TRANSISTORS

Transistors. Transistor is an active component and that is establishing in all over electronic circuits. They are used as amplifiers and switching apparatus.

As the amplifiers, they are used in high and low level, frequency stages, oscillators, modulators, detectors and in any circuit need to perform a function. In digital circuits they are used as switches.

There are a huge number of manufacturers approximately the world who produces semiconductors (transistors are members of this family of apparatus), so there are exactly thousands of different types. There are low, medium and high power transistors, for functioning with high and low frequencies, for functioning with very high current and or high voltages. This article gives an overview of what is a transistor, different types of transistors and its applications.

Different Types of Transistors.

The transistor is an electronic equipment. It is made through p and n type semiconductor. When a semiconductor is placed in center between same type semiconductors the arrangement is called transistors. We can say that a transistor is the combination of two diodes it is a connected back to back. A transistor is a device that regulates current or voltage flow and acts as a button or gate for electronic signals. Transistors consist of three layers of a semiconductor device, each capable of moving a current. A semiconductor is a material such like that germanium and silicon that conducts electricity in a “semi-enthusiastic” way. It’s anywhere between a genuine conductor such as a copper and an insulator (similar to the plastic wrapped roughly wires).

Transistor Symbol

A diagrammatic form of n-p-n and p-n-p transistor is exposed. In circuit is a connection drawn form is used. The arrow symbol defined the emitter current. In the n-p-n connection we identify electrons flow into the emitter. This means that conservative current flows out of the emitter as an indicated by the outgoing arrow. Equally it can be seen that for p-n-p connection, the conservative current flows into the emitter as exposed by the inward arrow in the figure.

There are so many types of transistors and they each vary in their characteristics and each has their possess advantages and disadvantages. Some types of transistors are used mostly for switching applications. Others can be used for both switching and amplification. Still other transistors are in a specialty group all of their own, such as phototransistors, which react to the amount of light shining on it to produce current flow through it. Below is a list of the different types of transistors; we will go over the characteristics that create them each up

Bipolar Junction Transistor (BJT)

Bipolar Junction Transistors are transistors which are built up of 3 regions, the base, the collector, and the emitter. Bipolar Junction transistors, different FET transistors, are current-controlled devices. A small current entering in the base region of the transistor causes a much larger current flow from the emitter to the collector region. Bipolar junction transistors come in two major types, NPN and PNP. A NPN transistor is one in which the majority current carrier are electrons. Electron flowing from the emitter to the collector forms the base of the majority of current flow through the transistor. The further types of charge, holes, are a minority. PNP transistors are the opposite. In PNP transistors, the majority current carrier is holes.

Field Effect Transistor

Field Effect Transistors are made up of 3 regions, a gate, a source, and a drain. Different bipolar transistors, FETs are voltage-controlled devices. A voltage placed at the gate controls current flow from the source to the drain of the transistor. Field Effect transistors have a very high input impedance, from several mega ohms (MΩ) of resistance to much, much larger values. This high input impedance causes them to have very little current run through them. (According to ohm’s law, current is inversely affected by the value of the impedance of the circuit. If the impedance is high, the current is very low.) So FETs both draw very little current from a circuit’s power source.

Thus, this is ideal because they don’t disturb the original circuit power elements to which they are connected to. They won’t cause the power source to be loaded down. The drawback of FETs is that they won’t provide the same amplification that could be gotten from bipolar transistors. Bipolar transistors are superior in the fact that they provide greater amplification, even though FETs are better in that they cause less loading, are cheaper, and easier to manufacture. Field Effect Transistors come in 2 main types: JFETs and MOSFETs. JFETs and MOSFETs are very similar but MOSFETs have an even higher input impedance values than JFETs. This causes even less loading in a circuit.

Heterojunction Bipolar Transistor (HBT)

AlgaAs/GaAs heterojunction bipolar transistors (HBTs) are used for digital and analog microwave applications with frequencies as high as Ku band. HBTs can supply faster switching speeds than silicon bipolar transistors mostly because of reduced base resistance and collector-to-substrate capacitance. HBT processing requires less demanding lithography than GaAs FETs, therefore, HBTs can priceless to fabricate and can provide better lithographic yield.

