc return path is created by placing an RF choke across the detector diode. This inductor offers a low-impedance path to ground at lower frequencies but at μwave frequencies it behaves like an open circuit.

The bypass capacitor grounds μwave frequencies and determines the upper limit of the signal bandwidth. It provides video capacitance to detector circuit that works even at 0 GHz, i.e. when input is a continuous wave. The video bandwidth is linked with minimum rise and fall time of the detector circuit, and the minimum width of detectable RF pulse.

The input impedance of a diode when it is switched on, is <50 Ω, so an impedance transformer that raises its impedance, precedes it.

For a certain range of power levels, the output voltage of a detector is proportional to its incident power. In linear operation, as per Ohm's law the voltage is proportional to the square-root of power. Thus, in the square-law region, power is proportional to the square of voltage. The ratio of output voltage to incident power is a constant in the square-law region for detector diode, typically value is 500 mV/mW.

Types of detector diodes: Schottky or Esaki tunnel diodes are used as detectors. The two ports of a detector are the RF port and the video port. A coaxial detector might have an SMA connector on the RF port and a BNC connector on its video port. The video port may not contain RF frequencies if its RF signal is rectified AM-modulated.