Resistance Measurements Using the B2900B/BL Series of SMUs

Introduction 

B2900B/BL Series of precision Source/Measure Unit is a compact and cost-effective bench-top Source/Measure Unit (SMU) with the capability to output and measure both voltage and current. They cover currents from 10 fA to 3 A (DC)/10.5 A (pulse) and voltages from 100 nV to 210 V, which enables you to make a wide range of current versus voltage (IV) measurements more accurately and quickly than ever before. The B2900B/BL Series of SMUs also supports a resistance measurement function that facilitates both low and high resistance measurements. These features make the B2900B/BL Series of SMUs the best solution for the accurate characterization of resistors and other devices. 

Making accurate resistance measurements is actually one of the more challenging areas of measurement science. Many factors can affect the accuracy of resistance measurement, including residual test lead resistance, thermal electromotive force, and leakage currents in the measurement path. The B2900B/BL Series of SMUs possesses a variety of features to address these measurement issues, including a remote sense function (4-wire connection), offset compensation, and a guard function. All of these will be described in greater detail in this application note. 

Also, you will learn how to perform accurate resistance measurement through two lab exercises

What is the B2900B/BL Series of SMUs?

An SMU combines the capabilities of a current source, a voltage source, a current meter, and a voltage meter along with the capability to switch easily between these various functions into a single instrument (please see Figure 1). This gives it the ability to evaluate the IV characteristics of devices across all four measurement quadrants without the need for any additional equipment. Besides being able to output and measure voltage or current very accurately, SMUs also possess a compliance feature that allows a limit to be placed on the voltage or current output to prevent device damage. The members of the B2900B/BL Series of SMUs are single or dual-channel SMU units that offer a wide range of IV measurement capability for a variety of two-terminal and three-terminal devices. They cover currents from a minimum of 10 fA to a maximum 3 A (DC)/10.5 A (pulse) and voltages from a minimum of 100 nV to a maximum of 210 V. In addition to their DC operation mode, the B2900B SMUs also has the ability to perform pulsed measurements in order to prevent device self-heating from distorting the measurement results. Finally, the B2900B/BL Series of SMUs also has a built-in resistance measurement function.

Best Resistance Measurement Solution 

The B2900B/BL Series of SMUs is the best solution for making accurate resistance measurements due to its intrinsic voltage/current sourcing and measurement capabilities. In addition, the B2900B/BL Series of SMUs has some special built-in functions specifically designed to improve resistance measurement. The following sections describe these features in detail.

Eliminating residual resistance effects 

A basic 2-wire connection is the most common scheme used for resistance measurements. In this configuration (shown in Figure 2a) the same pair of test leads is used to both force current and measure voltage. This arrangement is suitable for resistance measurements as long as the residual lead resistance is negligible compared to the resistance of the device under test (DUT). 

However, for very low resistance measurements where the residual lead resistance is comparable to the DUT resistance, the 2-wire measurement will give erroneous measurement results (please see Figure 2b). In this case, a 4-wire connection scheme (remote sensing) can be used to eliminate this error. A 4-wire measurement uses one pair of leads to force current and the other pair of leads to monitor voltage. This eliminates cable resistance effects so that only the voltage drop across the DUT is measured (please see Figure 2c). In addition, the 4-wire measurement scheme of the B2900B/BL Series of SMUs keeps the voltage between the sense points (A and B in Figure 2b) at exactly the specified voltage Vset, thereby ensuring that your device is characterized exactly under the measurement conditions you specify. The B2900B/BL Series of SMUs supports both connection schemes and it is easy to switch between them.