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Curve tracer measurements

Updated December 10, 2005

Click here to go to our main microwave measurements page

It never ceases to amaze us that many microwave engineers don't know how to use a curve tracer, because it's "too complicated!" There are only so many management positions in your company, so maybe you should expand your engineering skills if you want to eat well later in life!

The curve tracer is one of the best pieces of test equipment in your laboratory if you are working with anything that contains semiconductors (doesn't everything except antennas?) You can use it to characterize data that you won't get on most manufacturers' data sheets, to troubleshoot devices that might be damaged or disconnected, or even as a very precise dual power supply.

Check out the following separate pages on the subject of curve tracer measurements.

Example 1 will show you how to measure I-V curves of a MESFET MMIC amplifier.

Example 2 will show you how to measure forward voltage (VF) of a Schottky diode embedded in a double-balanced mixer, without delidding it.

Example 3 will show you how to determine if a black box contains a voltage regulator, using a curve tracer, and measure its voltage value.

Click here to learn how to stop curve tracer oscillations

Click here to learn how to modify your Tektronix 370B curve tracer so it will operate with the lid open. Our lawyers have advised us to call you a sissy if you don't do this!

Here's a beauty we saw recently on Ebay for a couple hundred bucks!

 

Here's a clickable outline of this page:

What are I-V curves?

Two-terminal versus three-terminal measurements

Static versus pulsed I-V curves

How does a curve tracer work?

What do all those knobs do?

Curve tracer models

Example I-V curve measurements

What are I-V curves?

When you plot the current/voltage relationship of an element, this is known as an I-V curve. Don't ask us why "I" is used to denote "current", that is a great mystery. But the convention is to hyphenate I-V so it doesn't get confused with Roman numerals.

A resistive element has an I-V curve that is described by Ohm's law to be a straight line. Semiconductors have I-V responses that bend, and a a whole lot more interesting. This is analog electronics at its best!

Before we go any further, you need to remember the two extremes of I-V measurements: A vertical I-V measurement means you are looking at a short circuit, and a horizontal I-V curve means you are looking at an open circuit. Every other resistor value is somewhere in between.

Two-terminal I-V curves show semiconductor diode characteristics such as forward conduction and reverse breakdown. Three-terminal I-V curves show "families of curves", which is really a two-dimensional representation of a three-dimensional surface. I-V curves are used every day for large signal modeling, and bias network design.

There are two categories of three-terminal devices for I-V measurements: Current-controlled devices include:

Bipolar junction transistors (BJTs)
Heterjunction bipolar transistors (HBTs)

Voltage controlled devices are different types of FETs, and include:

MESFETs
PHEMTs
MHEMTs
MOSFETs

I-V curves are taken so that we can extrapolate what happens to a high frequency signal when it is applied to the device that produced the curve. This is known as large signal modeling, and is a huge topic all by itself. The leap of faith that must be made is that a high frequency signal, say 10 GHz sine wave, will experience the same I-V characteristic that we measure with a curve tracer. This is not exactly the case, but it is often a reasonable approximation.

Coming soon!

Static versus pulsed I-V curves

"Static" or "DC" I-V curves refer to curves that are traced at a low rate,on the order of 100 Hertz (100 traces per second). The problem with static I-V curves is twofold. First, the self heating that goes on as the device is swept through the curves is substantial enough to actually change the curves. This becomes a bigger problem with large devices that dissipate appreciable power. For power FETs, the I-V curves start to bend downward at high power levels.

The second problem with static I-V curves is more insidious than power dissipation. Suppose the RF curves don't look like the static curves, "just because"? Semiconductor physicists give us many answers to why this is, including "surface states" and "traps". But the goal of the microwave engineer is not to ask why the static I-V curves are all wrong, but to figure out a way to measure them in as near as possible to the RF condition. This is known as "pulsed I-V".

Someday we will add a page or two on this topic!

How does a curve tracer work?

A curve tracer is at the same time a voltmeter and an ammeter. Here's a block diagram that we pinched from a Tektronix manual, we hope they don't mind...

There are various models of curve tracers, but there is only one manufacturer that you should consider. That is Tektronix. Modern units such as the 370B have a display that can digitize an image. This is far more convenient than the old CTs that recorded data with a Polaroid camera. Not only was this a big mess, but the image was the opposite of what would be most useful, it had a dark field with white line traces. Which ended up a smudged mess when you Xeroxed it. Someone we know once asked Polaroid if they could develop an instant "negative" film for this type of application. Surely they could have sold 1,000,000 packages of it. But they weren't interested... and now the company that was founded by Edwin Land, and true Yankee genius, is all but gone.

