Blog

Understanding Diode Terminology   Oct. 22, 2020

diodes.JPG

RF and microwave diodes come in multiple different offerings, ranging from PIN and varactor to Schottky diodes. Manufacturers often specify these diodes by parameters which can be measured without the need for a diode to be permanently connected to an external circuit.

What do the terms, abbreviations and symbols on RF and microwave component datasheets mean? To make it easier, we’ve compiled a list of common diode terminology you’ll find on datasheets:

 

Diode Parameter

Symbol

Definition

(Avalanche) Breakdown Voltage

VB or VBR

The reverse bias voltage at which a diode junction enters avalanche breakdown. For most types of RF and microwave diodes, this region of operation is to be avoided.

(Minority)(Carrier) Lifetime

TL or t

The mean time a free charge carrier exists in a diode junction immediately after its bias has transitioned from a forward bias current to a reverse bias voltage. Minority carrier lifetime is related to the amount of charge a diode stores while forward biased and is typically defined for a specific combination of forward bias current and peak reverse current.

Capacitance Ratio

CTx/CTy

The ratio of the capacitance a diode produces with applied reverse bias voltage VX to its capacitance with applied reverse bias voltage VY

Forward Bias Voltage

VF

Voltage polarity which is applied to a diode junction which produces current flow. For example, the positive side of a voltage connected to the anode of a diode and the negative side of the voltage applied to the diode’s cathode is the forward bias condition.

Forward Current

IF

The current that flows in a diode when a forward bias voltage is applied.

I Layer Length

W

Also known as I layer width. The mechanical thickness of the intrinsically-doped layer of a PIN diode. This parameter heavily influences many of the electrical properties of a PIN diode, including switching time, distortion performance, series resistance, capacitance, etc.

Junction Capacitance

CJ

The capacitance of a diode which results from the diode’s depletion region acting as dielectric layer separating two conductive regions, the anode and the cathode layers. This capacitance is produced by the semiconductor die only.

PIN (or p-i-n) Diode

 

A three-layer diode, consisting of a heavily-acceptor-doped anode layer and a heavily-donor-doped cathode layer sandwiched around a very lightly-donor-doped layer known as the intrinsic layer

Reverse (Leakage) Current

IR

The current that flows between a diode’s anode and cathode terminals when a reverse bias voltage is applied. This is a non-ideal characteristic of a semiconductor diode and is generally orders of magnitude larger than reverse saturation current.

Reverse Bias Voltage

VR

The voltage polarity which is applied to a diode junction to prevent current flow. For example, the positive side of a voltage source connected to the cathode of a diode and the negative side of the voltage source applied to the diode’s anode is the reverse bias condition.

Reverse Recovery Time

TRR

The interval required for the reverse bias current to substantially decay, immediately after the forward bias current is changed to a reverse bias voltage. TRR is similar in concept to TL. The primary difference is the magnitude of the forward and reverse biases are typically much larger for TRR than for TL measurements.

Reverse Saturation Current

IS or IO

The tiny current which flows when semiconductor diode junction is under reverse bias. This current comprises thermally-generated free minority carriers (electrons in the anode layer, holes in the cathode layer) which flow in the reverse bias direction. This current is temperature dependent and its magnitude is typically in the nanoamperes order of magnitude.

Schottky Diode

 

A diode whose rectifying junction is formed by intimate contact of a metal and a doped semiconductor. A Schottky diode operates with majority carriers only.

Series Resistance

RS

For PIN diodes: The RF resistance presented primarily by the diode’s I-layer as a function of a specified DC forward bias current

For Schottky diodes: The slope of the diode’s current vs. voltage characteristic at a specified forward bias current

For Varactor diodes: The RF resistance presented primarily by the diode’s un-depleted cathode layer as a function of a specified DC reverse bias voltage

Total Capacitance

CT

The parallel combination of a semiconductor diode’s junction capacitance (CJ) and the parasitic capacitance of a package in which the die resides.

Varactor Diode

 

A pn junction device whose cathode layer has a tightly controlled doping concentration. The thickness of the diode’s depletion layer is a function of the magnitude of the applied reverse bias voltage, the area of the diode’s junction and the doping concentration of the cathode layer.

 

For more help with definitions of RF and microwave diode parameters, contact your MACOM applications engineering team here: https://www.macom.com/support.


Read Full Post
Component Attach: Solder vs. Epoxy?   Apr. 06, 2020

Solder Epoxy Image.jpg

Choosing the proper attachment material when attaching a component to a printed circuit board or other material involves a diligent review of the properties of the component being attached. Some variables to consider include:

               > Material properties of the device
               > Properties of the mating material 
               > Surface conditions, such as plating on both the device and mating material
               > Subsequent assembly processes
               > Thermal dissipation requirements
               > Post environmental conditions 

Solders and epoxies can be procured in various forms.  Solder for instance can be in the form of preforms, solder paste and wire.  Epoxies can be selected as a pre-form or in a dispensable form.  Epoxies can have a range in thermal conductivity and in some cases have a higher thermal conductivity than some solders.

