Power device threshold voltage drift measurement method and system

Abstract

The invention relates to the field of transistor testing, in particular to a method and a system for measuring threshold voltage drift of a power device. According to the method for measuring the threshold voltage drift of the power device, a plurality of specific pulse signals with set pulse periods are continuously applied to the power device, the pulse waveform of the specific pulse signals comprises a gate stress stage and a measurement stage, the gate stress stage is to apply constant gate stress to the gate of the power device, and the measurement stage is to apply constant gate stress to the gate of the power device. In the measurement stage, a grid pulse signal is applied to the grid of the power device, and a drain pulse signal is applied to the drain of the power device; obtaining a drain current-grid voltage transfer characteristic curve in the measurement stage; extracting initial voltage according to the initial transfer characteristic curve; and taking a difference value between the threshold voltage and the initial voltage value as a threshold voltage drift value. According to the invention, accurate measurement brought by rapid switching can be ensured while rapid measurement is ensured, and the influence of a spontaneous heating effect on a power device is well avoided.

Description

Technical Field

[0001] The present invention relates to the field of transistor testing, and in particular to a method and system for measuring a threshold voltage drift of a power device. Background Art

[0002] Silicon carbide (SiC) MOSFET devices are used in high-temperature, high-voltage, and high-power working fields such as new energy vehicles and national defense industry due to their advantages such as fast switching speed, low leakage source current, and high power density. However, the reliability of SiC MOSFET devices, especially the threshold voltage V TH The drift problem is still an important factor limiting its further development. Under different bias stresses (gate stresses), SiC MOSFET will show complex drift characteristics. Finding a suitable test method to accurately measure the threshold voltage drift is an indispensable link.

[0003] Nowadays, there are three main methods for testing the threshold voltage drift of power devices, namely the voltage scanning method, the non-relaxation method and the fast pulse method. Among them, the voltage scanning method is a traditional method widely used in Si device threshold voltage measurement, and it is already a very mature technology in the Si system. This test method is relatively simple to operate, but when applied to SiC MOSFET, the slow test speed and scanning delay of this method have become the main factors affecting the accuracy of SiC MOSFET threshold voltage testing.

[0004] To solve this problem, a non-relaxation online testing technology was proposed. This testing method measures the threshold voltage V under the condition of uninterrupted gate stress. TH The drift of the non-relaxation method can compensate for the influence of fast traps underestimated by the traditional voltage scanning method. GS , so that the gate voltage V GS Greater than the threshold voltage V TH , the power device is working in the on state, and the drain voltage V is measured in real time DS and drain current I DS The threshold voltage V is obtained indirectly by using the Saatchi & Tang equation in the linear region. TH The threshold voltage V at any time TH The threshold voltage V at the initial moment TH (0) minus the threshold voltage drift value.

[0005] The non-relaxation method uses the formula conversion idea to achieve the threshold voltage V THThe real-time measurement solves the problem that the traditional voltage scanning method needs to scan after the gate voltage is removed. However, since this test method requires long-term stress application, it is easy to cause irreversible damage to the power device, and sampling must be completed while applying stress, which will greatly increase the experimental error.

[0006] The fast pulse method is based on the traditional voltage scanning method. It uses a pulse voltage signal instead of a DC voltage signal to measure the drain current-gate voltage (I DS -V GS ) transfer characteristic curve to extract threshold voltage drift information. Compared with the traditional scanning method, the pulse signal shortens the test time, the measurement results are more accurate, and the damage to the device is reduced. However, its fastest measurement time can only reach the microsecond level, and there are a large number of fast traps with nanosecond time constants at the SiO2 / SiC interface or near the interface area, which cannot be measured. In addition, when switching from the gate stress stage to the test stage, once the bias is eliminated, the threshold voltage V TH will quickly return to its predicted value, so V is measured at a slower switching speed. TH It will also lead to an underestimation of the threshold voltage drift. Summary of the invention

[0007] The purpose of the present invention is to provide a method and system for measuring the threshold voltage drift of a power device, so as to measure the threshold voltage drift characteristics of the power device under the action of an adjustable gate bias at the nanosecond level, and to reduce the influence of an external measurement signal on the electrical characteristics of the power device during the measurement process.