The terminals are labeled Collector, Base and Emitter. For a FET you should pretend Collector is Drain, Base is Gate, and Emitter is Source. For three terminal parts, the I-V characteristic is between two of the terminals, with some control over the curve through the third terminal. There are two types of three-terminal devices: current-controlled, and voltage controlled. The xxx knob can be used to generate either voltage of current steps between the Base terminal (Gate) and Emitter terminal (source).

What do all those knobs do?

If you really want to become an expert on curve tracer measurements, there is no substitute for studying your curve tracer's manual. Here we will review some of the most important controls for microwave device measurements, and let you figure out the rest.

Display controls

These include the horizontal and vertical axis controls. Here you select the voltage per division on the horizontal (voltage axis) and the current per division on the vertical (current axis).

Note that inverting the display does NOT invert the voltage outputs of a curve tracer. Some engineers will invert both the display and the voltage polarity during FET breakdown measurements, so that the curve will start in the lower left corner and not the upper right.

Step generator controls

Here is where you control either the voltage or current steps that will generate the family of curves. You can select current of voltage (use current for a bipolar transistor and voltage for a FET). You can select polarity, either positive or negative steps. You can select the number of steps, and even add an offset (the family doesn't have to start at zero mA or V) of up to 10 times the step size. You have a choice of pulse modes, but we recommend you leave the pulse mode off.

Collector supply controls

Here is where you set the maximum power that your device could see (think of it as a circuit breaker). You can limit the collector voltage to 16, 80, 400 or 2000 volts. Unless you are working with GaN, chances are the lowest setting (16 V) is plenty.

Danger Will Robinson! don't mess with the 2000 volt scale unless you know how to handle high voltage! When the little red light is on, you can get the shock of a lifetime. Our advice is to never exceed the 80 volt scale, then you can't get killed at work and your kids won't come in to a pile of money and spend it all on stupid video games...

The maximum peak power is set by adding a series resistor in front of the DUT (the resistor is internal to the CT). Older CTs let you choose the resistor value, on the 370B there are 23 different series resisters, depending on the max power and voltage that you select (look them up in the manual if you care). Series resistor setting also provides a way to "slow down" the I-V curves when you increase the collector supply; this degree of control which is what makes curve tracer measurements extremely safe for fragile semiconductors.

Curve traver rule of thumb: Use the lowest power setting (highest series resitance) that is practical for the device you are characterizing.

The variable collector supply is located near the output terminals. This is the knob that wither takes data successfully or does irreparable damage to your hardware. This is the last knob you set when taking I-V curves, and the first one you turn down after you capture the data. Turn it slowly, make sure your data makes sense before you turn it up all the way.

Output controls and configuration

Here is where you select the type of curves you are after, such as two-terminal or three terminal. The configuration knob tells you how the BASE, EMITTER and COLLECTOR are connected within the CT.

The CT has left and a right side inputs, these are the same. You can connect two devices at once, and switch back and forth. Although there are five inputs on each side, unless you are measuring something with considerable resistance in the measurement equipment (really long, small diameter wires), you don't need to worry about using the "sense" leads.

Curve tracer models

Tektronix newest model is the 370B. This is a great improvement over the older versions in that it "images" the data for you and stores it in a bmp file. You can output the file to a floppy disc, and fit maybe 20 I-V curves on a single disc if it doesn't have any other junk on it. But for $50,000, you'd expect a hard drive and a USB port, like Agilent is installing on all of their high-end gear. But this "old school" output has the advantage of instant bootstrapping, and uses no virus-defeatable Microsoft software, so it might just be a good trade.

Old models include the 576 and 577. These are just fine to use, but you are stuck using Polaroid camera to get your image. Or you can use the "Flintstone bird" to draw the image with his beak:

Wrrrraaaaaak!

Example I-V curve measurements

So far we have three complete examples posted:

Depletion mode MESFET I-V curves

Schottky diode forward voltage

Voltage regulators

Here's some potential topics we can get into later if there is interest:

Enhancement-mode MESFETs

PHEMTs (as opposed to MESFETs)

HBTs

GaN HBTs

P-channel MOSFET
This is not truly a microwave semiconductor, but it has applications as a current switch.

N-channel MOSFET
Also used mostly for its DC switching characteristics

 


 
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