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Chapter 6: Problem 7

a. Describe in your own words why \(I_{G}\) is effectively \(0 \mathrm{~A}\) for a JFET transistor. b. Why is the input impedance to a JFET so high? c. Why is the terminology field effect appropriate for this important three- terminal device?

Short Answer

Expert verified
(a) Gate current \(I_{G}\) in JFET is effectively zero because the gate is reverse-biased, restricting the current flow. (b) Input impedance of JFET is very high, as input gate current is near zero from being reverse-biased. Using Ohm's law, this implies a high resistance. (c) The term 'Field Effect' is appropriate for JFET because its operation involves controlling current flow using electric fields, thus 'Field Effect Transistor'.

Step by step solution

01

Understanding \(I_{G}\) Current in JFET

JFET stands for Junction Field Effect Transistor. It is a type of Field Effect Transistor (FET) that works on the principle of controlling the flow of current through semiconductor channels using an electric field. The \(I_{G}\) represents the current flowing through the gate. In the JFET structure, the gate is reverse-biased which means it forms a depletion region and hence, no current flow occurs. The only current which could flow are the minority carriers, but due to the reverse bias, this current is very minor, often disregarded, especially in comparison to the drain current \(I_{D}\). So, the effective current at gate (\(I_{G}\)) is considered as zero.
02

Explaining High Input Impedance in JFET

The input impedance refers to the ratio of the input voltage to the input current in a circuit or a device. In a JFET, the input stage is a reverse-biased diode which effectively restricts the input current, making it nearly zero as argued in first part. From Ohm’s law (\(V = IR\)), if the current (\(I\)) is near zero, the impedance (\(R\), resistance) tends to be very high (assuming a finite input voltage \(V\)). This is why the input impedance of a JFET is very high.
03

Field Effect in JFET

The terminology 'Field Effect' refers to the phenomenon where electric fields are used to control the behaviour of the device. In case of JFET, electric fields are used to control the conductivity and hence, the current flowing through the channel between the source and drain. The level of this current in the channel is regulated by the voltage applied to the gate. The name 'Field Effect' signifies this principle of operation in which the current is controlled by an electric field, hence the name 'Field Effect Transistor'.

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Most popular questions from this chapter

Given \(I_{D}=4 \mathrm{~mA}\) at \(V_{G S}=-2 \mathrm{~V}\), determine \(I_{D S S}\) if \(V_{P}=-5 \mathrm{~V}\).

Given a \(Q\) -point of \(I_{D_{Q}}=3 \mathrm{~mA}\) and \(V_{G S}=-3 \mathrm{~V}\), determine \(I_{D S S}\) if \(V_{P}=-6 \mathrm{~V}\).

What are the major differences between the collector characteristics of a BJT transistor and the drain characteristics of a JFET transistor? Compare the units of each axis and the controlling variable. How does \(I_{C}\) react to increasing levels of \(I_{B}\) versus changes in \(I_{D}\) to increasingly negative values of \(V_{G S}\) ? How does the spacing between steps of \(I_{B}\) compare to the spacing between steps of \(V_{G S}\) ? Compare \(V_{C_{\text {st }}}\) to \(V_{P}\) in defining the nonlinear region at low levels of output voltage.

A \(p\) -channel JFET has device parameters of \(I_{D S S}=7.5 \mathrm{~mA}\) and \(V_{P}=4 \mathrm{~V}\). Sketch the transfer characteristics.

In what ways is the construction of a depletion-type MOSFET similar to that of a JFET? In what ways is it different?
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