[0008] The method for measuring the threshold voltage drift of a power device of the present invention comprises: Continuously applying a specific pulse signal of a plurality of set pulse periods to the power device, wherein the pulse waveform of the specific pulse signal includes a gate stress stage and a measurement stage, wherein the gate stress stage is to apply a constant gate stress to the gate of the power device for a first set time, and the measurement stage is to apply a gate pulse signal to the gate of the power device and a drain pulse signal to the drain of the power device for a second set time; Obtaining a drain current-gate voltage transfer characteristic curve during the measurement phase; Obtaining an initial transfer characteristic curve when no gate stress is applied; Extracting a threshold voltage according to the drain current-gate voltage transfer characteristic curve; extracting an initial voltage according to the initial transfer characteristic curve; The difference between the threshold voltage and the initial voltage value is used as a threshold voltage drift value.

[0009] Furthermore, the first set time is not less than 20 nanoseconds.

[0010] Furthermore, the second set time is less than 1 microsecond.

[0011] Furthermore, the gate pulse signal is a DC or AC pulse signal with a pulse voltage periodically increasing, and the drain pulse signal is a DC pulse signal with a pulse voltage not less than zero.

[0012] Furthermore, the constant gate stress is negative gate stress or positive gate stress.

[0013] Further, the extracting threshold voltage includes: In each pulse cycle, data sampling is performed at 75% of the second set time.

[0014] Furthermore, a method based on transfer characteristic curve is used to extract the threshold voltage.

[0015] Furthermore, the method of extracting the threshold voltage drift value based on the transfer characteristic curve includes: In the drain current-gate voltage transfer characteristic curve, when the drain-source current of the power device reaches a set current value, the corresponding gate-source voltage is the threshold voltage drift value.

[0016] Furthermore, the set current value is not greater than 10 -5 A.

[0017] The present invention also provides a power device threshold voltage drift measurement system, which is used to implement the power device threshold voltage drift measurement method described in any one of the above technical solutions, including a measuring instrument and an auxiliary test circuit board, wherein the measuring instrument is used to provide a pulse signal of a set frequency to the power device, and the auxiliary test circuit board is used to connect the power device and the measuring instrument.

[0018] Compared with the prior art, the present invention has at least the following beneficial effects: The present invention solves the problem of inaccurate threshold voltage drift value test results of power devices caused by slow test speed in the prior art. The test time can reach hundreds of nanoseconds and the rise / fall time of the pulse edge can be as short as tens of nanoseconds. This ensures fast measurement while also achieving accurate measurement brought by fast switching.

[0019] In addition, the present invention has little impact on power devices, and the gate stress applied to the power devices can be as short as tens of nanoseconds, which effectively avoids the impact of self-heating effects on power devices. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A schematic structural diagram of an embodiment of a method for measuring a threshold voltage drift of a power device according to the present invention; Figure 2A measurement circuit diagram of a power device threshold voltage drift measurement system according to the present invention; Figure 3 for Figure 1 The measurement principle diagram of the power device threshold voltage drift measurement method in FIG. Figure 4 A pulse waveform diagram when the gate stress used in the measurement of the power device threshold voltage drift measurement system of the present invention is negative gate stress; Figure 5 A pulse waveform diagram when the gate stress used in the measurement of the power device threshold voltage drift measurement system of the present invention is positive gate stress; Figure 6 This is a measured pulse waveform diagram obtained by the power device threshold voltage drift measurement system of the present invention during measurement; Figure 7 A drain current-gate voltage transfer characteristic curve obtained by the power device threshold voltage drift measurement system of the present invention when the gate stress is negative gate stress; Figure 8 A drain current-gate voltage transfer characteristic curve obtained by the power device threshold voltage drift measurement system of the present invention when the gate stress is positive gate stress; Fig. 9 A threshold voltage variation diagram extracted by the power device threshold voltage drift measurement system of the present invention; Fig.10 A diagram of changes in threshold voltage drift values ​​obtained by the power device threshold voltage drift measurement system of the present invention. DETAILED DESCRIPTION

[0021] The power device threshold voltage drift measurement method and system of the present invention will be described below in conjunction with a schematic diagram, wherein a preferred embodiment of the present invention is shown, and it should be understood that those skilled in the art can modify the present invention described herein and still achieve the advantageous effects of the present invention. Therefore, the following description should be understood as being widely known to those skilled in the art and not as a limitation of the present invention.

