FAQs

  • Does DC = 0 Hz in RF & Microwave PIN diode product Datasheets?

    Never!

    The minimum operating signal frequency for RF & Microwave diode product Datasheets, such as those for PIN diodes, PIN diode switches, PIN limiter diodes, PIN limiter modules, etc., does not extend downwards to 0 Hz. At and below some low frequency which is roughly correlated to the PIN diode’s carrier lifetime, these diodes act as rectifiers rather than as charge-controlled, RF & Microwave resistances.

    Datasheets which show “DC” as the minimum operating signal frequency generally do so to emphasize the very wide operating signal bandwidth capability of the product. For example, an R F& Microwave diode product might perform very well in the Ka Band (26 – 40 GHz) range but can also perform its fundamental function at frequencies which are decades lower in frequency, thus the claim for operation at “DC”.

     

  • I am designing a PIN diode switch that must handle high power. How much power can the switch handle?

    There is one primary determinant of power handling for a PIN switch: junction temperature. 

    The maximum junction temperature a PIN diode switch can handle is specified in the Absolute Maximum Ratings table of its Datasheet.  For most Silicon, GaAs and AlGaAs PIN diodes, the maximum rated junction temperature which produces a mean time between failures (MTBF) value of 1 million hours is 175 °C.  IMPORTANT: Check the Datasheet to Confirm this Value 

    Junction temperature (TJ) can be found by

    A junction temperature formula

    where

    TJ

    =

    Junction temperature, in °C

    TC

    =

    Case or contact temperature, in °C

    PD

    =

    Dissipated power, in W

    ΘJC

    =

    Thermal resistance from the diode junction to its outermost cathode terminal, in °C/W

     

    The value of TC is the temperature of the surface to which the cathode of the PIN diode is attached.  This surface could be the top of a microstrip transmission line, the ground plane or substrate of a transmission line, a heat sink, etc.  It is a surface whose temperature is known.

    ΘJC is the thermal resistance from the junction of the PIN diode to its outermost cathode mounting surface.  For an unpackaged die, qJC is the thermal resistance from the junction to the outermost layer of cathode metallization.  For a packaged PIN diode, qJC is the thermal resistance from the junction to the cathode terminal of the package.

    TJ may also be found by

    A second junction temperature formula

    where

    TA

    =

    Ambient temperature, in °C

    ΘJA

    =

    Thermal resistance from the diode junction to the ambient surroundings of the system in which the PIN diode is contained, in °C/W

     

    ΘJA is the thermal resistance from the junction of the PIN diode to the ambient environment surrounding the system in which the PIN diode is contained.  ΘJA is the sum of ΘJC and ΘCA.   ΘCA is the thermal resistance of the system to which the diode is mounted, measured from the surface to which the diode is mounted to the ambient surroundings of the system.  Since the diode manufacturer has no control over ΘCA, PIN diode manufacturers typically do not specify ΘJA.

     

     

  • I am designing a PIN diode switch that must handle high power. How much reverse bias voltage do I need to keep the PIN diode out of conduction when the high power signal is present?

    Several factors determine the minimum required reverse bias voltage required to keep a PIN diode out of conduction when a large RF signal is incident upon the diode:

    • The power level of the RF signal
    • The frequency of the RF signal
    • The envelope of the RF signal (CW, pulsed, etc.)
      • If the RF signal is pulsed, what is its duty cycle?
    • The standing wave ratio present at the diode

    The magnitude of the minimum required reverse bias voltage (|VDC|) is found from

    FAQ2.JPG

    Where,

    VDC

     

    =

    Magnitude of the minimum DC reverse bias voltage required to maintain the diode in its nonconducting state

    VRF

     

    =

    Magnitude of the RF signal voltage (including the effects of VSWR)

    fMHz

     

    =

    Lowest RF signal frequency, expressed in MHz

    D

     

    =

    Duty factor of the RF signal (D=1 for CW signals)

    Wmils

     

    =

    Thickness of the PIN diode I layer, expressed in mils (thousandths of an inch)

     

    Contact MACOM application engineers for an Excel spreadsheet which implements this equation: https://www.macom.com/support/contact-us/technical-support-form

    Source: Caverly, R. H and Hiller, G., “Establishing the Minimum Reverse Bias for a p-i-n Diode in a High-Power Switch”, IEEE Transactions on Microwave Theory and Techniques, Vol.38, No. 12, December 1990

     

     

  • What is the operating frequency range for this RF & Microwave diode?

