Getting faster results when checking common parameters of experimental devices

Because this test is often performed using pulsed test signals, the user must make certain that the voltage has settled to a final value before taking the reading. The newest SMUs combine fast settling with fast digitisers to allow the designer to verify these breakdown measurements are valid.

Another important parameter is the drain and gate leakage currents. On older silicon technologies, the leakage could be many micro-amps, but the newer silicon carbide and gallium nitride materials exhibit much lower leakage. Measuring nano-amps or even pico-amps accurately requires both an instrument that has sufficient sensitivity and the proper cables and connections. In this context, “proper” cables means triaxial cables and a driven guard, features which were never available with earlier generations of high voltage test equipment. Triaxial cables not only minimise stray leakage currents, but they shield the measurement from noise and allow the device to settle much more quickly.

RDS(ON) is an important measurement but often a difficult one to make accurately. Typical sources of error in this measurement include the sensitivity (or lack thereof) of the voltage measurement, the noise level, and the difficulty of compensating for drops in the test leads. Given that RDS(ON) could be as low as a few milliohms, very little voltage is generated even when the currents are as high as 50A or more. Curve tracers simply can’t provide sufficient voltage sensitivity. In some cases, users have attempted to create test solutions by integrating power supplies and sensitive voltmeters, but these solutions are typically limited by synchronisation and settling time problems. Most devices can only handle high currents for very short periods, perhaps just hundreds of milliseconds. All it takes is a little extra stray inductance in the test cables to extend the measurement’s settling time, creating inaccurate results.

The threshold voltage of the transistor has some interesting test challenges associated with it as well. For example, one way of measuring threshold voltage is to hold the drain at a fixed bias while sweeping the gate and measuring the resultant drain current. However, the drain actually needs to be in a pulse mode to prevent the device from heating up, which would cause a shift in the device’s characteristics. In any pulse test, in order to make accurate, repeatable measurements, it is imperative for users to make certain that the entire test system has reached a settled state. Recently, we encountered an interesting problem with an inaccuracy in the gate voltage measurement caused by the residual resistance in the test leads connected to the source of the device. The large pulse of current flowing from the drain to the source caused the source terminal voltage to increase with respect to the gate, thereby reducing the gate drive and causing an error. After repositioning the gate sense leads to be directly on the source, we were able to eliminate the error.

You can read the rest of this article in the September issue of Electronic Specifier Design by clicking here.