[0022] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. The advantages and features of the present invention will become more apparent from the following description and claims. It should be noted that the drawings are in very simplified form and in non-precise proportions, and are only used to facilitate and clearly assist in illustrating the purpose of the embodiments of the present invention.

[0023] The following is attached to the instruction manual Figure 1 To Attachment Fig.10 , the power device threshold voltage drift measurement method of the present invention is described.

[0024] In one embodiment, if Figure 1As shown, the power device threshold voltage drift measurement method comprises the following steps: S100: continuously applying a specific pulse signal of a plurality of set pulse periods to the power device, wherein the pulse waveform of the specific pulse signal includes a gate stress stage and a measurement stage, wherein the gate stress stage is to apply a constant gate stress to the gate of the power device for a first set time, and the measurement stage is to apply a gate pulse signal to the gate of the power device and a drain pulse signal to the drain of the power device for a second set time; S200: obtaining a drain current-gate voltage transfer characteristic curve in a measurement phase; S300: obtaining an initial transfer characteristic curve when no gate stress is applied; S400: extracting a threshold voltage according to the drain current-gate voltage transfer characteristic curve; S500: extracting an initial voltage according to the initial transfer characteristic curve; S600: taking the difference between the threshold voltage and the initial voltage value as a threshold voltage drift value.

[0025] The significant advantages of the power device threshold voltage drift measurement method of the present invention are: The power device threshold voltage drift measurement method of the present invention solves the problem of inaccurate threshold voltage drift value test results of power devices caused by slow test speed in the prior art. The test time can reach hundreds of nanoseconds and the rise / fall time of the pulse edge can be as short as tens of nanoseconds. This ensures fast measurement while also achieving accurate measurement brought by fast switching.

[0026] The power device threshold voltage drift measurement method of the present invention has little influence on the power device, and the gate stress applied to the power device can be as short as tens of nanoseconds, which effectively avoids the influence of self-heating effect on the power device.

[0027] In step S100, in the gate stress stage, a constant gate stress V is applied to the gate of the power device. GS,stress The gate stress is reflected in the form of gate bias voltage. The longer the gate stress is applied, the easier it is to damage the power device. Therefore, the time for applying the gate stress needs to be shortened. Preferably, the first set time is not less than 20 nanoseconds. In the measurement phase, a gate pulse signal V is applied to the gate of the power device. GS , apply a drain pulse signal V to the drain of the power device DSAnd last for a second set time. The shorter the measurement time, the higher the measurement efficiency. Preferably, the second set time is less than 1 microsecond. Theoretically, the pulse period is the sum of the first set time and the second set time. In the actual test process, it is found that in the process of switching from the gate stress stage to the measurement stage, there will be a switching time for the pulse voltage applied to the gate. The switching time can be set by a measuring instrument. The actual value of the pulse period is the sum of the first set time, the second set time and the switching time. The shorter the switching time, the higher the accuracy of the measurement result.

[0028] In the measurement phase, the drain current-gate voltage transfer characteristic curve of the measurement phase is obtained by measuring the characteristic changes of the drain current and the gate voltage of the power device.

[0029] In one embodiment, in step S100, the gate pulse signal V GS is a DC or AC pulse signal with a pulse voltage period increasing. Preferably, the gate pulse signal V GS is an AC pulse signal whose pulse voltage increases periodically from negative to positive. The drain pulse signal V DS is a DC pulse signal with a pulse voltage not less than zero. GS The cycle is increased in steps, for example, in 0.1V steps.

[0030] In one embodiment, in step S100, the constant gate stress is a negative gate stress or a positive gate stress. During the test, the gate stress is reflected in the form of a gate bias voltage, so by applying gate bias voltages of different values, the drain current-gate voltage transfer characteristic curve under different gate bias voltages can be obtained.

[0031] In one embodiment, in step S400, when extracting the threshold voltage, the sampling point of the data is from the platform area after the pulse signal is stabilized. Preferably, in each pulse cycle, sampling is performed at 75% of the second set time, that is, sampling is performed at 75% of the duration of the test phase. In other embodiments, sampling can also be performed at 70% of the duration of the test phase.

