Characterizing High Power Semiconductors Requires New Technologies

June 1, 2012
The next generation of power semiconductors exhibits high current densities and breakdown voltages, which has put pressure on test instrument manufacturers to provide adequate testing support. Help is on the way, thanks to new instruments from Agilent.

Able to withstand large voltages with fast switching speeds allows wide bandgap materials such as SiC and GaN to promise the required performance for emerging high power applications. Crucial to employing these devices is the ability to test them at their specified voltage and power rating. This requires the handling of breakdown voltages up to 3000 V or even higher, more than 100 A, and junction capacitances for dc biases up to 3000 V. A curve tracer produces I-V (current-voltage) curves that provide a good assessment of device behavior. Therefore, a curve tracer is the ideal instrument to quickly determine the characteristics of these high power, discrete semiconductors.

SiC and GaN have potentially higher breakdown voltages and smaller leakage currents than conventional power devices. Therefore, characterization of SiC and GaN devices requires an appropriate curve tracer with high breakdown voltage measurement capability, as well as the ability to measure leakage currents at high voltage biases. For more information on curve tracers, see “A Look At Curve Tracers.”

High Power Instrumentation

Blocking characteristics that comprise leakage current and breakdown voltage characteristics are useful in obtaining basic physical insights into device behavior. However, older equipment lacks sufficient resolution and accuracy for leakage characterization. It is typically limited to the µA level. If the breakdown voltage is more than 3kV, there is no instrument that can measure it. Some researchers have had to design and build a custom test system to measure ultra-high voltage. However, a custom test system may have safety and traceability issues.

On-resistance of the newer power semiconductors is now as low as a few milliohms and continues to decrease, which causes other measurement problems. Accurate measurement of on-resistance requires precise constant current applied to the device using an accurate voltmeter with a full Kelvin connection. However, older generation curve tracers don’t have current source capability, which makes it difficult to accurately perform characterization.

From a reliability viewpoint, I-V characteristics near safe operating area (SOA) are important for high voltage devices. When a high voltage is applied to a device, its impedance rapidly changes and medium current, such as 300mA flows through the device. High power due to high voltage (e.g 1000V) and medium current (e.g 100mA) requires a narrow pulse (less than 50µs) to avoid device self-heating that would impact accurate characterization.

Capacitance inherent to a device or inductance of measurement cables can cause measurement issues, which are not isolated easily just by checking measured results. Therefore, actual waveforms are the best method to check to aid problem investigation.

Also, a conventional curve tracer or a custom test system built by multiple measurement instruments may lack a switch to automatically change the measurement resource between high voltage and high current. This requires researchers or operators to have to manually change connections between the measurement equipment and the device, which sometimes cause trouble.

New High Power Test Technology

To meet the requirements of the new generation of high power semiconductors, Agilent Technologies Inc. has enhanced its B1505A Power Device Analyzer/Curve Tracer to significantly increase its voltage and current range (Fig. 1). These enhancements allow the B1505A to measure a device’s characteristics from sub-pA up to 10 kV/1500 A, with a fast pulse down to 10 microseconds. Agilent also introduced the B1505AP, a preconfigured version of the B1505A that includes all modules, cables and accessories necessary to ensure a quick startup. Both the enhanced B1505A and new B1505AP are intended for power-device researchers and manufacturers performing power-device characterization and failure analysis. These instruments can also be used for incoming device inspection and failure analysis.

Characteristics of the B1505A and B1505AP include:

  • Up to 1500 A/10 kV I-V measurement range
  • Medium current measurement with high voltage bias (500 mA at 1200 V, for example)
  • µΩ on-resistance measurement capability
  • Sub-pA level current measurement, traceable to international standards
  • High power pulsed measurement down to 10 µs
  • Two independent A/D converters on each channel enable simultaneous high-speed current and voltage measurement
  • Peak powers of 22.5 kW for high current and 900 W for high voltage
  • Temperature measurement capability (ambient and self-heating)
  • Oscilloscope view allows accurate visualization of current and voltage pulse waveforms
  • Quantitatively evaluate GaN current collapse phenomena (see “What Is GaN Current Collapse?
  • “Stoplight” feature defines forbidden regions graphically so measurement stops if the trace enters the forbidden area.
  • Auto record function that stores measured data in real time, which can be recalled later even when device gets accidentally destroyed
  • Two standard test fixtures: 3,000V/40A and 10,000V/1500A
  • Application Test Mode supports over 40 user-modifiable high power application tests

Current source mode aids in accurately measuring power semiconductor on-resistance. Current is continuously monitored and adjusted to set value by the internal feedback circuit when used as current source. Voltage is measured accurately with a Kelvin connection. Because set current flows regardless of contact resistance variations, on-resistance can be measured very accurately. In contrast, use of the voltage source mode involves immeasurable contact resistance that causes a voltage drop, which deteriorates the measurement.

