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


    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




    Junction temperature, in °C



    Case or contact temperature, in °C



    Dissipated power, in W



    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




    Ambient temperature, in °C



    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






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




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




    Lowest RF signal frequency, expressed in MHz




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




    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



    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





    reverse saturation current in A



    base of the natural number system ≈ 2.7128



    charge of an electron ≈ -1.602 x 1019 C



    forward voltage of the diode junction in V



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



    junction temperature in Kelvins



    a proportionality constant


    For a typical PIN diode, the IV characteristic is


    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.  





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