BJT and FET Transistor Trivia Questions & FAQ

FET is unipolar & voltage driven device but BJT is bipolar & current driven device.

FET is the Field Effect Transistor. It is 3 terminal voltage controlled device. Its terminals are drain, source and gate. Gate is the controlling terminal.

Consider an n channel device. The gate (p material) is diffused. At zero gate voltage there is no reverse voltage at the channel. So as Vds (drain source voltage) increases current Ids also increases linearly. As the voltage is increased, at a particular voltage, pinch off occurs .This voltage is known as pinch off voltage. After pinch off drain current remains stationary .If we apply a gate voltage (negative voltage) the pinch off occurs early.

a) No minority carriers, b) High input impedance, c) It is a voltage controlled device, d) Better thermal stability

The modes of operation can be described in terms of the applied voltages (this description applies to NPN transistors; polarities are reversed for PNP transistors): Forward-active (or simply, active): The base?emitter junction is forward biased and the base?collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, ?F, in forward-active mode. Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector. This mode corresponds to a logical "on", or a closed switch. Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch.

A switch is a device that is used to 'open' or 'close' a circuit. Opening a circuit means creating a break in the circuit, preventing current flow and thus, turning it 'off'. Closing a circuit, on the other, means completing the circuit path, thereby allowing current to flow around it and thus, turning it 'on'.

The bipolar transistor, whether NPN or PNP, may be used as a switch. Recall that the bipolar transistor has three regions of operation: the cut-off region, the linear or active region, and the saturation region. When used as a switch, the bipolar transistor is operated in the cut-off region (the region wherein the transistor is not conducting, and therefore makes the circuit 'open') and saturation region (the region wherein the transistor is in full conduction, thereby closing the circuit).

The bipolar transistor is a good switch because of its large transconductance Gm, with Gm = Ic/Vbe where Ic is the collector-to-emitter (output) current and Vbe is the base-emitter (input) voltage. Its high Gm allows large collector-to-emitter currents to be easily achieved if sufficient excitation is applied at the base.

To illustrate this, the simplest way to use an NPN bipolar transistor as a switch is to insert the load between the positive supply and its collector, with the emitter terminal grounded. Applying no voltage at the base of the transistor will put it in the cut-off region, preventing current from flowing through it and through the load In this state, the load is 'off?. Applying enough voltage at the base of the transistor will cause it to saturate and become fully conductive, effectively pulling the collector of the transistor to near ground. This causes a collector-to-emitter current to flow through the load that's limited only by the impedance of the load. In this state, the load is 'on'.

One limitation of this simple design is that the switch-off time of the transistor is slower than its switch-on time if the load is a resistor. This is because of the stray capacitance across the collector of the transistor and ground, which needs to charge through the load resistor during switch-off. On the other hand, this stray capacitance is easily discharged to ground by the large collector current flow when the transistor is switched on. There are, of course, other better designs for using the bipolar transistor as a switch.