Written by hirobert and mkaiserm
This blog post explains how to implement a leakage test using a common piece of equipment, a source measure unit (SMU) and introduces a simplified circuit based on high voltage amplifier components which forces voltage and measures current. In this blog, you will learn about how to apply these two methods to MOSFET leakage testing.
MOSFETs are a very common component found in switching and automotive applications, supporting low voltage or high voltage swings and with a wide range of current drive. The number of high power applications is increasing, generating additional demand for Power MOSFETs. To produce this growing number of Power MOSFETs, more manufacturing test capacity is required. Many companies are developing products to serve the Power MOSFET testing need in addition to established semiconductor ATE vendors.
On the Power MOSFET production line, each device is tested to ensure that it meets the datasheet specifications. Leakage is a key performance metric which can be challenging to test in high voltage devices.
Let’s take Infineon Power MOSFET IPA100N08N3 as an example. Figure 1 shows the electrical characteristics from IPA100N08N3 datasheet.
Figure 1
Gate leakage current is usually determined by forcing a voltage and measuring the resulting current. The leakage current can vary due to the quality of the oxide or the physical property of the materials.
For IPA100N08N3, when VGS, VDS=0V, the I_{GSS} max value should be below 100nA. This I_{GSS} leakage test determines what is a good part to be shipped to customers versus a bad one to be discarded.
Generally speaking, MOSFET gate driver voltage range is +20V, and GaN may be even lower. Figure 2 shows basic topology to test gate leakage current value.
Figure 2
First the Source and Measure Unit (SMU) method is described and then a simplified force voltage, measure current circuit is introduced.
SMU
SMU is a fourquadrant source and sink operation equipment with measure functions, which adopts FVMI (force voltage, measure current) and FIMV (force current, measure voltage) modes. It can source a specified voltage and measure the current. It can also source a specified current and then measure the voltage through the DUT.
SMU’s can achieve a very low current range (nA, pA) with a resolution of fA, even aA. Therefore, there is no doubt that SMU’s can measure MOSFET leakage current accurately. Figure 3 gives the specification from Tektronix SMU2450 as an example.
Figure 3
Figure 4 shows the typical topology to test leakage current. Note that three SMU channels are required to implement the MOSFET leakage test.
Figure 4
FVMI mode: Set an output voltage typically from a DAC, amplify by power stage, measure current through selected sense resistor for the range, provide feedback on the voltage applied in DUT to the output voltage point, to make the DUT voltage is equal to the set voltage.
FIMV mode: Set an output current typically from a DAC, amplify by power stage, measure current through selected sense resistor for the range, provide feedback the voltage of sense resistor to output current setting point, to make sure the current flowing through DUT is equal to the set current.
Figure 5 shows a typical analog loop control SMU block diagram—for more information see Parametric Measurement  Analog Devices.
Figure 5
Some recommend parts are shown in Table 1:
Block Diagram Name 
Function 
Part Number 
Description 
VREF 
Voltage Reference 
LM399,ADR45xx D version 
0.5ppm/°C temperature drift 
SP DAC 
DAC 
AD5761, AD5781 
precision output DAC, 16bit to 18bit, for setpoint 
CMP DAC 
DAC 
AD5685R 
14bit DAC, for setting compliance 
I ADC 
ADC 
AD717X series 
low speed, high precision sigma delta ADC 
V ADC 
AD7982,AD4000 series 
high speed, high precison SAR ADC 

Current sense

HV Buffer 
ADHV47021,ADA47001, LTC6090/1 
Precision HV amp to buffer the input signal 
Difference Amp 
LT6375, LT19973, LT5400 + HV Buffer 
Difference amp to remove the common mode voltage 

Voltage sense 
HV Buffer 
ADHV47021,ADA47001, LTC6090/1 
HV amp to buffer the input signal (Optional) 
Difference Amp 
LT19972, LT6375 
Difference amp to remove the common mode voltage 

Driver Amp, AMP 
HV Amp 
ADHV47021, ADA47001, LTC6090/1 
High voltage Amp(drive output stage) 
There is also digital loop control SMU which requires an ADC with higher precision and speed than the analog loop control SMU. A digital loop control SMU example is shown in Figure 6.
Figure 6
For SMU solutions based on AD5522 PMU (Parametric Measurement Unit) IC, see ADI Multichannel Source Measurement Unit (SMU) Solution Guide. A system block diagram from this guide is given below:
Simplified Force Voltage, Measure Current Circuit
Figure 7 is the basic block to test leakage current.
Figure 7
Voltage source V1 sets the voltage to the Amplifier (ADHV47021) noninverting input node, and by simple OpAmp theory, the inverting input will match it (V_{+}=V_{}). The leakage current of the MOSFET gate will go through the resistance R1 if the Amplifier bias current is low enough compared with MOSFET leakage current and the common mode voltage of Amplifier is bigger than the voltage applied to MOSFET plus voltage of Sense resistance R.
I_{GSS} =(VoutV1)/R
Figure 8 and Figure 9 are the LTSpice simulation graphs with ADHV47021. A sense resistor of 10MOhm is recommended to measure a leakage current of 10nA. The voltage on the sense resistor varies from 100.00801mv to 99.981308mv, which shows that ADHV47021 can measure 10nA easily due to its low bias current (+2pA level at 25°C) at DC voltage Sweep from 20V to 20V.
Figure 8
Figure 9
Figure 10 and Figure 11 show a recommended signal chain and board implementation for leakage current test based on high voltage amplifier components.
Figure 10
Figure 11
In summary, we have discussed two methods to test the leakage current, using a standard SMU and simplified force voltage, measure current circuit. The functionality of a SMU is very powerful and this equipment can be used in many kinds of tests, but the solution is very complex and the cost is high. The simplified circuit using high voltage amplifier components is an easy way to achieve the desired result but with limited function and low cost.
For additional precision signal chain recommendations, please refer to analog.com/precision. Precision High Voltage signal chain examples are coming soon.
REFERENCES
[1] Datasheet IPA100N08N3 G (infineon.com)
[2] Keithley Low level voltage measurements handbook
[3] Tektronix SMU 2450 datasheet
[4] LTspice XVII simulation software
[6] ADI Multichannel Source Measurement Unit (SMU) Solution Guide
[7] Parametric Measurement  Analog Devices
[8] Precision Technology Signal Chains  Analog Devices, Inc.