    This question cannot be answered without information about the intended purpose of the circuit in which the diode will be used as well as information about the acceptable performance specifications for it. 

    For example, a PIN diode is often used as a switching element.  The insertion loss and isolation the diode produces are determined by diode parameters (capacitance and series resistance) along with the signal frequency. 

    For a shunt PIN diode, the magnitude of the vector sum of the diode’s series resistance when the diode is forward biased and its inductive reactance, determines the isolation the diode produces.  The diode’s capacitive reactance when it is not conducting largely determines its insertion loss.  Both of these diode properties are affected by the signal frequency.  The questions “How much isolation is enough?” and “How much insertion loss is too much?” determine whether a given diode is suitable for use at a specific signal frequency.

    A varactor diode is commonly used as the tuning element in a resonant circuit in a voltage controlled oscillator.  Such circuits also contain inductances or their equivalents which, in concert with the varactor capacitance, determine resonant frequency.  This constitutes a single-equation-with-two-unknowns situation, in which the value of the inductance is as important as the varactor’s capacitance.  Again, the suitability for use of a specific diode at a given frequency is decided by more than the diode’s properties alone.

     

     

  • What is MSL?

    MSL stands for “Moisture Sensitivity Level”. Nonhermetic semiconductor packages intended to be surface mounted (SMDs) can absorb moisture from the ambient atmosphere during the assembly process which can cause problems when these packages are attached to a printed circuit board (PCB) using surface mount technology.

    When these packages are attached to a PCB they are heated to a temperature which can flow metallic solder, typically in excess of 200 °.  These elevated temperatures will cause moisture which was trapped in the package’s encapsulant epoxy to vaporize and expand, which can cause damage to the encapsulant epoxy and the internal components of the packaged device.  This damage due to expanding water vapor (and other factors) is known as ‘popcorning’ due to the physical appearance of the damage it can produce.

    The IPC/JEDEC-J-STD-020 standard describes several classification levels for such packages, according to the level of protection which is required to prevent damage from ‘popcorning’.  These levels are defined by the amount of time a component may be stored prior to soldering to a PCB while subject to specific temperature and relative humidity conditions (30 °C/85% RH at Level 1, 30 °C/85% RH at all other levels) and by the preventative measures which are required to be employed immediately before component attachment to a PCB.

    MSL Level

    Floor Life

    Required Preventative Measures

    MSL 1

    Unlimited

    none

    MSL 2

    1 year

    if floor life is exceeded, bake immediately prior to component attachment to PCB

    MSL 2A

    4 weeks

    if floor life is exceeded, bake immediately prior to component attachment to PCB none

    MSL 3

    168 hours

    if floor life is exceeded, bake immediately prior to component attachment to PCB none

    MSL 4

    72 hours

    if floor life is exceeded, bake immediately prior to component attachment to PCB none

    MSL 5

    48 hours

    if floor life is exceeded, bake immediately prior to component attachment to PCB none

    MSL 5A

    24 hours

    if floor life is exceeded, bake immediately prior to component attachment to PCB none

    MSL 6

    -

    bake immediately prior to component attachment to PCB

     

    The IPC/JEDEC J-STD-033 standard describes handling, packing, shipping and the use of surface mount plastic packages which may be susceptible to damage caused by trapped moisture.

    MACOM specifies the appropriate MSL level for SMD products on the products’ Datasheets.

    Hermetically sealed packages are not subject to popcorning.

     

  • What voltage should I use to control a PIN diode?

    The answer to this question has two parts: 1) reverse bias control and 2) forward bias control

    Reverse bias control is discussed in the FAQ  That articles describes how to determine the minimum required reverse bias voltage to keep a PIN diode out of conduction when there is a large RF signal incident on the diode.

    For the forward bias condition, this question must be restated: how much current should I use to control a PIN diode?  A PIN diode is a charge-controlled device.  Its impedance is determined by physical characteristics of the diode, such as its I layer thickness, its junction area, the resistivity of its three layers, the semiconductor material which comprises the diode, etc., as well as the concentration of free charge carriers injected into its I layer from the forward bias current applied to the diode.

