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.

GUNN DIODE

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Gunn diode

(JB Gunn discovered Gunn Effect at IBM in ’62 in GaAs, InP, and CdTe).

Gunn diode (Transferred electron device) is used in high-freq electronics. It has three regions: two heavily N-doped layers sandwiching a thin layer of lightly doped N-type material. Application of voltage across the sandwich creates a very high electrical field in the thin middle layer, forcing a current to flow through it and altering its conducting properties. Its resistivity increases, preventing further conduction and causing the current to fall. It means a Gunn diode has a region of negative differential resistance. This negative differential resistance fetches major application of Gunn diode in oscillators at microwave  freq and above. Apply DC to bias the Gunn diode into its negative resistance region, which cancels the positive resistance of the load circuit, thus creating a circuit with zero resistance, which will produce spontaneous oscillations.

Its oscillation freq governed by middle layer can be tuned with an electronic resonator in the form of a waveguide, microwave cavity or YIG sphere. Gunn diode mounted inside a cavity cancels the loss resistance of the resonator, and it oscillates at its resonant frequency, which can be tuned mechanically, by adjusting the size of the cavity. Gunn diodes are often used as oscillators in W-band (75-110 GHz). Gallium arsenide Gunn diodes are made for frequencies up to 200 GHz, gallium nitride materials can reach up to 3 THz.

In addition to the valence and conduction bands, GaAs has a sub-band at energy higher than the conduction band. The sub-band is empty until energy is supplied to elevate electrons to it. Such energy can come from K.E. of ballistic electrons. Those electrons in the conduction band which are moving with sufficient kinetic energy can reach this band. In GaAs, the mobility or drift velocity in the third band is lower than the mobility in the usual conduction band, so with a small increase in forward voltage, more and more electrons can reach the third band and the current decreases. It creates a region of negative incremental resistance in the V–I characteristics.

(These electrons may acquire the requisite energy from a strong electric field, or they may be injected with the right energy by a cathode. For the latter, the cathode material has to be chosen carefully; chemical reactions at the interface need to be controlled during fabrication and additional monoatomic layers of other materials inserted. With forward voltage applied, the Fermi level in the cathode moves into the third band, and reflections of ballistic electrons starting around the Fermi level are minimized by matching the density of states and using the additional interface layers to let the reflected waves interfere destructively.)

When the potential applied to the diode is high enough, the charge carrier density along the cathode becomes unstable, and will develop small slices of low conductivity and high field strength which move from the cathode to the anode. It is not possible to balance the population in both bands, so there will always be thin slices of high field strength in a general background of low field strength. So in practice, with a small increase in forward voltage, a slice is created at the cathode, resistance increases, the slice takes off, and when it reaches the anode a new slice is created at the cathode to keep the total voltage constant. If the voltage is lowered, any existing slice is quenched and resistance decreases again.

The laboratory methods that are used to select materials for the manufacture of Gunn diodes include angle-resolved photoemission spectroscopy.

At low E-field most of the electrons will be located in the lower energy central valley Γ. At higher E-field, most of the electrons will be transferred in to the high-energy satellite L and X valleys where the effective electron mass is larger and hence electron mobility is lower than that in the low energy Γ valley. Since the conductivity is directly proportional to the mobility, the conductivity and hence the current decreases with an increase in E-field or voltage in an intermediate range, beyond a threshold value V_th as shown in Fig. This is called the transferred electron effect and the device is also called ‘Transfer Electron Device (TED) or Gunn diode’. Thus the material behaves as –ve resistance device over a range of applied voltages and can be used in microwave oscillators.

            

 where Cj is diode capacitance;

                       Rj is diode resistance; 
                      Rs is sum of lead, ohmic contacts, and bulk resistance of the diode,
                     Cp is package capacitance and Lp is package inductance. 
                    The value of negative resistance lies between –5 and –20 Ω.

It is an inexpensive oscillator of microwave power. Its I-V characteristics show that for voltages between 0V and ~1V, it behaves as a linear resistor. An increase in voltage takes it to threshold where the current stops increasing. Further increase in voltage takes the diode to negative resistance region where the curve slopes downward, which means that it is getting ready to oscillate. The operating point is usually about 4 times the threshold voltage. It often oscillates in W-band (75-110 GHz). 

Applications: As amplifiers, oscillators at µwave freq (highest output power of any semicon device). Used for airborne collision avoidance radar, anti-lock brakes, sensors to monitor the flow of traffic, car radar detectors, pedestrian safety systems, distance traveled recorders, motion detectors, slow-speed (22m/sec) sensors, traffic signal controllers, automatic door openers, automatic traffic gates, process control equipment to monitor throughput, burglar alarms and equipment to detect trespassers, sensors to avoid derailment of trains, remote vibration detectors, rotational speed tachometers, moisture content monitors.