Material properties between the device being attached and the mating material require choosing a bonding material that can accommodate those coefficient of thermal expansion (CTE) differences.  In the case of using solder, if there is a large difference in CTE between the mating items then a soft solder should be selected. 

In terms of epoxy there are a wide range of epoxies that could be used to help with CTE differences.  In terms of thermal dissipation requirements, typically solder has been the material of choice. However with recent advances in epoxies such as sintered epoxies, the thermal conductivity ratings for the epoxy are in some cases 2X that of standard solders. 

Post processing assembly steps and total cost of manufacturing can also greatly affect the most appropriate material to use.  Solders, depending on the component type allow the user to select either no clean flux-based options or solders with a flux that require post assembly cleaning. For instance, for bottom terminated components (BTC) a no clean solder paste should be used. However, if post processing is needed where the flux becomes an issue trying to clean this is more difficult and costly.  If there are post steps such as post die attach or wire bonding that require the surfaces to be clean such as in the case of a hybrid design, then the fluxes will need to be either burnt off completely during reflow or post cleaned. When reviewing the plating on the device being attached in some cases such as raw dice, devices are offered with different plating options to allow the device to be either soldered or epoxied. 

As we can see careful review of all the properties and overall manufacturing process are key to making the right component attach choice. With the goal of having a greater first pass success rate, these quick checks and decisions can help save both time and money.

 

 


Read Full Post
Getting the Voids Out   Mar. 23, 2020

Getting the Voids Out Image.png

Reducing solder voids is a pervasive challenge affecting the electronics industry.  With higher thermal dissipation requirements placed upon the attachment of a device to a circuit board the number of voids within the solder joint becomes critical.  There are various considerations that need to be taken into account, including 1) the design of the substrate 2) the surface condition of the part being attached 3) the type of solder selected 4) the reflow profile 5) pattern and volume of paste deposition, among others.  Let’s dive further into some of these considerations:

Substrate design can have a large effect on the resulting level of solder voiding within the solder joint connection.   Most land pads use thermal vias to dissipate heat while parts requiring even higher thermal dissipation capability utilize embedded heat slugs.  These thermal dissipation structures can impact how the solder connection is formed during reflow, causing temperature differences in areas that have vias or slugs vs. areas that do not.

Solder paste selection and flux type can also impact the final overall solder void connection.  In general, for bottom terminated components (BTC) a ‘no clean flux’ within the paste should be used.   That said, many solder paste manufacturers have their own formulations to help with reducing the amount of solder voiding.   Careful attention should be paid to selecting the right solder paste/flux formulation and to follow the supplier’s recommended solder profile.   In addition, the Association Connecting Electronics Industries (IPC) has a standard IPC-7530A: Guidelines for Temperature Profiling for Mass Soldering Processes to assist with understanding the reflow process and setting up reflow profiles.

The solder paste application process can also play a role in how many solder voids are formed.  In most cases stencil printing is performed.  There are various methods to print the paste in terms of pattern to allow flux volatiles to escape.    Balancing the volume of paste deposited on the I/Os vs. the ground paddles on the mating substrate, can also impact the amount of voiding.         

MACOM application note S2083 is available to customers to assist in the recommended land pad design and stencil design for our products.  In addition, for BTCs, IPC also has available document IPC-7093: Design and Assembly Process Implementation for Bottom Termination (BTC) Components to help customers with the overall construction and post assembly of BTC components onto a second level assembly. 

 

For more information and supplemental reading:

1. M. Johnson, E. Eilenberg, P. Hogan, J. Aldrich, A. Reyes, “Comparison of Laminate Construction Methods on Fabrication, Junction Temperature and Second Level Assembly”, 2019 IPC Apex Expo, January 2019, San Diego, CA United States.

2. M. Johnson, “Techniques to Reduce Solder Voiding Under BTC Components”, 2018 IMAPs New England, May 2018, Boxborough, MA United States.

 

 


Read Full Post
Non-Linear Transmission Line Comb Generators Part-2: Solutions to the Low Phase Noise Problem   Jan. 21, 2020

Part 2.png

Overview

In this two part series from MACOM, we will delve into Non-Linear Transmission Line (NLTL) Comb Generators.  In part 1 we considered the phase noise problem and introduced a potential solution to the problem. In this second part of the blog series, we will explore NLTL comb generation, compare it to its predecessor comb generation using Step Recovery Diodes and see how the NLTL comb generation approach can enable improved sensitivity and lower bit error rates in communication systems.

