Curve Tracer

The curve tracer is one of the best pieces of test equipment in your laboratory if you are working with anything that contains semiconductors. 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.

What are I-V curves?
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.

Block diagram of a typical curve tracer (taken from tektronix)

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.

Source: http://www.microwaves101.com/encyclopedia/curvetracer.cfm