    At DC, the PIN diode’s current vs. voltage (IV) characteristic is that of a PN junction diode.  If we ignore the DC resistance of the diode which is typically very small, the diode’s forward current (IF) is described by the exponential equation

    FAQ4.JPG

    where

    Isat

    =

    reverse saturation current in A

    e

    =

    base of the natural number system ≈ 2.7128

    q

    =

    charge of an electron ≈ -1.602 x 1019 C

    VF

    =

    forward voltage of the diode junction in V

    k

    =

    Boltzmann’s constant ≈ 1.3807 x 10 -23 J/K

    T

    =

    junction temperature in Kelvins

    n

    =

    a proportionality constant

     

    For a typical PIN diode, the IV characteristic is

    Picture1.png

    We can clearly see from this relationship that for some values of forward voltage, a tiny change in forward voltage can produce an immense change in forward bias current.  It is much more precise to control the current supplied to the diode rather than to directly control its forward voltage.

     

     

  • PCB Design Considerations for Low Thermal Resistance

    When designing a printed circuit board (PCB) for low thermal resistance, various factors need to be taken into account. Some of the factors include the thermal dissipation requirements for the component, material properties of the component, the mating surface in terms of coefficient of thermal expansion (CTE), and the thickness of the PCB and mating pad size.  

    There are various ways to dissipate heat such as using via arrays, embedded heat slugs, etc. 

    Vias can either be non-conductive filled, conductive filled, or even solid copper depending on the thickness of the PCB and the diameter of the via.

    • Non-conductive filled vias offer the highest thermal resistance relying primarily on the barrel of the via to dissipate heat. 
    • Conductive fill materials can also be used to fill vias however when trying to maximize the number of vias on a pad in many cases the via diameter chosen may be much smaller than what many fabricators can fill accurately.  Smaller via diameters (i.e., 0.008 inches [0.200 mm] and under) may result in voiding within the via due to the particle sizes within the conductive fill and thus may not offer much improved thermal dissipation over a non-conductive filled via. 
    • If the PCB thickness is 0.020 inches [0.500 mm] or thinner solid copper vias are possible which will lower the thermal resistance compared to non-conductive or conductive filled vias.

    When using a via array the size of the pad will dictate the array size.

    Thermal heat slugs offer the maximum potential for lowering the thermal resistance between the component and the outside world.   When using thermal heat slugs extra attention should be taken into account in terms of how well the heat slug is installed in terms of co-planarity with the primary and secondary side of the PCB, thermal differences between the heat slug location and the remainder of the PCB as well as any potential CTE mismatches that may occur.  

     

     

     

  • What does t mean?

     

     t is also known as minority carrier lifetime or TL

    Minority carrier lifetime is the mean time a free charge carrier exists in a diode junction after the diode has been forced to transition from its conducting to its nonconducting state.  During this interval the diode’s impedance remains low, until almost all of the free charge carriers have been conducted out of the junction or recombined.

  • What does CJ mean?

    CJ stands for junction capacitance. 

    Junction capacitance is the capacitance of a die.  Junction capacitance does not include the effects of parasitic capacitance which might be contributed by a package or other environment in which the die is installed.

     

  • What does CT mean?

     

    CT stands for total capacitance. 

    Total capacitance is the capacitance measured between the terminals of a packaged diode.  In general, total capacitance is the sum of the junction capacitance (CJ) of the die and the parasitic capacitance of the package (CPKG), since these two components are connected in parallel with each other,

    formula.JPG

     

     

  • What does IF mean?

     

     IF stands for forward current. 

    Forward current is the current that flows through a diode when a forward bias voltage is applied. 

     

  • What does IR mean?

     

    IR stands for reverse current, also known as reverse leakage current. 

    Reverse leakage current is the current that flows between the terminals of a diode when a reverse bias voltage is applied.  For voltages significantly smaller than the diode breakdown voltage, most of the reverse leakage current flows through the passivation layer of the diode (around the actual diode junction).  When the diode is in avalanche breakdown, most of the reverse leakage current flows through the diode junction.

     

     

     

  • What does TL mean?

    TL stands for minority carrier lifetime. 

    Minority carrier lifetime is the mean time a free charge carrier exists in a diode junction after the diode has been force to transition from its conducting to its nonconducting state.  During this interval the diode’s impedance remains low, until almost all of the free charge carriers have been conducted out of the junction or recombined.

    In some cases, minority carrier lifetime may be called t.  

  • What does tRR mean?

     

    tRR stands for reverse recovery time.  It is a measure of the time required for a diode to cease conduction after reverse bias has been applied. 

    This nonideal characteristic of semiconductor diodes, especially those used in high-power rectification circuits, causes switching losses.