Fig. 2 shows an Ultra High Current Unit (UHCU) that consists of Ultra High Current Expander and Medium Current SMUs (source measurement units). The UHCU is an integrated unit in B1505A and a designer can regard it as a single measurement resource. The plot in Fig. 3 shows that the UHCU can measure less than 1 mΩ.

When making on-wafer measurements, extension cables are necessary to connect the test equipment and prober. In this case, the drive voltage has to be high enough to overcome the IR voltage drop in the extension cable. The B1505A’s collector supply voltage is 60V, which has enough margin, even when IR voltage drop is large due to high current. A specially developed extension cable for B1505A has low residual resistance, which further ensures sufficient drive voltage. Its co-axial structure provides low inductance, which minimizes current settling time. Fig. 4 shows actual measurement results over 200A for a MOSFET on a wafer.

Fig. 5 shows The B1505A’s 10kV architecture. It utilizes two wide dynamic range SMUs, one on the high side driving a high-speed voltage amplifier and the other on the low side measuring current with pA resolution. It can produce 10kV output pulses with a 100µs width. Fig. 6 shows an actual ultra-high voltage measurement using the B1505A.

Pulsed medium current in hundreds of mA at high voltage can be generated using the architecture shown in Fig. 7. A fast pulse is applied to the device by controlling the switch in the current expander circuit. The pulse width can continuously varied from 10µs to 1ms. Fig. 8 and Fig. 9 show actual I-V measurement results and pulse waveforms. 500mA is measured at 1200V bias in this example. Pulse width is as narrow as 10µs, which cannot be achieved using anold architecture instrument.

Further Efficiency Improvements

There are several advanced technologies developed for the enhanced B1505A, which improves device characterization efficiency and significantly reduces total test cost. One is an auto-record function for ultra-high voltage measurements. Besides a compliance function to avoid device damage, B1505A records all measurement results in real time into the internal memory. This data can be recalled from the memory, just like rewinding and playing a video for further analysis even when the device is accidentally destroyed.

Another feature is the oscilloscope view function. You can verify actual waveforms being applied to the device at any specified measurement point on the I-V curve. Checking the waveforms in both current and voltage at multiple device terminals can greatly improve problem isolation when measurement results are strange.

Automation is another key feature to drastically enhance the efficiency. The high voltage/high current selector in the N1265A and N1259A test fixtures allows multiple parameters of automated measurement without changing cable connection. Used together with a semi-automatic wafer prober, multiple parameters on multiple dice on a wafer are automatically measured. By using a built-in automatic analysis function together with this sequence, even complicated parameters can be extracted automatically.

The enhanced Agilent B1505A employs the Microsoft Windows® 7 operating system and has various selectable measurement modules, including:

  • High-current SMU (20A/20V)
  • High-voltage SMU (8 mA/3000 V)
  • High-power SMU (1 A/200 V)
  • Medium-power SMU (0.1 A/100 V)
  • Medium-current SMU (1 A/30 V)
  • Multi-frequency capacitance measurement unit
  • Ultra-high current unit (1500 A/60 V)
  • Ultra-high voltage unit (20 mA/10 kV)
  • High-voltage medium current unit (2.5 A/1500 V and 1.1 A/2200 V)

The B1505A software environment allows designers to check device characteristics and detect device faults with the convenience of a curve tracer. The B1505A supports rotary knob control of the independent sweep variable for intuitive and real-time evaluation of parameters such as breakdown voltage. You can automatically store measurement setup information and data in the B1505A’s built-in hard disk drive. Data can then be transferred to USB memory sticks or other portable storage devices. It is also easy to print graphical measurement data and to copy and paste it into reports to summarize analysis results.

Proper test fixturing is needed to ensure safety and to support the wide variety of power device package types. A previous limitation of curve tracers was that some power devices could not be evaluated due to their size, or it was necessary to jury-rig an adapter in order to test the device. However, the B1505A’s test fixture can accept various devices, regardless of their size or shape via a large fixture adapter with customizable fixture modules. In addition, the test fixture’s built-in interlock mechanism ensures that high voltages and currents can be applied to test devices safely.

