Enhancement-Type MOS-Transistors

In an more use enhancement-type MOSFET, a channel can only be formed and maintained when the device is in operation (Fig. 5.9), and then only when a definite value of gate voltage is applied in the correct polarity.

Fig. 5.9. N -channel enhancement-type MOSFET (a) and its graphical symbol (b)

In the absence of such a voltage, no channel exists, the source and drain of the n ⁺-type are separated solely by a p -type semiconductor substrate, and one of the p-n ⁺ junctions is reverse-biased. In this con­dition the resistance from source to drain is very high, and the MOSFET is turned off. If, however, we apply a positive voltage to the gate, the electric field set up at the gate will sweep conduction electrons out of the source and drain regions and the p- typesubstrate towards the gate. When the gate voltage exceeds some threshold value U_T (several volts), the electron concentration in the surface layer grows so much that it exceeds the hole concentration, thereby giving rise to the inversion of conduction and to the inversion layerwhich serves as a thin (or narrow) n -type channel so that the MOSFET is turned on. The higher the positive gate voltage, the higher the channel conduc­tivity and the greater the drain current. Thus, this type of MOSFET can only operate in the enhancement mode-this is corroborated by its output characteristics shown in Fig. 5.10a and the transfer characteristic in Fig. 5.10b. If an n -type substrate is used, p -type channel will be induced.

Fig. 5.10. Output (a) and transfer characteristics of an n -channel enhancement-type MOSFET

The parameters of MOSFETs are analogous to those of JFETs.

Their d. c. input resistance at low frequency is the insulation resistance of the gate and is 10¹²-10¹⁵Ω. High-power MOSFETs have been de­veloped for which the transconductance is 0,1A/V and greater and which can operate at frequencies of several hundreds megahertz.

That MOSFETs can be fabricated by planar-epitaxial technology with relative ease, and this simplifies the manufacture of integrated circuits.

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