Depletion-Type MOS-Transistor

The more recently developed and probably more important devices are insulated-gate FETs or IGFETs. The metal gate is insulated from the semiconductor channel by a thin layer of oxide. This device is termed a metal on silicon dioxide type or the metal-oxide-semiconductor field-ef­fect transistor, or MOSFET.

The structure of and a graphical symbol for the MOSFET are shown in Fig. 5.7. The substrate is a high-resistance p -type silicon bar in which two low-resistance n⁺ -type regions are produced. One of them is the source, and the other is the drain of the device.

Fig. 5.7. MOSFET: (a) structure, and graphic symbols for (b) n -channel MOSFET and (c) p- channel MOSFET

The source and the drain are separated by an n -type layer where a conducting channel is formed. Its length from source to drain is usually several micrometres. The Si0₂ layer (shown shaded) is 0.1 or 0.2 um wide. The device substrate is usually connected to the source electrode, and its potential is taken as datum (zero), and so is the source potential.

An n -type conducting channel formed with a gate voltage of zero. If we apply a voltage between drain and source, a current constituted by electrons will be flowing through the channel. No current will be flowing through the substrate because one of the p-n junctions of drain is reverse-biased. If we apply to the gate a voltage which is negative with respect to the source and, in consequence, with respect to the substrate, a transverse electric field will be set up in the channel, which will sweep conduction electrons out of the channel into the source and drain regions and into the substrate. The channel is thus depleted of electrons, its resistance is increased, and the drain current I_D is reduced. For pinch-off voltage U_SG = U_P the drain current is I_D = 0. This is termed depletion-mode operation, and a MOSFET operating thus is referred to as a depletion-type MOSFET.

Likewise some FETs can operate in both the enhancement and depletion modes. This can clearly be seen from their output (drain) cha­racteristics shown in Fig. 5.8a and the transfer characteristic of Fig. 5.8b.

Fig. 5.8. Output (a) and transfer (b) characteristics of an n -channel depletion-type MOSFET

If the substrate is an n -type material, the channel must be a p -type, and the polarity of voltages must be reversed (Fig. 5.7c).

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