Transcript Slide 1
Chapter 6 Field-Effect Transistor Dr.Debashis De Associate Professor West Bengal University of Technology Outline Introduction The Field-effect Transistor Construction of the JFET Biasing of the JFET Current–Voltage Characteristics Transfer Characteristics of the JFET Construction and Characteristics of the MOSFET Complementary MOS Real-life Applications of the FET INTRODUCTION The invention of the BJT has brought a great twist in the modern era of semiconductor technology. This device, along with its field-effect counterpart, known as the field-effect transistor (FET), has had a huge impact on virtually every area of modern life. Practical field-effect transistors were first made in the form of JFET in 1953 and MOSFET in 1963. The field-effect transistor has taken various forms like that The junction field-effect transistor (JFET), The metal semiconductor field-effect transistor (MESFET), The metal-insulator-semiconductor field-effect transistor (MISFET), The metal-oxide-semiconductor field-effect transistor (MOSFET). THE FIELD-EFFECT TRANSISTOR The FET is a single carrier device and is often called the unipolar transistor because the carriers involved in the operation are either electrons or holes. The FET is also a semiconductor device in which the output quantity is controlled by an electric field, which is often the input quantity. The phenomenon where the conductivity of the semiconductor is modulated by an electric field applied normally to the surface of the semiconductor is called field effect, and this principle is brought into operation by extending the depletion region deep into the bulk of the semiconductor. Junction Field-effect Transistor (JFET):- In a junction FET, the control voltage modulates the depletion width of a reverse-biased p –n junction which, in turn, varies with the various parameters of the device. Insulated Gate Field-effect Transistor (IGFET):- The IGFET is also called the metal-oxide-semiconductor field-effect transistor (MOSFET). In this, the metal gate electrode is separated from the semiconductor by an insulator. Metal-semiconductor Field-effect Transistor (MESFET):- If the MOS junction is replaced by a direct metal-semiconductor contact, i.e., a Schottky barrier, it is called metal-semiconductor FET (MESFET). A MESFET is similar to a JFET except for the following differences:(i) It has a single gate (ii) The gate is formed by a metal-semiconductor junction THE FIELD-EFFECT TRANSISTOR Comparison between the BJT and the FET CONSTRUCTION OF THE JFET The JFET is a three-terminal device whose one terminal is capable of controlling the current between the other two. In JFETs, the width of a junction is used to control the effective crosssectional area of the channel that conducts current. JFETs are basically of two types: n-channel and p-channel. Construction of n-channel JFET CONSTRUCTION OF THE JFET The n-channel JFET consists of a uniformly doped n-type silicon semiconductor bar with ohmic contacts at both ends and semiconductor junctions made on either sides of the bar. The top portion of the n-type channel is connected through the ohmic contact to a terminal called the drain (D) while the lower end is connected to the terminal referred to as the source (S ). The two p-type materials, fabricated on the two sides, are connected together and then to the third terminal called gate (G). The source terminal gets its name from the fact that the carriers contributing to the current flow move out from the external circuit into the semiconductor at this electrode. The carriers travel through the bulk of the semiconductor and are subsequently collected at the drain electrode. The gate is called so because it controls the flow of charges though the bulk. CONSTRUCTION OF THE JFET With proper biasing of the device, current is allowed to flow from the source and gets collected at the drain terminal of the bar. As the drain current (ID) flows through the channel, a reverse-bias between the two p-regions and the channel reduces the effective width of the channel. As the effective width of the conducting channel has a striking effect on the resistance of the channel, the current flowing through it also varies correspondingly. The electrons in the n-region move from the source to the drain region; they are carried by majority carriers that drift through the channel. The majority carriers enter the channel region through the source terminal and leave the channel through the drain terminal, in agreement with the naming of the drain and the source terminals. In the absence of any applied potentials at the two p –n junctions, the JFET is under no-bias condition. The result is a depletion region at each junction that resembles the same region of a junction diode under no-bias conditions. For normal operation, the current enters the channel through the drain and as it flows through the channel the voltage drop constantly decreases. This can be a linear variation for low values of current. CONSTRUCTION OF THE JFET The FETs have considerably higher input impedance as compared to BJTs. It is evident that the control voltage is applied to a reverse-biased junction. So, the FET has a negative temperature coefficient at high current levels. This characteristic leads to a more uniform temperature distribution over the device area and prevents the FET from thermal runaway, which is a major point of concern in bipolar transistors. The device is thermally stable, even when the active area is large and when many such devices are connected in parallel. Because FETs are unipolar devices, they hardly suffer from any minority-shortage effects, and thus, have higher switching speeds and higher cut off frequency. This makes their operation rather smooth. In addition to this they are square-law devices, i.e., inter-modulation and cross-modulation products are much smaller than those of a bipolar transistors. BIASING OF THE JFET Let us now study the operation of the device with respect to the characteristics of an n-channel JFET. We shall consider two different cases in this regard. When VGS 0 V, VDS some positive value: a positive voltage VDS has been applied across the channel and the gate has been connected directly to the source to establish the condition VGS 0 V. State of the device with zero source–gate voltage and positive drain–source voltage