[0032] In one embodiment, in step S400, a method based on a transfer characteristic curve is used to extract the threshold voltage drift value. Specifically, in the drain current-gate voltage transfer characteristic curve, when the drain-source current of the power device reaches a set current value, the corresponding gate-source voltage is the threshold voltage drift value. Preferably, the set current value is not greater than 10 - 5 A. For example, the set current value is 10 -5A, in other embodiments, the set current value can also be 10 -6 A. During extraction, a threshold voltage value is extracted from each drain current-gate voltage transfer characteristic curve of the power device to determine the drain-source current I of the device. DS Reach 10 -5 A corresponds to the gate voltage V GS As the threshold voltage V TH In other embodiments, the maximum transconductance method or the separated CV method may be used to extract the threshold voltage drift value.

[0033] The threshold voltage is an important parameter that characterizes the stability of the device's threshold voltage. The threshold voltage can be obtained from the drain current-gate voltage transfer characteristic curve.

[0034] In linear coordinates, the drain-source current I is generally defined as DS The gate voltage V corresponding to the point just rising GS is the threshold voltage V TH , while the gate voltage V at a specific current is defined in logarithmic coordinates GS is the threshold voltage V TH The experimental results show that due to different definitions, the threshold voltage V obtained in linear coordinates TH Compared with the threshold voltage V in logarithmic coordinates TH In actual research, the threshold voltage V in logarithmic coordinates is used. TH Drift is more instructive, so the drain-source current I of the power device is specified here. DS Reach 10 -5 A corresponds to the gate voltage V GS Defined as the threshold voltage V TH .

[0035] Since the threshold voltage of the power device can be obtained by performing only one measurement operation, the measurement speed of the power device threshold voltage drift measurement method of the present invention is very high.

[0036] Due to the threshold voltage drift V TH is the difference between the threshold voltage and the initial voltage value, so it is also necessary to obtain the initial transfer characteristic curve when no gate stress is applied. Specifically, a plurality of pulse signals with the same pulse period are continuously applied to the power device, and the pulse signal only has a measurement phase, a gate pulse signal is applied to the gate of the power device, and a drain pulse signal is applied to the drain of the power device and lasts for the entire pulse period.

[0037] After obtaining the initial transfer characteristic curve, the initial voltage is extracted, and then the difference between the threshold voltage and the initial voltage value is used as the threshold voltage drift value. V TH .

[0038] The step of obtaining the initial transfer characteristic curve when no gate stress is applied may be performed before applying the constant gate stress or after applying the constant gate stress, and is not limited to the order marked by the above step numbers.

[0039] In one of the embodiments, in order to avoid that the threshold voltage drift caused by the gate stress applied in the previous pulse cycle has not been fully recovered, which will affect the measurement result of the next pulse cycle, after each pulse cycle, the threshold voltage is restored to the initial voltage before the next pulse cycle is performed.

[0040] The present invention also provides a power device threshold voltage drift measurement system, which is used to implement the power device threshold voltage drift measurement method described in the above technical solution, including a measuring instrument and an auxiliary test circuit board, wherein the measuring instrument is used to provide a pulse signal of a set frequency to the power device, and the auxiliary test circuit board is used to connect the power device and the measuring instrument.

[0041] The power device threshold voltage drift measurement system of the present invention is described below in conjunction with a specific test process.

[0042] First, build a measurement system. Specifically, the measurement system includes a measuring instrument and an auxiliary test circuit board. The measuring instrument can be a 4200-SCS parameter analyzer. The auxiliary test circuit board is used to connect the power device and the measuring instrument, that is, the source, drain and gate of the power device are connected to the auxiliary test circuit board, and then the auxiliary circuit test board is connected to the measuring instrument. In this embodiment, the power device is a SiC MOSFET, and the measuring instrument is integrated with a bias voltage source, a pulse generator and an oscilloscope, which are used to provide a pulse signal of a set frequency to the power device. The scanning mode of the pulse signal is step scanning. The measurement circuit diagram of the measurement system is shown in the figure below. Figure 2 As shown, the test schematic is as follows Figure 3 As shown, the source of the power device is grounded and the gate (V gate ) and the drain (V drain ) are respectively indirectly connected to the pulse generator through the auxiliary test circuit board. In other embodiments, the power device may also be GaN HEMTs.