A Look At Curve Tracers

A curve tracer is the primary instrument used to analyze the characteristics of discrete semiconductors, ranging from diodes to SiC or GaN devices. In operation, the curve tracer applies voltage to the main terminals (drain-source, collector-emitter, anode-cathode, etc.) of the device under test (DUT). Three-terminal semiconductors require a stepped signal applied to the DUT’s control terminal. Bipolar transistors use a series of current steps applied to their base whereas MOSFET transistors use a series of voltage steps applied to their gate. Sweeping through the control signal input steps, generates a family of I-V (current vs. voltage) curves (Fig. 10). The range of the input steps in these I-V curves depends on the specified values for the device, for example, 1 to 5V in 0.5V steps for a power MOSFET. This family of curves makes it easy to evaluate the power semiconductor’s performance.

When displayed on a screen, the operator can control and monitor the voltage applied to the device, polarity of the applied voltage, and the load resistance inserted in series with it you can use I-V (current-voltage) data to study anomalies, locate maximum or minimum curve slopes, and perform reliability analyses.

When originally introduced, curve tracers were analog instruments that employed a CRT (cathode ray tube) display. Today, curve tracers have become digitized and use flat panel LCDs to present data. Digital techniques provide the ability to store and retrieve measurement data.

What Is GaN Current Collapse?

High-voltage GaN transistors improve the trade-off characteristics between the on-resistance and the breakdown voltage dramatically, thanks to GaN’s high carrier mobility and high critical electric field. However, initial devices experienced current collapse: a temporary reduction of drain-current immediately after the application of high voltage. This limited the device’s output power at high frequencies. Plus, an increase in current collapse was usually an indication of device degradation.

Current collapse is caused by electron trapping due to defects in the GaN layer and the interface between the passivation film and the AlGaN layer. Therefore, the electric field at the gate edge strongly affects the collapse due to the acceleration of channel electrons. Current collapse phenomena cause an increase in on-resistance, RDS(ON), which degrades the application’s power efficiency. This RDS(ON) increase caused by the current collapse phenomena strongly depends on the gate-edge electric field, and is caused by electron trapping due to high electric field acceleration. Also called dynamic RDS(ON), it is a phenomenon whereby a device’s on-resistance increases after being subjected to a drain bias. The magnitude of the increase depends on the drain-side gate edge electric field, under which electrons are accelerated and a small number remained trapped in the EPI layer, or at the EPI surface. Several companies and institutions have reported suppression of dynamic RDS(ON) by surface passivation. Proper field-plating can also reduce the peak electric field, thereby suppressing dynamic RDS(ON).

Better On-Resistance

It has been found that dynamic RDS(ON) was much improved with GaN devices built on conductive silicon substrates rather than those built on non-conductive sapphire substrates. This improvement is due to the conductive substrate acting as a field plate on the back side. With a conductive substrate, dynamic RDS(ON) is influenced by the GaN EPI layer thickness.

Dynamic RDS(ON) was a particular focus of the Efficient Power Conversion (EPC) Phase One reliability program. As EPC’s devices have been designed to mitigate dynamic RDS(ON), data show significantly greater stability than other similar devices.

EPC products use an optimized EPI structure and EPI thickness on conductive silicon substrates to minimize dynamic RDS(ON). The EPI surface is passivated with a high quality Si3N4 layer. This configuration enables EPC’s GaN transistors to be used as bare die, without additional packaging. The transistors employ wafer-level solder land-grid-arrays with optimized field plate structures.

Advantages of this design include:

  • Elimination of plastic packages and the related performance issues
  • Improved reliability
  • Reduced cost

Dynamic RDS(ON) was evaluated with a TESEC tester on experimental devices from EPC. RDS(ON) was measured pre- and post- stress while the drain-source voltage was stepped from low voltage to 30% above the rated maximum drain-source voltage. RDS(ON) values were found for various drain biases for a 2.5 ms duration for 40V, 100V, and 200V products. The 40V devices, EPC1014 and EPC1015, were biased to 40V, 48V and 52V sequentially. The 100V devices, EPC1001 and EPC1007, were biased to 60V, 100V, and 130V sequentially. The 200V devices, EPC1010 and EPC1012, were biased to 100V, 200V, and 260V sequentially. The degree of dynamic RDS(ON) was similar for all product types. The increase in RDS(ON) for the main population was about 10% with a tail at higher or lower values.

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