High Field Transport

(Updated April 20, 2000)

Measurement of High Field Electron Transport in Silicon Carbide

Imran Khan and J. A. Cooper, Jr.

Supported by the Auburn Space Power Institute with funding from the SiC Consortium and Wright Laboratories

Single crystal wafers of SiC have only been commercially available since around 1990, and many of the fundamental electrical parameters of the material have not been accurately measured as yet. This presents a serious problem for device designers, since it is difficult to predict the performance of device structures if the basic electrical parameters are not known. One of the most important electrical parameters is the relationship between electron drift velocity and electric field. At low fields, velocity is proportional to field, v = µ E, where µ is the electron mobility. At higher fields (say greater than 1 V/µm), the velocity increases sub-linearly with field, and at sufficiently high fields the velocity saturates, becoming essentially independent of field. Measurements conducted by vön Muench and Pettenpaul [1] in 1977 indicate that the saturation velocity of electrons in 6H-SiC is around 2x10⁷ cm/s, approximately twice as high as other common semiconductors such as silicon or GaAs.

In this project, we have made the first measurements of the drift velocity of electrons in 6H-SiC as a function of temperature, and the first measurements ever for electrons in 4H-SiC [2]. The experimental structure, shown in Fig. 1, makes use of a narrow constriction in an isolated n-type conducting layer. In our devices, the n-layer doping is 1.3x10¹⁷ cm^-3 and the n-layer thickness is 1.1 µm in some samples and 4 µm in other samples. Several different constriction sizes are utilized: 5 x 5 µm, 10 x 10 µm, 5 x 10 µm, and 5 x 15 µm. We measure the steady-state current flowing through the constriction as a function of voltage. From the current density and a knowledge of the doping and cross-sectional area of the constriction, we calculate the electron drift velocity. Knowing the voltage drop and the length of the constriction, we calculate the electric field. We then plot electron drift velocity as a function of field.

Figure 1. Isometric view of the velocity-field test structure. This structure allows us to determine the drift velocity at a given field by measuring the current in the narrow n-type constriction at the center of the mesa.

In the process of conducting these experiments, we became aware of a number of pitfalls in the measurement procedure. One of the most serious is transient heating of the device during measurement. This heating is most severe at the highest fields, and can lead to a dramatic reduction in apparent drift velocity. To minimize heating effects, short (<1 µs) single-shot pulses are applied across the constriction and the current is recorded at the earliest point in the pulse, before significant heating has occured. A second pitfall is associated with end resistance on either side of the constriction. If the end resistance is ignored, the electric field is overestimated by up to a factor of two.

After accounting for these effects, we obtained velocity-field relations for electrons in 4H and 6H-SiC, as shown in Figs. 2 and 3 below. Table 1 gives an empirical equation for the velocity-field curve, along with parameters for 4H and 6H-SiC at various temperatures. The saturated drift velocity at room temperature is essentially the same in 4H and 6H-SiC, and is close to the value reported by vön Muench and Pettenpaul [1] for 6H-SiC.

Figure 2. Velocity-field relationship for electrons in 4H-SiC at two temperatures. Solid lines are generated using the equation and parameters given in Table 1 below.

Figure 3. Velocity-field relationship for electrons in 6H-SiC at three temperatures. Solid lines are generated using the equation and parameters given in Table 1 below.

Table 1. Parameters of the velocity-field equation v(E) = µ E /[1+ ( µ E / v_sat )^a ]^1/a for 6H and 4H-SiC at three temperatures. Solid lines in Figs. 2 and 3 are generated using this equation with parameter values in this table.

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[1] W. v. Muench and E. Pettenpaul, "Saturated Electron Drift Velocity in 6H Silicon Carbide,"J. Appl. Phys., 48, 4823 (1977).
[2] I. A. Khan and J. A. Cooper, Jr., "Measurement of High-Field Electron Transport in Silicon Carbide," IEEE Trans. on Electron Devices, 47, 269 (2000).

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