A Study of the Interelectrode Capacitances of SiC and GaN Power Semiconductor Devices Using Multiple Current Probes

When it comes to high switching speed, low on-state resistance, high operating temperature, etc, a considerable deal of research has been drawn to SiC/GaN-based devices over the past few years. This results in smoother functioning for high-power applications with even lower operating costs as compared to the traditional silicon (Si) based semiconductor devices. As a result, SiC and GaN support applications in reusable energy, HEMTs, high power industrial equipments, etc.

Power Semiconductor devices have voltage-dependant interelectrode capacitances that have a crucial role in the commutation on switching losses as well as electromagnetic compatibility (EMC).

Fig 1a: Normally-on GaN HEMT   Fig 1b: Normally-off» SiC JFET

The above figure 1a shows the normally-on condition of GaN HEMT with capacitances C_gd, C_gs, and C_ds and figure 1b shows the normally-off condition with capacitances C_gd and C_gs. During the experiment, it was noted that C-V_DS curves were always given for one V_GS voltage value. It is impossible to get accurate information in linear coordinates in high V_DS voltage due to the strong nonlinearity of the interelectrode capacitances.  Therefore, researchers and engineers must be able to characterize these capacitances easily, which can be applied to modeling of power devices.

Two-current-probe method Principle

It is essential to devise a new arrangement that can be easily operated by researchers to characterize the capacitances in Sic and GaN power semiconductor devices. Hence, the arrangement to compute an unknown impedance Z_x is portrayed in figure 2.

Figure 2- Basic Setup for two current probe measurements

In this figure a vector network analyzer, a current receiving probe, and a current injection probe are used. With the help of an equivalent transformer model for current probes, equation 1 can be formulated as

Equation 1- Impedance Equation

Here, network analyzers probe and measure S-parameters S₁₁ and S₂₁. Whereas  K and Z_setup are the two parameters that describe the coupling effect between current probes and the connecting wires, and the impedance of the measurement configuration respectively.

In order to determine the value for these parameters, we replace Z_X in equation 1 with two precision standard resistors to obtain two equations. By resolving these equations to find  K and Z_setup, this arrangement can be used to find unknown impedance Z_x.

Measurement Results

Figure 4 shows the measured capacitances of the JFET using multi-probe method. SiC JFET interelectrode capacitances show a correlation both with V_DS and V_GS, as shown in the results.

Fig 3: Capacitances measured

Under 40 volts, the surfaces could be seen to almost overlap, implying that low voltages could easily be detected and passed through the probes. Moreover, the multiprobe method allows for a higher V_DS voltage than the impedance analyzer with a 40V bias V_DS  voltage limit. 

Three-current-probe method Principle

The main issue with the two-current probe method is the accuracy limitation which is subdued by the new three-current probe method. This uses one additional current probe with one CIP and two CRPs to directly compute inter-electrode capacitances as shown in figure 4a.

     Figure 4a- Preliminary Setup

However for this setup to work, it is essential to form an equivalent circuit of measurement as shown in figure 4b. Here, the signal source of port 1 is V₁ whereas Vp₁, Vp_2, and Vp₃ are resultant signal voltages measured at their respective ports.

      Figure 4b- Equivalent Circuit

Keeping the output and input impedance of all ports at 50, Z_p1, Z_p2, and Z_p3 are input impedances of injection and receiving probes respectively. The mutual inductance between the probe and circuit is represented by M₁, M_2, and M₃, whereas the Z_w, Z_w1, and Z_w2 depict the parasitic impedance of wire connections. According to Ohms's law, voltage V₁ induces a current I_W in the circuit through the injection probe.

From figure 4b, a matrix(shown in figure 5) can be formulated that uses Kirchoff's voltage law to find unknown impedances Z_x1 and Z_x2.

Figure 5- Formulated Matrix

The unknown impedances Z_x1 and Z_x2 can be represented as shown in figure 6.

Figure 6- Impedance Equation

The network analyzer measures the S parameters whereas with the help of six precision standard resistors coupling effects Q₁ and Q₂ between the probes and measurement configuration can be obtained.

Measurement Results

The below figures 5a and 5b show the C_ds impedance and phase measurement results when V_DS equals to 0V and 20V respectively.

Fig 5a: C_ds measurement result  Fig 5b: C_gd measurement result

As demonstrated in the results, both C_ds and C_gd are close to noise levels at 1MHz, which makes calculating the capacitance at 1MHz difficult.  GaN HEMTs have such small interelectrode capacitances that their measured impedance around 1MHz is quite big, leading to measurement insufficiency.

Conclusion

A technical study on the use of multi-probe method in measuring voltage-dependent inter-electrode capacitances of SiC/GaN semiconductor devices was conducted. During the experiment, a basic setup was done wherein the proposed method has the advantage of isolating the measurement equipments from the DC bias power source. During the «Normally-off» condition of the SiC JFET, the method allows the characterization of the inter-electrode capacitances. This characterization takes place in high bias V_DS voltage and the measurement results prove the accuracy of this method. Contrarily, the measurement results for «Normally-on» GaN HEMT may not occur, as the inter-electrode impedance around 1MHz makes the received current by the CRP so small that the results are almost noise. In order to counter these inconveniences, two CRPs are wound with more turns in order to increase their measurement sensitivity. The results obtained proved that this modification greatly enhances the measurement precision for small inter-electrode capacitance values. This application can be successfully used in traditional silicon power devices as well along with their inter-electrode characterization.