Introduction of BJT, JFET and MOSFET

BJT (Bipolar Junction Transistor)

A BJT is a current controlled device (i.e. output characteristics of a device are controlled by base current and not by base voltage)

However in field effect transistor (FET) the output characteristics are controlled voltage (i.e. Electric field) and not by input current.

In BJT both holes and electrons play parts in conduction.

The input circuit of BJT is forward biased and hence has low input impedance. A BJT uses a current into its base to control a large current between collector and emitter. BJT gain is characterized by current gain.

A bipolar junction transistor is formed by joining three sections of semiconductors with alternative different doping. The middle section (base) is narrow and one of the other two regions (emitter) is heavily doped. The other region is called the collector.

 Two variants of BJT are possible: NPN (base is made of p-type material) and PNP (base is made of n-type material).

JFET (Junction Field Effect Transistor)

A JFET is a three terminal semiconductor device in which current conduction is by one type of carrier (i.e. electrons or holes). It is controlled by means of an electric field between the gate electrode and the conducting channel of the device. The JFET has high input impedance and low noise level. A JGET consists of a P-type or N-type silicon bar containing two PN junctions at eh sides. The bars form the conducting channel for the charge carriers. If the bar is of N- type it is called N- channel JFET and if the bar is P- type it is called a P- channel JFET. The two PN- junction forming diodes are connected internally and are taken out known as gate.

JFET has essentially three terminals.

Gate (G)                                              Source (S)                                           Drain (D)

Schematic symbol of JFET:

N- Channel JFET                                                                                                                P- channel JFET

In a JFET there is only one type of carrier:

                Hole in P- type channel

                Electron in N- type channel

For this reason it is also called unipolar transistor. As the input circuit of a JGET is reverse bias, the device has high input impedance. As the Gate is reverse bias, it carries very small current. JFET can also be said as voltage driven device. JFET use a voltage on gate terminal to control the current. JFET gain is characterized by the ratio of change in output current (drain current) to input voltage (gate voltage).

Working principle of JFET

When voltage (V_DS) is applied between drain and source terminal and voltage on gate is zero, the two PN-junctions at eh sides of the bar establish depletion layer. The electron will flow from source to drain through on channel between depletion layers.

When reverse voltage (V_GS) is applied between the gate and the source, the width of the depletion layer is increased; this reduces the width of conducting channel thereby increasing the resistance of N- type bar.

Importance of JFET

A JFET acts like a voltage controlled device (i.e. input voltage (V_GS) controls the output current. This is different from BJT where input current controls the output current. Thus JFET is a semiconductor device acting like vacuum tube.

Output characteristics of JFET

The curve between drain current (I_D) and drain source voltage of a JFET at constant gate source voltage (V_GS) is known as output characteristics of JFET.

The following points may be noted from this characteristic:

At first, the drain current (I_D) rises rapidly with drain source voltage (I_D) but then becomes constant. The drain source voltage above which drain current becomes constant is known as pinch off voltage.

After pinch off voltage, the channel width becomes so narrow that the depletion layer almost touches each other. The drain current passes through the small passage between these layers. Therefore increase in drain current is very small with (V_DS) above pinch off voltage. Consequently drain current remains constant.

The characteristic resemble that of a pentode valve.

Important terms in the analysis of JFET circuit

Short gate drain current (I_DSS): it is the drain current with source short-circuited to gate (i.e. voltage gate source (V_GS) = 0) and drain voltage (V_DS) equal to pinch off voltage. It is sometimes called zero bias current.

                Pinch off voltage: it is the minimum drain source voltage at which the drain current essentially becomes constant.

Gate source cut off voltage: it is the gate source voltage where the channel is completely cut off and the drain current becomes zero.

Application of JFET:

                As a buffer amplifier:

A buffer amplifier is a stage of amplification that isolates the preceding stage from the following stage. Because of the high input impedance and low output impedance, a JFET can act as an excellent buffer amplifier.

                As a phase shift oscillators:

               

As RF amplifier:

In communication electronics, we have to use JFET RF amplifier in a receiver instead of BJT amplifier because of following reasons:

                The nose level of JFET is very low than BJT.

                Since JFET is a voltage controlled device, is will well respond to low current signal provided by antenna.

• As Displacement sensors

• As High input impedance amplifiers

• As Low-noise amplifiers

• As Differential amplifiers

• As Constant current sources

• As Analogue switches or gates

• As Voltage controlled resistors              

Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

Metal oxide semiconductor field effect transistor is an important semi-conductor device and is widely used in many circuit applications. The input impedance of a MOSFET is much more than that of JFET because of very small gate leakage current. The MOSFET can be used in any of the circuits covered for the JFET. Therefore, all the equations apply equally well to the MOSFET and JFET in amplifier connections.

The constructional details of N- channel MOSFET is similar to JFET except with the following modification:

                There is only a single P- region. This region is called substrate.

                A thin layer of metal oxide (usually silicon dioxide) is deposited over the left side of the channel. A metallic gate is deposited overt the oxide layer. As silicon dioxide is an insulator, therefore, gate is insulated from the channel. For this reason, MOSFET si sometimes called insulated gate FET.

                Like JFET, a MOSFET has three terminals (Source, Gate, and Drain)

Working principle of MOSFET

Instead of gate diode as in JFET, here gate is formed as a small capacitor. One plate of this capacitor is the gate and the other plate is the channel with metal oxide as the dielectric. When negative voltage is applied to the gate, electrons accumulate on it. These electrons repel the conduction band electrons in the N- channel. Therefore, lesser number of conduction electrons is made available for current conduction through the channel. The greater the negative voltage on the gate, the lesser is the current conduction from the source to drain. If the gate is given positive voltage, more electrons are made available in the N- channel, consequently, current from source to drain increases. The following points may be noted:

                In a MOSFET, the source to drain current is controlled by the electric field of capacitor formed at the gate.

                Unlike the JFET, and MOSFET has no gate diode. This makes it possible to operate the device with positive or negative gate voltage.

                As the gate forms the capacitor, therefore, negligible gate current flows whether positive or negative voltage is applied to the gate. Consequently, the input impedance of MOSFET is very high, ranging from 10,000 Mῼ to 10, 00,000 Mῼ.