[0043] Second, the connection of the measurement system. Specifically, the power device to be tested is connected to the auxiliary test circuit board, the drain of the power device is connected to the 4225-PMU2 terminal of the 4200-SCS parameter analyzer through the auxiliary test circuit board, and is set to voltage bias, the gate of the power device is connected to the 4225-PMU1 terminal of the 4200-SCS parameter analyzer through the auxiliary test circuit board, and is set to pulse voltage, and the pulse voltage value and voltage step are further set, and the source of the power device is connected to the GNDU terminal of the 4200-SCS parameter analyzer through the auxiliary test circuit board.

[0044] Third, pulse waveform setting. After turning on the power switch, select the pulse signal, and adjust the pulse voltage amplitude, minimum value, pulse width, pulse edge time and pulse period in the pulse parameter setting interface. Select the selected pulse signal as an ultra-fast pulse signal. When the gate stress of the power device is negative gate stress, the pulse voltage diagram is as follows: Figure 4 As shown, when the gate stress of the power device is positive gate stress, the pulse voltage schematic diagram is as follows Figure 5 shown.

[0045] Taking into account the measurement time (i.e., the second setting time) and the extraction threshold voltage V TH The required current sensitivity, that is, a longer measurement time corresponds to a lower current sensitivity, and a shorter measurement time can test a higher current sensitivity. According to the instrument manual, after comprehensively considering various factors, the measurement time is set to 1 microsecond, such as Figure 4 and Figure 6 As shown, the gate stress application time is set to 40 nanoseconds, and the pulse rising / falling edge time is set to 20 nanoseconds, wherein the above times are the minimum limits of the instrument, and the smaller the above time is, the more accurate the measurement is.

[0046] Fourth, apply a pulse signal. Figure 6 As shown, the pulse signal first applies a gate bias voltage of +15V to the gate of the power device. GS,stress As positive gate stress, a voltage of 0V is applied to the drain terminal of the power device to cause a threshold voltage drift of the power device. This stage is the gate stress stage and lasts for 40 nanoseconds.

[0047] Then switch to the measurement phase, apply a pulse signal with a voltage of 0.2V to the drain end of the power device, and apply a pulse signal with a voltage step of 0.1V to the gate of the power device. The sampling point of the data comes from the platform area after the pulse signal stabilizes. In this embodiment, sampling is performed at 75% of the duration of the test phase, thereby obtaining a dynamic drain current-gate voltage transfer characteristic curve. According to the results obtained from the actual test, the gate stress time of 40 nanoseconds can meet the requirements. Although the time of the rising / falling edge of the pulse signal is set to 20 nanoseconds, the actual test result requires about 60 nanoseconds. By reducing the setting time, the time of the rising / falling edge of the pulse signal can be reduced to further reduce the error.

[0048] In order to avoid the threshold voltage drift caused by the gate stress applied in the previous pulse cycle not being fully recovered, which will affect the measurement result of the next pulse cycle, the next pulse cycle is carried out after the threshold voltage is restored to the initial voltage after each pulse cycle.

[0049] Fifth, threshold voltage extraction. In this embodiment, a method based on a transfer characteristic curve is used to extract the threshold voltage V TH , the drain-source current I DS Reach 10 -5 A corresponds to the gate-source voltage V GS is the threshold voltage V TH .

[0050] Since only one measurement operation is required to obtain the threshold voltage of the device, the measurement speed using this method is very fast. Figure 7 and Figure 8 As shown, by applying different values ​​of gate bias voltage V GS,stress , different gate bias voltages V GS,stress Based on the drain current-gate voltage transfer characteristic curve under the condition of , a threshold voltage value can be extracted from the pulse transfer characteristic curve of each power device to specify the drain-source current I of the power device. DS Reach 10 -5 A corresponds to the gate-source voltage V GS is the threshold voltage V TH , the extracted threshold voltage V TH With different gate bias voltage V GS,stress The changing picture Fig. 9 shown.

