Question

71. In many kinds of integrated circuits. the preferred element of amplification/switching is nor the bipolar transistor discussed in the chapter, but the MOSFET (metal oxide semiconductor field effect transistor). Thecompany diagram shows one in its "normally off" state: Conduction electrons cannot flow from the n-type source, which is analogous to the emitter. "over the bump" in the ptype region to the n-type drain. analogous to the collector. (Annpn arrangement is shown. but just as for the bipolar transistor, a pnp would yield the complementary device.) The important difference is that rather than a direct electrical contact to the p-type region, as in the base of the bipolar, the centre lead, the gate, is a conductor bonded to the p-type region but separated by a thin insulating layer.

(a) Explain how applying a bias to the gate can cause this device to tum on. Should the gate bias voltage be positive or negative (relative to the source)? Why is the control mechanism referred to as "field effect"?

(b) The MOSFET is often said to be a "unipolar" device because valence holes (conduction elections in the pnp device) do not play the important role that they do in the bipolar. Explain. Would you expect a significant current through the gate due to electron-hole recombination in the p-type region? Why or why not?

(c) A low-input-impedance device is one in which there are large oscillations in input current for small oscillations in the input voltage. Correspondingly, a highinput-impedance device has a small input currentfor a large input voltage. Bearing in mind that the voltage across the forward-biased base-emitter diode of a bipolar transistor is always about$\frac{{\mathbf{E}}_{\mathbf{g}\mathbf{a}\mathbf{p}}}{\mathbf{e}}$ , while the input current is proportional to the output current, would you say that the bipolar transistor has low or high input impedance? What about the MOSFET?

Short answer

a) The gate to base must be forward biased and as the field of the gate to bias decides the current flow therefore the name is field effect.

b) The current in MOSFET is due to the type charge carriers the holes are not important and the significant amount of current will not flow through the gate due to electron hole recombination.

c) The BJT has low input impedance the MOSFET has very high input impedance.

Step-by-step solution

Given data

The given diagram is shown in Figure 1

Definition of Gate and MOSFET

A logic gate is a key switching component in digital circuits that regulates when an input pulse can reach the output. It generates a single output from one or more inputs.

A MOSFET, or metal oxide field effect transistor, is a three-terminal device with a high input impedance for its gate, source, and drain. Depletion type MOSFETs and enhancement type MOSFETs are the two types of these.

Draw the Diagram and explain

(a)

The given diagram is shown in Figure 1

MOSFET stand for metal oxide filed effect transistor, it is a three terminal device as gate, source and drain they have high input impedance.

These are of two type, depletion type MOSFET and other is enhancement type MOSFET.

The gate to bias voltage should be positive biased as this voltage decides the width of the depletion region.

When the gate is forward biased it penetrates the oxide layer and creates a path for the MOSFET current.

The oxide material is of the same material as of the transistor's source and drain. As the current flow is decided by the field in the oxide layer therefore the name is field effect transistor.

Therefore, the gate to base must be forward biased and as the field of the gate to bias decides the current flow therefore the name is field effect.

Explain how a significant amount of current will flow from the gate because of the electron hole recombination in the p type region

(b)

MOSFET stand for metal oxide filed effect transistor, it is a three terminal device as gate, source and drain they have high input impedance.

These are of two type, depletion type MOSFET and other is enhancement type MOSFET.

The base of the MOSFET is used to turn off the device when the reverse biased is applied. The current flows from the transistor from drain to source and there are only one type of carriers that is the n type.

Therefore the current through the gate due to electron hole recombination is not significant and the holes are not important.

Therefore, the current in MOSFET is due to the n type charge carriers the holes are not important and the significant amount of current will not flow through the gate due to electron hole recombination.

Explain howBJT has low or high output or low input impedance, also determine the case for MOSFET

(c)

MOSFET stand for metal oxide filed effect transistor, it is a three terminal device as gate, source and drain they have high input impedance.

These are of two type, depletion type MOSFET and other is enhancement type MOSFET.

The BJT has very low input impedance and the input and the output current in the BJT are equal.

The MOSFET has very high value of the input impedance as it has a thin layer between the gate and PNP region the large input voltage reduces the input current in the MOSFET.

Therefore, the BJT has low input impedance the MOSFET has very high input impedance.