SRD Comb Generation

Many systems require signals at frequencies which are not easily generated or in some cases impossible to generate directly.  The widely-employed mitigation strategy has been to apply a locally-generated, low-frequency signal known as the ‘fundamental frequency’ to a circuit containing nonlinear impedance, which translates energy from the fundamental signal frequency to its harmonics.  Step recovery diodes (SRDs) have been used extensively to multiply the frequency of a signal.  The step recovery diode is a pn junction device, typically silicon, comprising 3 layers: the p layer anode, a lightly-doped n layer and a heavily-doped n layer, the latter two of which comprise the cathode of the diode.

The SRD goes into conduction when the positive (forward bias) alternation of an input signal is applied.  Charge carriers (holes) from the p layer and the n layers (electrons) flow, which produces low diode impedance.  Immediately after the polarity of the input signal reverses to the negative-bias polarity, the population of these charge carriers is as large as it was under forward bias, so the diode’s impedance remains low.  A non-zero interval is required for the charge carriers to be conducted out of the diode. 

When the population of the free charge carriers is small, the recombination of holes and electrons becomes the dominant mechanism responsible for the impedance of the diode.  When recombination is completed, the diode’s impedance has completed the transition from its low value to its maximum value.  In effect, the diode current “snaps off”. In Figure 1 below, the typical current versus time plot is shown.

Figure 1.jpg

Figure 1 Typical Current vs. Time Plot

 

A typical step-recovery-diode frequency multiplier circuit is shown in Figure 2

Figure2.jpg

Figure 2 Typical Step-Recovery Diode Frequency Multiplier Circuit

 

The RF current which flows through the SRD, D, also flows through the inductor Li.  As the SRD current snaps off, the current through inductor Li also very rapidly decreases to zero.  This sudden decrease in current through the inductor creates an impulse-like voltage waveform which is rich in even and odd harmonics.

Recombination is a stochastic process.  Recombination can only occur when a hole and an electron pair are in “the right place at the right time”.  It is not difficult to imagine that this process does not occur identically, cycle after input cycle, but rather occurs with random variations.  This produces jitter of the impulse-like waveform in the time domain, which is equivalent to phase noise in the frequency domain.

 

NLTL Comb Generation

MACOM’s family of nonlinear transmission line (NLTL) comb generators produce harmonics of an input signal in an entirely different manner than that employed in the SRD comb generator.

Recall that the equivalent circuit of a transmission line is composed of a ladder network of series inductors and shunt capacitors.  This structure is shown below.

Structure2.jpg

In a nonlinear transmission line comb generator, the shunt capacitors are replaced with Schottky junction varactor diodes.  The capacitance of a varactor diode is inversely proportional to the reverse-bias voltage across the diode.  At small reverse-bias signal voltage, the varactor diode produces maximum capacitance.  As the reverse voltage increases in magnitude, the capacitance of the varactors decreases in a nonlinear manner. 

Cacit.jpg

 

The capacitance of the Schottky varactors is a virtually instantaneous function of the amplitude of an incident signal.  As an input signal propagates through the transmission line structure, the phase velocity of the lower-voltage portion of its negative alternation is lower than that of its higher-voltage portion due to the higher varactor capacitance produced by the Schottky varactors.(3)  Cascaded L-C sections of the transmission structure cause a portion of the waveform to be increasingly vertical, as plotted versus time, as additional L-C sections are traversed.  This distortion of the waveform produces harmonics of the fundamental input signal frequency.

Schottky varactor diodes are utilized in the NLTL comb generator because they are majority carrier devices - there are no minority charge carriers in Schottky diodes so there is no stochastic carrier recombination to produce flicker noise and other random fluctuations which are unavoidable in pn junction devices like SRDs.  Thus an NLTL comb generator produces much less additive phase noise than does a SRD comb generator, by as much as 10 to 15 dB.  Figure 3 below compares the additive phase noise for offset frequencies up to 1 MHz for the MLPNC-7103S1-SMT580 NLTL comb generator at 12 GHz output frequency when driven with a 500 MHz input signal at 22 dBm input power versus the output of a SRD comb generator with at the same output frequency with the same input signal conditions.

 

Fig3.png

Figure 3

Conclusion

MACOM offers a family of low-phase-noise NLTL comb generators in surface mount or connectorized modules which have best-in-class phase noise, conversion loss and frequency coverage performance.  These products enable significantly improved sensitivity in radar and communications receivers and lower bit error rates in vector-modulated systems.  For more information on MACOM’s Comb Generators: https://www.macom.com/products/frequency-generation/nltl-gaas-comb-generators

 

References

3. Breitbarth, J., “Design and Characterization of Low Phase Noise Microwave Circuits”, University of Colorado, 2006


Read Full Post

All Blog Posts