How to measure p-n junction diode junction capacitance? - Page 1
Forum section: electronics / projects

author: Subhadeep
author_rank: Newbie
timestamp: May 21, 2016, 03:41:08 pm
content: Hello, I'm Subhadeep, I'm an engineering student as well as a electronic hobbyist and very new in this forum. I'm trying to make a p-n junction capacitance measurement device.so far I know about diode junction capacitance is in different bias conditions(forward/reverse) the depletion region width varies and the diode shows transition capacitance during reverse bias and diffusion capacitance during forward bias.when a sine wave is applied to a capacitor there is a phase shift between voltage and current, current always leads the voltage by 90degree.my question is that can i proceed to measure the junction capacitance of a p-n junction diode using sine wave and how? please help me with this..thank you..

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 22, 2016, 01:02:06 am
content: It would be a lot easier to measure a frequency. Try the usual circuits for varicaps.http://electronics.stackexchange.com/questions/167051/how-to-properly-connect-and-drive-varicap-diodes

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 01:12:34 am
content: Quote from: Subhadeep on May 21, 2016, 03:41:08 pmHiThe normal approach is to apply a signal to the junction that is a small fraction of the bias voltage. If you have 2V on the junction, you apply 200 mv p-p. You can vary the test signal to evaluate the sensitivity to the voltage.Bob

author: Kleinstein
author_rank: Super Contributor
timestamp: May 22, 2016, 08:31:09 am
content: Because of the voltage dependent capacitance the AC amplitude needs to be relatively small. In the forward direction it's more like 10 mV. The forward direction is more difficult anyway, as there is a DC current superimposed. So the phase shift will not be 90 degree. This is why using an oscillator an measuring the frequency does not work well in that range. So it's more like the basic setup of apply an DC+AC voltage and measure the current. To separate the current in the capacitive and conductive (in phase) part one needs to use some kind of pase sensitive analysis. This could be analog (e.g. using synchronous rectification) or digital by analyzing the digitized signal in software.

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 12:48:08 pm
content: Quote from: Kleinstein on May 22, 2016, 08:31:09 amHi.... or for the forward direction, do it the normal way and get the data with a network analyzer. Then fit the S parameters to extract the capacitance.Bob

author: danadak
author_rank: Super Contributor
timestamp: May 22, 2016, 02:13:14 pm
content: You could always build a simple RF osc and switch the diode in and out of thetank circuit and measure the frequency change, and solve for C.http://vidyasagar.ac.in/journal/maths/vol17/JPS-v17-19.pdfhttp://www.docircuits.com/public-circuit/461/measuring-reverse-bias-diode-capacitancehttp://avionics.nau.edu.ua/files/doc/Lab%20III%20Capacitance%20of%20a%20PN%20junction%20(1).docRegards, Dana.

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 03:28:14 pm
content: Quote from: danadak on May 22, 2016, 02:13:14 pmHiExcept for two issues:1) You really don't know what the p-p voltage on the junction will be. That's pretty critical in this case.2) Since there *is* resistance associated with the equivalent circuit, you very much need to know how this impacts your oscillator. It's the good old R in series with C is a different C than C in parallel with R for the same net impedance at a frequency fun and games.Bob

author: rfeecs
author_rank: Frequent Contributor
timestamp: May 22, 2016, 05:00:28 pm
content: It's a classic C-V measurement. Used routinely in semiconductor characterization. Look it up.Here's one reference:http://www.tek.com/sites/tek.com/files/media/document/resources/C-V%20Fundamental019%29%20020409.pdf

author: rfeecs
author_rank: Frequent Contributor
timestamp: May 22, 2016, 05:49:21 pm
content: Here's another reference on the measurement, "Agilent Impedance Measurement Handbook":http://cp.literature.agilent.com/litweb/pdf/5950-3000.pdfSection two tells you more than everything you want to know about how the measurement is done in multiple ways.Paragraph 5.4 is specifically about diode measurement.

