Bipolar Transistor

What is a Bipolar Transistor?

The Bipolar Junction Transistor (BJT) is a solid-state device for amplifying, controlling, and generating electrical signals.

The transistor was invented in 1947 at the Bell Telephone Laboratories. This invention is the foundation for the solid-state microelectronics revolution that still continues even today. Bipolar transistors effectively replace the huge vacuum tubes. Their advantages are:

  • solid body and small size.
  • low heat generation.
  • relatively small power requirements.

This makes the miniaturization of complex circuitry possible .

Where are the Bipolar Transistors Used?

Transistors played an essential role in the advancement of electronics. They are used in a wide area of electronic equipment, ranging from pocket calculators to industrial robots and communications satellites. In addition to their use as amplifiers, bipolar transistors are also key components for oscillators, digital and analog circuits.

Without bipolar transistors, the portable radios, computers, night vision technology, low pollution automobiles, video cameras, cellular phone, touch tone dialing, photocopy machines, space exploration, modern manufacturing equipment, electronic games, car instrumentation panels, and many more, would not exist.

Integrated Circuits - A New Era of Transistor Evolution

As the semiconductor technology improved, the transistor became smaller,faster, cheaper, and more reliable. The ability to organize numerous transistors and other electronic components on a single tiny silicon wafer completed with wiring, led to the invention of integrated circuits. They are also called microchips or simply - chips.

These microchips took the transistor innovation to an exciting new level and spurred the evolution of the Information Age. The integrated circuits make modern computers and other contemporary state-of-the-art electronic equipments possible.

Bipolar Transistor Structure

The bipolar transistor is build in a single semiconductor crystal from layers with different types of conductivity.

Basically the bipolar transistor is fabricated by first forming a p-type region in the n-type substrate. Subsequently a n+ region (very heavily doped n-type) is formed in the p region. Metal contacts (Al) are made to the top n+ and p regions through the windows opened in the oxide layer (an insulator) and to the n region at the bottom.

An idealized, one-dimensional structure of the bipolar transistor can be considered as a section of the device along the dashed lines in the figure.

One-Dimensional Idealization

The figure shows an idealized one-dimensional structure of the bipolar transistor.

The bipolar transistor has three regions: the emitter, the base, and the collector;

  • The emitter is heavily doped and emits free charge carriers.
  • The base controls the flow of charges. It has light doping and is very thin.
  • The collector collects the charge carriers from the base.

The transistor is called bipolar because both the electrons and holes are involved in the conduction process. In n-type semiconductor, the majority charge carriers are free electrons. In p-type, the principal charge carriers are positively charged holes.

Emitter & Collector Junctions

The transistor structure in the figure is not drawn to a precise scale. The base is actually extremely thin.

Physically the bipolar transistor comprises two, closely coupled pn junctions. This structure gives the name: - bipolar junction transistor (BJT).

The pn junction between the emitter and the base is called emitter-base junction or simply - emitter junction. The collector-base or collector junction is the one between the base and the collector. There are two depletion layers - de for emitter junction and dc for collector junction. For each one of them, the barrier potential is approximately 0.7 V at 25 oC for silicon devices.

Bipolar Transistor Types

Two types of bipolar transistors exist - NPN and PNP. The letters correspond to the semiconductors type for emitter, base and collector. Both transistor types operate the same way but have opposite voltage polarities and currents directions.

The transistor is similar to two diodes - one at the emitter-base junction and another at the collector-base junction. However, these are not simply two back to back connected diodes. In a transistor both pn junctions share one region - the base.

Bipolar Transistor Schematic Symbols

The figure shows the schematic symbols for a transistor and the relation between the symbol electrodes and the transistor's structure. The arrow indicates that this electrode is the emitter. An arrow into the base indicates PNP. An arrow out of the base indicates an NPN.

As in all semiconductor symbols, the arrow is in the direction of the hole current. For PNP transistor the emitter arrow shows holes movement from the P emitter to the N base. For NPN transistor the arrow is in the opposite direction showing holes movement from the P base to the N emitter.

Bipolar Transistor Packages

The figure shows some of the existing packages (not relative in sizes) for bipolar transistors. They are available with plastic or metal cases.

Large power transistors have the collector connected to the metal case. The transistor case is then fastened to the chassis of an electronic equipment. This increases the effective surface area of the device making cooling easier. To prevent the collector from shortening to the chassis a thin mica spacer is used between the transistor case and the chassis.

Why are Bipolar Transistors Important?

The bipolar transistor is one of the most important semiconductor devices. It is an active semiconductor device - element that supplies the rest of the circuit with energy. It allows a small signal to control a much larger, high powered one.

The figure demonstrates how a transistor functions in a circuit. The signal from the microphone is very weak and the loudspeaker produces a very quiet sound. When the signal is supplied to a transistor it transforms a weak incoming current into a stronger copy of itself. A larger amplified signal in the speaker leads to a higher volume.

Biased Transistor - Common Base Connection

The bipolar junction transistor is a normally off device. It needs dc voltages applied to both pn junctions to start conducting. The figure shows a biased NPN transistor. Note that the supply voltage for each electrode is labeled with double subscripts, as in VEE and VCC. This notation is a standard practice. The left side of the circuit is called the emitter or input circuit , and the right side is called the collector or output circuit .

The circuit arrangement in the figure is known as a common-base configuration because the base is common to both the emitter and collector circuits.

Active Mode

The emitter-base junction is forward biased and the collector-base junction is reverse biased. This mode of operation is called the active mode.

In the active mode NPN transistors require negative supply voltage VEE at the emitter and positive dc supply voltage VCC at the collector. For a PNP transistor, all voltage polarities are the opposite. RE and RC are resistors in emitter and collector circuits.