    The concept of reverse recovery time for semicondctor diodes is similar in concept to minority carrier lifetime, tLGenerally, the magnitude of the reverse bias for tRR measurements is much larger than that used for tL measurements.

  • What does VB (or VBR) mean?

     

     VB and VBR both stand for reverse avalanche breakdown voltage.  This is the reverse bias voltage at which a diode junction enters avalanche breakdown, which is the condition in which potentially large currents can flow in the reverse bias direction. 

    In general, for a pn or Schottky junction, this condition is not destructive as long as the reverse current flow is limited to relatively small magnitude, typically less than 100 microamperes. 

    However, for PIN diodes with I layer thickness greater than approximately 40 microns, the avalanche breakdown condition can almost immediately destroy the diode junction, even for reverse currents as small as 10 microamperes.

    In general, for RF/microwave diodes, operation in the avalanche breakdown condition should be avoided.

     

  • What does VF mean?

     

     VF stands for forward voltage. 

    The forward voltage of a diode junction is the voltage developed across the diode when it is biased into forward conduction.  Forward conduction occurs when the voltage applied to the anode of the diode is more positive than the voltage applied to the cathode of the diode.

     

     

     

  • What does VR mean?

     

     VR stands for reverse bias voltage. 

    Reverse bias voltage is a voltage applied to a diode junction in which the voltage on the anode of the diode is more negative than the voltage applied to the cathode of the diode. 

    The diode is held in its nonconducting state when a reverse bias voltage whose magnitude is less than its avalanche breakdown voltage is applied.

     

     

  • What is a PIN diode?

     

    A PIN diode (also known as a p-i-n diode) is a two-terminal device which comprises three layers: the heavily-acceptor-doped (p-type) anode layer, the heavily- donor-doped (n-type) cathode layer, separated by an intrinsically-doped layer known as the “I” layer.

    The RF impedance of the PIN diode is determined by its geometry as well as by the polarity and the magnitude of an applied bias signal.  A forward bias current reduces the instantaneous resistance of the I layer and a reverse bias voltage forces the diode’s impedance to its maximum value, best modelled as a large resistance in parallel with the diode’s junction capacitance.

    Since a PIN diode produces a bias-controlled impedance whose range of impedance variation can be 3 orders of magnitude or more, it is widely used as a switching element or as a variable attenuation element.

    MACOM PIN diodes may be produced using silicon (Si), gallium arsenide (GaAs) or aluminum gallium arsenide (AlGaAs).

     

  • What is a varactor diode?

     

     

    The word “varactor” is a contraction of the words “variable reactor”.  A varactor diode is a pn junction diode whose junction capacitance varies as a function of the reverse bias voltage applied to the diode.

    The depletion region which forms at the junction of the anode and cathode layers of the diode acts as the dielectric layer of a parallel plate capacitor, in which one of the conductive plates is formed by the heavily-p-type-doped anode layer and the other conductive plate is formed by the cathode layer of the diode.

    As the reverse bias voltage applied to the diode from an external source is increased, the cathode-side edge of the depletion layer sweeps through the cathode layer, away from the junction, thereby increasing the thickness of the dielectric layer.  The rate at which the dielectric layer changes thickness can be influenced by the doping profile of the cathode layer.

    Varactor diodes are classified as abrupt junction or as hyperabrupt junction devices, depending on the dopant concentration profiles.  Hyperabrupt varactors produce much larger available change in capacitance versus change in reverse bias voltage than abrupt junction varactors.

    MACOM varactor diodes may be produced using silicon (Si) or gallium arsenide (GaAs).

     

  • Why is the performance of an NLTL comb generator degraded when I attach my band pass filter to the output of the comb generator?

     

    The phase noise and conversion loss performance of a nonlinear transmission line comb generator (NLTL CG) are sensitive to the load impedance presented to the NLTL comb generator.  Harmonics of the input signal generated by the NLTL CG must be absorbed by the load attached to the NLTL CG to prevent reflections causing distortion and noise.

    If the output of the NLTL CG must be filtered, the filter should be a triplexer rather than a band pass, high pass or low pass filter.  In the case where a single harmonic or a band of harmonics are to be selected, a triplexer comprising a band pass filter designed to pass the desired harmonic(s), a low pass filter for the lower harmonics and a high pass filter for the higher harmonics should be implemented. 

    The outputs of the low pass and high pass filter sections should each be terminated in an absorptive load, whose impedance is approximately 50 Ω.  The input impedance in the pass band of the band pass filter should also be approximately 50 Ω.

     

     

  •  

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