author: German_EE
author_rank: Super Contributor
timestamp: May 22, 2016, 07:29:09 pm
content: The article here describes the use of LEDs as varicaps, maybe not so useful for your application but the measurement technique is interesting:http://www.hanssummers.com/varicap/varicapled.html

author: SeanB
author_rank: Super Contributor
timestamp: May 22, 2016, 07:36:00 pm
content: Quote from: German_EE on May 22, 2016, 07:29:09 pmI guess if you redo the tests using 1W leds you will get higher capacitance, just because of the much larger physical die in the package. Reverse voltage will be a limit though, along with leakage current.

author: danadak
author_rank: Super Contributor
timestamp: May 22, 2016, 11:02:07 pm
content: 1) You really don't know what the p-p voltage on the junction will be. That's pretty critical in this case.2) Since there *is* resistance associated with the equivalent circuit, you very much need to know how this impacts your oscillator. It's the good old R in series with C is a different C than C in parallel with R for the same net impedance at a frequency fun and games.This pretty much indicates in rev bias model is pretty much just C.https://inst.eecs.berkeley.edu/~ee105/sp04/handouts/lectures/Lecture20.pdfFor sure this is not a simple design. As far as P-P Vjunc that is under user control, agc and circuit parameter scaling cometo mind in any linear oscillator. Varactor tuned oscillators pretty well understood. Of course the "normal" diode is not targetedfor varactor performance, but measurement principles the same. One has to do a model/error analysis to see what can be achieved.It was just a suggestion.I think the diode in controlled reverse bias in a phase shift network maybe one of the easier ways to do the measurement.Maybe there are some agilent/hp notes on how their semi analyzers do semiconductor measurements. They are the mastersat this craft.https://arxiv.org/ftp/arxiv/papers/1011/1011.3463.pdfhttp://www.tek.com/sites/tek.com/files/media/document/resources/C-V%20Fundamental019%29%20020409.pdfRegards, Dana.

author: uncle_bob
author_rank: Unknown
timestamp: May 22, 2016, 11:33:23 pm
content: Quote from: danadak on May 22, 2016, 11:02:07 pmHiExcept that there seems to be a desire from the OP to do it both forward and reverse biased.....Bob

author: danadak
author_rank: Super Contributor
timestamp: May 22, 2016, 11:52:42 pm
content: Asleep at the switch, thanks for pointing that out.Looks like ap notes discussing small signal C-V approach are the way to go.In this the conductance is calculated as an approximation of the simple I-V curveslope -http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-012-microelectronic-devices-and-circuits-spring-2009/recitations/MIT6_012S09_rec16.pdfInteresting enough that would simply lower Q in a resonant tank. Maybe not a big dealin an oscillator approach ? Out of my league.....Of course that does not handle the Cj problem in the measurement.Regards, Dana.

author: sohail anwar
author_rank: Newbie
timestamp: November 19, 2018, 11:22:20 am
content: What is PN Junction (Diode):A PN Junction Diode is a two-terminal semiconductor device. It’s made up from a small piece of semiconductor material (usually Silicon), it allows the electric current to flow in one direction while opposes the current in other direction. In the Forward Bias, the diode allows the current to flow in uni-direction. On the other hand, when the diode is reverse biased it opposes the electric current to flow. A PN Junction Diode is a semiconductor device with two opposite region such as (P-type region and N-type region).The P-region is called as the anode and is connected to a positive terminal of a battery and it has Holes in majority carrier and electrons in minority carrier.The N-region is called as the cathode and is connected to the negative terminal of a battery and it has Electrons as a Majority carrier while holes as Minority carrier.When the P-type semiconductor material is joined with the N-type semiconductor material, a P-N Junction is formed, hence resulting P-N Junction is also called as a P-N Junction Diode.Source:http://www.studentsheart.com/pn-junction-diode/

