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European Journal of Applied Sciences – Vol. 12, No. 5

Publication Date: October 25, 2024

DOI:10.14738/aivp.125.17525.

Chanana, R. K. (2024). Interrelated Channel Current and Oxide Leakage Current Density in a MOSFET Device. European Journal of

Applied Sciences, Vol - 12(5). 21-22.

Services for Science and Education – United Kingdom

Interrelated Channel Current and Oxide Leakage Current Density

in a MOSFET Device

Ravi Kumar Chanana

Self-Employed Independent Researcher, Gr. Noida-201310, India

ABSTRACT

This research paper discusses the observed lower Fowler-Nordheim (FN) onset

electric field in the thermal SiO2 grown on the (112̅0) surface of the 4H-SiC

semiconductor than on the (0001) oriented surface. This forms the metal-oxide- semiconductor (MOS) device which is the essential part of the MOSFET device as a

transistor. First, the FN onset field is revised to a new value of 4.6 MV/cm, and

then the mechanism for the lowering of the field is described.

Keywords: MOS device, Fowler-Nordheim tunnelling, MOSFET, effective masses.

SHORT COMMUNICATION

It has been observed that the Fowler-Nordheim (FN) onset field for metal-oxide- semiconductor (MOS) device having thermal SiO2 on the (112̅0) surface, known as the a-face

of the 4H-SiC semiconductor, is about 4.6 MV/cm. The observation as reported by the

research group is of the increased current density in the 4.5-5.5 MV/cm range [1]. Enlarging

the figure and using a ruler point to a value of 4.6 MV/cm for the increased current density, as

reported here by the author. It has also been calculated theoretically by the author to be 4.8

MV/cm when using the transverse electron effective mass of 0.42m in 4H-SiC [2]. N.T. Son et

al., first found the values of longitudinal and transverse electron effective masses in 4H-SiC to

be 0.29m and 0.42m [3]. They later revised the values to 0.30m and 0.45m [4]. The revised

value of 0.45m for the transverse electron effective mass in 4H-SiC is attributed to improved

crystal quality [4]. The author of the present study has first used 0.42m for the transverse

electron effective mass in 4H-SiC to get the FN onset field of 4.8 MV/cm. Now, using the

0.45m revised value yields an FN onset field in the MOS device on the (112̅0) surface of 4H- SiC of 4.6 MV/cm, utilizing the same theory and calculating procedure used earlier [2]. This

matches the observed value [1].

It is known that in an n-channel MOSFET device fabricated on a p-type semiconductor, the

electrons form the inversion channel from the source to drain of the device when a positive

gate voltage is applied. The n-type source and drain of the MOSFET have a high electric field

with the drain as the anode to source electrons from the source towards the drain giving the

drain current. The higher the mobility of the electrons, the higher is the drain current. Also, it

has been shown recently that the lower electron effective mass in silicon, for example, yields a

higher saturated drift velocity of electrons [5]. Higher drift velocity of electrons is directly

proportional to higher mobility of electrons and thus to higher drain current in the MOSFET

channel. So, if the channel in the MOSFET is oriented in the direction of lower electron

effective mass, then the channel current is larger. This has been demonstrated to be true in

4H-SiC based n-channel MOSFET that the channel oriented in the [0001] direction having a

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European Journal of Applied Sciences (EJAS) Vol. 12, Issue 5, October-2024

lower electron effective mass of 0.30m by growing the SiO2 on the (112̅0) surface [6]. A

trench MOSFET on the (0001) oriented surface of 4H-SiC gives the same result because the

walls of the trench along which the electrons flow are oriented in the [0001] direction and

form the (112̅0) surface. Increased current in the channel of the MOSFET is known to be

caused by increasing the gate voltage of an n-channel MOSFET. Therefore, if the channel

current is higher due to the lower electron effective mass, then the vertical gate voltage is

effectively higher than the original vertical gate voltage of the device. This means that the

leakage current density in the oxide given by Cox (dV/dt) will be higher with V as the gate

voltage. Here, Cox is the oxide capacitance per unit area. At the FN onset field in the oxide, the

vertical field due to Vwill yield a higher current density and so the onset field for electron

tunneling reduces to a lower value. This is what is happening in the 4H-SiC n-channel

MOSFET device as proposed by the author in this study, where the onset vertical field is

reduced to 4.6 MV/cm. The device fabricated on the (0001) oriented surface has a FN onset

vertical field of 5.6 MV/cm and the electron flows in the direction perpendicular to the [0001]

direction [2].

CONCLUSION

It is concluded by the above study that, an attempt to increase the electron channel mobility

and therefore the current in a MOSFET device by orienting the electron flow in the lower

effective mass direction, increases the oxide leakage current density. This consequently

reduces the FN onset field in the oxide, and further, the electric breakdown field strength in

the oxide.

References

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of nitridation on the reliability of 4H-SiC (112̅0) MOS device, Applied Physics Express, 2022. 15, 041002.

[2]. R.K. Chanana, N-channel 4H-SiC MOSFET device on (112̅0) oriented epitaxial surface, IOSR-J of Electrical and

Electronics Engineering, 2022. 17(3), p. 20-22.

[3]. N.T. Son, W.M. Chen, O. Kordina, A.O. Konstantinov, B. Monemar, E. Janzen, D.M. Hofman, D. Volm, M.

Drechsler, B.K. Meyer, Electron effective masses in 4H-SiC, Appl. Phys. Lett., 1995. 66(9), p. 1074-1076.

[4]. N.T. Son, P.N. Hai, W.M. Chen, C. Hallin, B. Monemar, E. Janzen, Hole effective masses in 4H-SiC, Physical

Review B, 2000. 61(16), p. R10544-546.

[5]. R.K. Chanana, Variation of saturated drift velocity of electrons in silicon with temperature, Journal of

Research in Engineering and Computer Sciences, 2024. 2(4), p. 38-40.

[6]. H. Yano, T. Hirao, T. Kimoto, H. Matsunami, K. Asano, Y. Sugawara, High channel mobility in inversion layers

of 4H-SiC MOSFET’s by utilizing (112̅0) face, IEEE Electron Device Letters, 1999. 20(12), p. 611-613.