TEST & MEASUREMENT

for the source voltage may not be the same as the voltage actually applied to the DUT; with multiple SMUs in parallel, the source offsets may add up to be quite significant, so using source readback provides a clearer picture of the level of voltage actually being sourced, not just the voltage that’s been programmed.

Figure 3. Pulse sweep that combined 4 SMUs and took an I-V curve on a P-N diode DUT

38

0.1V (10A _ 0.01 ohm) in amplitude and 300 microsecond width. Combining four SMUs in parallel to pulse 40A across the same DUT resulted in a waveform of 0.4V magnitude with excellent synchronization (low jitter) between the channels (Figure 2). Pulse consistency was verified using the same test setup and pulse waveform.

With the pulse performance verified, the engineers programmed a pulse sweep that combined four SMUs and took an I-V curve on a P-N diode DUT (Figure 3). Note the correlation of the one-SMU DC sweeps up to 3A with the one-SMU pulse sweep up to 10A. Then, the engineers extended the achievable I-V curve up to 40A. This experiment verified the validity of combining four SMU channels and pulsing to achieve 40A on two- terminal devices (resistor and diode). With certain modifications, this technique is equally valid when applied to testing a three-terminal device, such as a high-power MOSFET. Several implementation factors are critical to maximizing the accuracy and precision of the results obtained using this multi- SMU pulsed sweep approach.

Using source readback

Table 1

Characteristic

DUT Current DUT Voltage

Maximum Source Current Maximum Voltage

An SMU has both source and measure functions built into the same unit, so it’s capable of reading back the actual value of the applied voltage using its measurement circuitry. The programmed value

Definition

IDUT

= ISMU1

VDUT IMAX

+ ISMU2

= VSMU1 = IMAX

= VSMU2 SMU1 + IMAX SMU2

Smaller of the two SMUs maximum voltage capabilities

Making four-wire measurements

Four-wire (Kelvin) measurements are necessary when doing high current testing because this technique bypasses the voltage drop in the test leads by bringing two very high-impedance voltage sense leads out to the DUT. With very little current flowing into the SENSE leads, the voltage seen by the SENSE terminals is the virtually same as the voltage developed across the unknown resistance. At 40A levels, even a small resistance, such as 10 milliohms in the test cable, can generate a voltage drop of 0.4V. So if the SMU is forcing 1V at 40A current and the cable resistance is 10 milliohms and there are two test leads, the DUT might only receive a voltage of 0.2V, with 0.8V dropped across the test cables.

Unlike source readback, which primarily impacts just the source values, making four-wire measurements will result in significantly better accuracy on both the sourced and measured values because they eliminate the voltage drop in the current-carrying wires that would otherwise affect the measurement.

Putting one source at each DUT node

It is common in many test sequences to perform voltage sweeps (force voltage) and measure current (FVMI). In the case where more than one SMU is connected in parallel to a single terminal of the device, the obvious implementation would be to have all of the SMUs in voltage-source mode and measure current.

However, three factors must be considered:  SMUs when sourcing voltage are in a very low-impedance state.

 DUTs can have impedances higher than an SMU that’s in voltage-source mode.

 The DUT’s impedance can be static or dynamic, changing during the test sequence.

Even when all SMUs in parallel are programmed to output the same voltage, small variations between SMUs related to the instruments’ voltage source accuracy mean that one of the SMU channels will be at a slightly lower voltage (millivolt order of magnitude) than the others. So, if three SMUs are connected in parallel to one terminal of a DUT, and

www.solar-pv-management.com Issue III 2010 Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56