This technology can also provide higher breakdown voltages and easier broadband impedance matching than GaAs FETs. In assessment with Si bipolar junction transistors (BJTs), HBTs show better presentation in terms of emitter injection efficiency, base resistance, the base-emitter capacitance, and cutoff frequency. They also present a good linearity, low phase noise and high power-added efficiency. HBTs are used in both profitable and high-reliability applications, such as power amplifiers in mobile telephones and laser drivers.

Darlington Transistor

A Darlington transistor sometimes called as a “Darlington pair” is a transistor circuit that is made from two transistors. Sidney Darlington invented it. It is like a transistor, but it has much higher ability to gain current. The circuit can be made from two discrete transistors or it can be inside an integrated circuit. The hfe parameter with a Darlington transistor is every transistors hfe multiplied mutually. The circuit is helpful in audio amplifiers or in a probe that measures very small current that goes through the water. It is so sensitive that it can pick up the current in the skin. If you connect it to a piece of metal, you can build a touch-sensitive button.

Schottky Transistor

A Schottky transistor is a combination of a transistor and a Schottky diode that prevents the transistor from saturating by diverting the extreme input current. It is also called a Schottky-clamped transistor.

Multiple-Emitter Transistor

A multiple-emitter transistor is specialize bipolar transistor frequently used as the inputs of transistor transistor logic (TTL) NAND logic gates. Input signals are applied to the emitters. Collector current stops flowing simply, if all emitters are driven by the logical high voltage, thus performing a NAND logical process using a single transistor. Multiple-emitter transistors replace diodes of DTL and agree to reduction of switching time and power dissipation.

Dual Gate MOSFET

One form of MOSFET that is a particularly popular in several RF applications is the dual gate MOSFET. The dual gate MOSFET is used in many RF and other applications where two control gates are required in series. The dual gate MOSFET is fundamentally a form of MOSFET where, two gates are made-up along the length of the channel one after the other.

In this way, both gates influence the level of current flowing between the source and drain. In effect, the dual gate MOSFET operation can be considered the same as two MOSFET devices in series. Both gates affect the general MOSFET operation and therefore the output. The dual gate MOSFET can be used in a lot of applications including RF mixers /multipliers, RF amplifiers, amplifiers with gain control and the like.

Junction FET Transistor

The Junction Field Effect Transistor (JUGFET or JFET) has no PN-junctions but in its place has a narrow part of high resistivity semiconductor material forming a “Channel” of either N-type or P-type silicon for the majority carriers to flow through with two ohmic electrical connections at either end normally called the Drain and the Source respectively. There are a two basic configurations of junction field effect transistor, the N-channel JFET and the P-channel JFET. The N-channel JFET’s channel is doped with donor impurities meaning that the flow of current through the channel is negative (hence the term N-channel) in the form of electrons.

Avalanche Transistor

An avalanche transistor is a bipolar junction transistor designed for process in the region of its collector-current/collector-to-emitter voltage characteristics beyond the collector-to-emitter breakdown voltage, called avalanche breakdown region. This region is characterized by the avalanche breakdown, an occurrence similar to Townsend discharge for gases, and negative differential resistance. Operation in the avalanche breakdown region is called avalanche-mode operation: it gives avalanche transistors the capability to switch very high currents with less than a nanosecond rise and fall times (transition times).

Transistors not particularly designed for the purpose can have reasonably consistent avalanche properties; for example 82% of samples of the 15V high-speed switch 2N2369, manufactured over a 12-year period, were capable of generating avalanche breakdown pulses with rise time of 350 ps or less, using a 90V power supply as Jim Williams writes.

Diffusion Transistor

A diffusion transistor is a bipolar junction transistor (BJT) formed by diffusing dopants into a semiconductor substrate. The diffusion process was implemented later than the alloy junction and grown junction processes for making BJTs. Bell Labs developed the first prototype diffusion transistors in 1954. The original diffusion transistors were diffused-base transistors. These transistors still had alloy emitters and sometimes alloy collectors like the earlier alloy-junction transistors. Only the base was diffused into the substrate. Sometimes the substrate produced the collector, but in transistors like Philco’s micro alloy diffused transistors the substrate was the bulk of the base.

