Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

METAL OXIDE SEMICONDUCTOR FIELD EFFECT TRANSISTOR (MOSFET)

MOSFET is the common term for the Insulated Gate Field Effect Transistor (IGFET). There are two basic terms of MOSFET (i) Enhancement MOSFET and (ii) Depletion MOSFET.

Principle

By applying a transverse electric field across an insulator, deposited on the semiconducting material, the thickness and hence the resistance of a conducting channel of a semiconducting material can be controlled.

In a depletion MOSFET, the controlling electric field reduces the number of majority carriers available for conduction, whereas in the enhancement MOSFET, application of electric field causes an increase in the majority carrier density in the conducting regions of the transistor.

ENHANCEMENT MOSFET

Construction

The construction of an N-channel enhancement MOSFET is shown in Fig. 1,50(a) and the circuit symbols for an N-channel and a P-channel enhancement MOSFET are shown in Fig. 1.50 (b) and 1.50(c) respectively. As there is no continuous channel in an enhancement MOSFET, this condition is represented by the broken line in the symbols.

Two highly doped N+ regions are diffused in a lightly doped substrate of P-type silicon substrate. One N+ region is called the source S and the other one is called the drain D. They are seperated by 1 mil (10-3 inch). A thin insulating layer of SiO₂ is grown over the surface of the structure of SiO₂ is grown over the surface of the structure and holes are cut into the oxide layer, allowing contact with source and drain. Then a thin layer of metal aluminium is formed over the layer of SiO₂. This metal layer covers the entire channel region and it forms the gate G.

The metal area of the gate, in conjunction with the insulating oxide layer of SiO₂ and the semiconductor channel forms a parallel plate capacitor. This device is called the insulated gate FET because of the insulating layer of SiO₂. This layer gives an extremely high input impedance for the MOSFET.

Operation

If the substrate is grounded and a positive voltage is applied at the gate, the positive charge on G induces an equal negative charge on the substrate side between the source and drain regions.

The direction of the electric field is perpendicular to the plates of the capacitor through the oxide. The negative charge of electrons which are minority carriers in the P-type substrate forms an inversion layer. As the positive voltage on the gate increases, the induced negative charge in the semiconductor increase. Hence, the conductivity increases and current flows from source to drain through the induced channel. Thus the drain current is enhanced by the positive voltage as shown in Fig. 1.51.

DEPLETION MOSFET

The construction of an N-channel depletion MOSFET is shown in Fig. 1.52(a) where an N channel is diffused between the source and drain to the basic structure of MOSFET.

The circuit symbols for an N-channel and a P-channel depletion MOSFET are shown in Fig. 1.52 (b) and 1.52 (c) respectively.

With V_GS = 0 and the drain D at a positive potential with respect to the source, the electrons (majority carriers) flow through the channel D to S. If the gate voltage is made negative, positive charge consisting of holes is induced in the channel through SiO₂ of the gate channel capacitor. The introduction of the positive charge causes depletion of mobile electrons on the channel.

Thus a depletion region is produced in the channel. The shape of the depletion region depends on V_GS and V_DS. Hence the channel will be wedge shaped as shown in Fig.1.52. When V_DS increased, I_D increases and it becomes practically constant at a certain value of V_DS, called the Pinch-Off voltage. The drain current I_D almost gets saturated beyond the pinch-off voltage.

Since the current in an FET is due to majority carriers (electrons for an N-type material) the induced positive charges make the channel less conductive, and I_D drops as V_GS is made negative.

The depletion MOSFET may also be operated in an enhancement mode. It is only necessary to apply a positive gate voltage so that negative charges are induced into the N-type channel. Hence the conductivity of the channel increases and I_D increases. As the depletion MOSFET can be operated with bipolar input signals irrespective of doping of the channel, it is also called as dual mode MOSFET. The volt ampere characteristics are induced in Fig. 1.51.

The curve of I_D versus V_GS for constant V_DS for is called the transfer characteristics of MOSFET and is shown in Fig. 1.53.