[0051] Due to the threshold voltage drift V TH is the threshold voltage V TH The difference between the initial voltage and the initial voltage value, so it is also necessary to obtain the initial transfer characteristic curve when no gate stress is applied. Figure 7 and Figure 8The black curve (Fresh) in the figure is the initial transfer characteristic curve. Specifically, a plurality of pulse signals with the same pulse period are continuously applied to the power device, and the pulse signal only has a measurement phase, a gate pulse signal is applied to the gate of the power device, and a drain pulse signal is applied to the drain of the power device and lasts for the entire pulse period.

[0052] After obtaining the initial transfer characteristic curve, the initial voltage is extracted, and then the threshold voltage V TH The difference between the initial voltage value and the threshold voltage drift value V TH , the threshold voltage drift value obtained V TH With different gate bias voltage V GS,stress The changing picture Fig.10 shown.

[0053] The significant advantages of the power device threshold voltage drift measurement system of the present invention are: The power device threshold voltage drift measurement system of the present invention solves the problem of inaccurate threshold voltage drift value test results of power devices caused by slow test speed in the prior art. The test time can reach hundreds of nanoseconds and the rise / fall time of the pulse edge can be as short as tens of nanoseconds. This ensures fast measurement while also achieving accurate measurement brought by fast switching.

[0054] The power device threshold voltage drift measurement system of the present invention has little influence on the power device, and the gate stress applied to the power device can be as short as tens of nanoseconds, thus effectively avoiding the influence of self-heating effect on the power device.

[0055] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for measuring threshold voltage drift of a power device, characterized in that: include: Continuously applying a specific pulse signal of a plurality of set pulse periods to the power device, wherein the pulse waveform of the specific pulse signal includes a gate stress stage and a measurement stage, wherein the gate stress stage is to apply a constant gate stress to the gate of the power device for a first set time, and the measurement stage is to apply a gate pulse signal to the gate of the power device and a drain pulse signal to the drain of the power device for a second set time; Obtaining a drain current-gate voltage transfer characteristic curve during the measurement phase; Obtaining an initial transfer characteristic curve when no gate stress is applied; Extracting a threshold voltage according to the drain current-gate voltage transfer characteristic curve; extracting an initial voltage according to the initial transfer characteristic curve; The difference between the threshold voltage and the initial voltage value is used as a threshold voltage drift value.

2. The method for measuring the threshold voltage drift of a power device according to claim 1, characterized in that: The first set time is not less than 20 nanoseconds.

3. The method for measuring the threshold voltage drift of a power device according to claim 1, characterized in that: The second set time is less than 1 microsecond.

4. The method for measuring the threshold voltage drift of a power device according to claim 1, characterized in that: The gate pulse signal is a DC or AC pulse signal with a pulse voltage periodically increasing, and the drain pulse signal is a DC pulse signal with a pulse voltage not less than zero.

5. The method for measuring threshold voltage drift of a power device according to claim 1, characterized in that: The constant gate stress is negative gate stress or positive gate stress.

6. The method for measuring the threshold voltage drift of a power device according to claim 1, characterized in that: The extracting threshold voltage comprises: In each pulse cycle, data sampling is performed at 75% of the second set time.

7. The method for measuring the threshold voltage drift of a power device according to claim 1, characterized in that: The threshold voltage is extracted using a method based on transfer characteristic curves.

8. The method for measuring the threshold voltage drift of a power device according to claim 7, characterized in that: The method of extracting the threshold voltage drift value based on the transfer characteristic curve includes: In the drain current-gate voltage transfer characteristic curve, when the drain-source current of the power device reaches a set current value, the corresponding gate-source voltage is the threshold voltage drift value.

9. The method for measuring the threshold voltage drift of a power device according to claim 8, characterized in that: The set current value is not greater than 10 -5 A.

10. A power device threshold voltage drift measurement system, characterized in that: A method for measuring the threshold voltage drift of a power device for implementing any one of claims 1 to 9, comprising a measuring instrument and an auxiliary test circuit board, wherein the measuring instrument is used to provide a pulse signal of a set frequency to the power device, and the auxiliary test circuit board is used to connect the power device and the measuring instrument.

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