Transistor Applications

The appropriate application of power semiconductors requires an understanding of their maximum ratings and electrical characteristics, information that is presented within the device data sheet. Good design practice employs data sheet limits and not information obtained from small sample lots. A rating is a maximum or minimum value that sets a limit on device’s ability. Act in excess of a rating can result in irreversible degradation or device failure. Maximum ratings signify extreme capabilities of a device. They are not to be used as design circumstances.

A characteristic is a measure of device performance under individual operating conditions expressed by minimum, characteristic, and/or maximum values, or revealed graphically.

Thus, this is all about what is a transistor and different types of transistors and its applications. We hope that you have got a better understanding of this concept or to implement electrical and electronics projects, please give your valuable suggestions by commenting in the comment section below. Here is a question for you, what is the main function of a transistor?

Bipolar Junction Transistor Biasing

Transistors are the most important semiconductor active devices essential for almost all circuits. They are used as electronic switches, amplifiers etc in circuits. Transistors may be NPN, PNP, FET, JFET etc which have different functions in electronic circuits. For the proper working of the circuit, it is necessary to bias the transistor using resistor networks. Operating point is the point on the output characteristics that shows the Collector-Emitter voltage and the Collector current with no input signal. The Operating point is also known as the Bias point or Q-Point (Quiescent point).

Biasing is referred to provide resistors, capacitors or supply voltage etc to provide proper operating characteristics of the transistors. DC biasing is used to obtain DC collector current at a particular collector voltage. The value of this voltage and current are expressed in terms of the Q-Point. In a transistor amplifier configuration, the IC (max) is the maximum current that can flow through the transistor and VCE (max) is the maximum voltage applied across the device. To work the transistor as an amplifier, a load resistor RC must be connected to the collector. Biasing set the DC operating voltage and current to the correct level so that the AC input signal can be properly amplified by the transistor. The correct biasing point is somewhere between the fully ON or fully OFF states of the transistor. This central point is the Q-Point and if the transistor is properly biased, the Q-point will be the central operating point of the transistor. This helps the output current to increase and decrease as the input signal swings through the complete cycle.

For setting the correct Q-Point of the transistor, a collector resistor is used to set the collector current to a constant and steady value without any signal in its base. This steady DC operating point is set by the value of the supply voltage and the value of the base biasing resistor. Base bias resistors are used in all the three transistor configurations like common base, common collector and Common emitter configurations.

Modes of biasing:

Following are the different modes of transistor base biasing:

1. Current biasing:

As shown in the Fig.1, two resistors RC and RB are used to set the base bias. These resistors establish the initial operating region of the transistor with a fixed current bias.

The transistor forward biases with a positive base bias voltage through RB.  The forward base-Emitter voltage drop is 0.7 volts. Therefore the current through RB is I_B = (V_cc – V_BE ) / I_B

2. Feedback biasing:

Fig.2 shows the transistor biasing by the use of a feedback resistor. The base bias is obtained from the collector voltage. The collector feedback ensures that the transistor is always biased in the active region. When the collector current increases, the voltage at the collector drops. This reduces the base drive which in turn reduces the collector current. This feedback configuration is ideal for transistor amplifier designs.

3. Double Feedback Biasing:

Fig.3 shows how the biasing is achieved using double feedback resistors.

By using two resistors RB1 and RB2 increases the stability with respect to the variations in Beta by increasing the current flow through the base bias resistors. In this configuration, the current in RB1 is equal to 10 % of the collector current.

4. Voltage Dividing Biasing:

Fig.4 shows the Voltage divider biasing in which two resistors RB1 and RB2 are connected to the base of the transistor forming a voltage divider network. The transistor gets biases by the voltage drop across RB2. This kind of biasing configuration is used widely in amplifier circuits.

5. Double Base Biasing:

Fig.5 shows a double feedback for stabilization. It uses both Emitter and Collector base feedback to improve the stabilization through controlling the collector current. Resistor values should be selected so as to set the voltage drop across the Emitter resistor 10% of the supply voltage and the current through RB1, 10% of the collector current.

Advantages of Transistor:

1. Smaller mechanical sensitivity.
2. Lower cost and smaller in size, especially in small-signal circuits.
3. Low operating voltages for greater safety, lower costs and tighter clearances.
4. Extremely long life.
5. No power consumption by a cathode heater.
6. Fast switching.