author: Marco
author_rank: Super Contributor
timestamp: November 19, 2018, 02:10:48 pm
content: Quote from: Subhadeep on May 21, 2016, 03:41:08 pmThis doesn't make sense, sure in theory there's a forward capacitive component to the impedance but unless you want to measure it at the double digit GHz range it's overwhelmed by resistive and inductive components.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: November 19, 2018, 03:56:26 pm
content: I suspect this can be done with many traditional bridges. Usually you apply a bias through a resistor using a T in the detector circuit. Easy to build a bridge circuit.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 19, 2018, 06:24:17 pm
content: The most general method: use a VNA with a bias tee.The VNA (Vector Network Analyzer) measures the phase and magnitude of transmitted and reflected AC waves from a device under test (DUT), through as many ports as it is equipped with. In this case, we expect a capacitance at the end of a transmission line, so we merely need to use the equation for a transmission line with a capacitor load, at each frequency tested, and can solve for capacitance in that way.We can also solve for resistance, and by varying frequency we can also synthesize an equivalent circuit for the DUT.Now, a VNA is rather expensive, if you don't have access to one. But this highest-level perspective gives us insight into what methods might be applied.A VNA is fundamentally a very precisereflectance bridge. A bridge has already been mentioned above, and that is also a very nice, general method for lower frequency applications. (At high frequencies, it's desirable to do things in an RF-and-transmission-lines method, which means, using a reflectance bridge as such. But there isn't much operational difference between that, and the old fashioned e.g. Wheatstone bridge, as long as one understands the compromises of each).We can also look at the fundamentals of what we're measuring. What is a capacitance? In AC steady-state, it is a reactance inversely proportional to frequency. This is actually a consequence of a more general statement, the fundamental capacitor equation: I = C * dV/dt. (If we have I and V as sinusoids at some frequency, then the AC steady-state condition follows instantly from this.) We don't need to use sinusoids here; we can just as well use sharp, piecewise waveforms, that may be easier to measure with digital circuitry, for example.Namely, if we apply a square-wave current, the time integral of that waveform is the voltage, a triangle wave. This is what most multimeters do: apply an alternating current (square wave), and measure the voltage. Typically the current will be switched when the voltage reaches a threshold, so the impedance (V / I) is held constant, and the frequency of oscillation is then inversely proportional to capacitance. Or the current is varied, in steps or continuously, to hold voltage and/or frequency in a typical range.Note that such a method loses something: we expect to measure capacitance, but if we measure resistance instead, we get nonsense results. We're also testing with a wideband signal (assuming linear components (constant C), the usual analysis methods apply (superposition, Fourier transform, Parseval's theorem, etc.), and we note the transform of a square wave has many harmonics), which means we can get very weird results for complex (RLC) networks.Some of this can be recovered (e.g., measuring the voltage waveform in quadrature with the current waveform, so we measure in-phase and out-of-phase components -- ESR and C), and some of it can be managed (modest size ESL can be avoided with a bandwidth-limited square wave test signal), but if you are expecting a network any more complex than that, a bridge or VNA method is probably best.Tim

author: rodpp
author_rank: Frequent Contributor
timestamp: November 20, 2018, 02:07:27 am
content: Quote from: Conrad Hoffman on November 19, 2018, 03:56:26 pmYes, this is the suggested method to measure diode capacitance in the Tektronix book Semiconductor Device Measurements:https://archive.org/details/tektronix_Semiconductor_Device_Measurements

author: cdev
author_rank: Super Contributor
timestamp: November 20, 2018, 03:04:12 am
content: Many diodes act as varactors, some to a remarkable extent. This effect can be leveraged to make some useful devices. For example, its quite easy to turn some LEDs into varactors and they perform as well or almost as well as real varactors. This can be demonstrated very easily by back biasing all the segments in an old seven segment LED display and applying a variable positive voltage by means of a voltage divider, by way of an RF choke to decouple the dc supply lead from the magnetic loop part. You may then find that with some parts - using this arrangement, you can tune a good chunk of the HF shortwave bands. Under the best conditions, some diodes behave just like a decent sized variable cap, plus this loop is remotely tunable. Use a big multi turn pot or a reduction gear on the shaft if you have one around because the tuning can be very sharp, with the peak easy to miss. This uses very little current so a back biased battery will last a long time used like this if you choose a very high value for the pot. (1 meg or more, even 10 megs will work) Turn it off when its not in use.