Principle of Operation

The bipolar transistor delivers a change in output current in response to a change in the input voltage uin. The ratio of these two changes has resistance dimensions.

The name "transistor" is derived from "trans resistor", meaning that it can transfer its internal resistance from low R in the forward biased emitter-base circuit to a much higher R in the reverse biased collector-base circuit.

The principle of operation of the bipolar transistors is based on processes occurring in two closely spaced and influenced each other pn junctions.

Physical Processes in Emitter

If VEE is greater than the barrier potential, the emitter emits or injects electrons into the base as shown in the figure. Since the emitter is doped more heavily than the base the forward current at the emitter-base junction is carried primarily by electrons.

Physical Processes in Base and Collector

Electrons entering the P base are the minority carriers in this region. Since the base is lightly doped and very thin, a very small number of electrons recombine with holes in the base. Most of the free electrons move on to the collector junction.

They appear as extra minority carriers there and are extracted into the collector by the reverse collector voltage. As a result, almost all electrons supplied by the emitter flow in the collector circuit.

Transistor Currents

There are three different currents in a transistor:

  • emitter current IE,
  • base current IB and
  • collector current IC.

Transistor Currents Comparison

The emitter current is the largest current because it is the source of free charges. Typically, 98 or 99 percent or more of the emitter charges provide collector current. For this reason, the collector current approximately equals the emitter current.

The remaining one to two percent of charges injected in the base return through the external base circuit to the emitter and constitute the base current. Hence the base current is very small. For most transistors IB is measured in microamperes.

Transistor Currents Relation

According Kirchhoff's current law, the sum of all currents into a point equals the sum of all currents out of the point. Applied to a transistor this law gives important relation among the three transistor currents: IE = IC + IB The emitter current is the sum of the collector current and the base current. Note that for NPN transistors the IE is out from its electrode, while the IC and IB are into their electrodes. The IC almost equals the IE.

The ratio of IC to the IE is given by parameter αdc.It is very close to unity and indicates that in common-base connection the output current is less than the input current.

Common Emitter Connection

In figure, the common or groundside of each voltage source is connected to emitter.

Because of this, the circuit is referred to as a common-emitter (CE) connection.

The circuit has two loops. The left loop is called the base or input circuit, and the right loop is called the collector or output circuit. The voltage between the base and the emitter is symbolised UBE, while between the collector and the emitter - UCE. VBB is supply voltage at the base and RB is resistor in base circuit.

Common Emitter Connection

By using different values of VBB or RB the IB can be changed.

The base current controls the current from the emitter to the collector. In response to a small change in the base current (by increasing the forward voltage), a large change in collector current could appear. This is called current amplification, a very useful circuit property. If a resistor is placed in the collector lead, voltage will be dropped across the resistor.

Thus a small change in base current is the instigator of a large change in collector voltage. This is a manifestation of transistor action, that is, control by a third terminal of the current at, or the voltage between, two other terminals.

The Current Gain

The variation of the base current produces equivalent variation in the collector circuit. The ratio of the collector current to the base current is called the current gain, symbolized as βdc or hFE. The CE current gain has a high value since the collector current is much larger than the base current. For low-power transistor, the βdc is typically 100 to 300.

By rearranging current gain equation in two equivalent forms the collector and the base currents can be calculated as shown above.

Common-Emitter Amplifier

Bipolar junction transistors are used primarily as amplifiers. An amplifier is a circuit that increases the peak-to-peak voltage, current, or power of a signal.

The common-emitter configuration is the most useful amplifier circuit, in which a small change in the input current requires little power but results in much greater current and voltage in the output circuit. Common-emitter configuration provides both current and voltage amplification and hence has high power amplification factor.

Parameters - Current Gain

The figure shows the volt ampere (VA) characteristics of a bipolar junction transistor in CE configuration.

Input and output characteristics are the most important characteristics for circuits design. For this reason they will be discussed in more detail.

The Input Characteristic

This is a graph of IB versus UBE. It is similar to the graph of an ordinary rectifier diode as shown in figure.

The voltage across the base resistor equals the difference between the source voltage VBB and the base-emitter voltage UBE. The base current can be found by applying Ohm's low to the base resistor : IB = (VBB - UBE)/RB

Input Characteristics Family

The figure shows an input characteristics family. The curve at the higher collector voltage has slightly less base current for a given UBE. This phenomenon results from an internal transistor feedback from the collector diode to the emitter diode.

At higher collector voltages, the depletion layer at the base-collector junction expands. This reduces the base width and less electrons injected from the emitter recombine in the base. This reduces the base current. The phenomenon is called the Early effect. It can be ignored because the gap between different input curves is quite small.

Output Characteristics

The figure shows an input characteristics family. The curve at the higher collector voltage has slightly less base current for a given UBE. This phenomenon results from an internal transistor feedback from the collector diode to the emitter diode.

A typical output current-voltage characteristic for the common-emitter configuration is shown in the figure, where the collector current IC is plotted against the collector-emitter voltage UCE for various base currents.

When UCE is zero, the collector diode is not reverse biased. Therefore the collector current is zero. for UCE between 0V and approximately 1V the collector current rises rapidly and later becomes almost constant. At higher voltages a breakdown occurs.

Regions of Operation - Active Region

The output characteristics has four distinct operating regions: active, cutoff, saturation and breakdown.

In the active region, the emitter is forward biased, and the collector is reverse biased. The changes in collector voltage have no effect on the collector current because even a small amount of reverse bias is enough to collect the available electrons from the base. Graphically, the active region is the almost horizontal part of the curve.

When transistors are used as amplifiers, they operate in the active region. Sometimes these circuits are called linear circuits because changes in the input signal produce proportional changes in the output signal.

Saturation and Cutoff Regions

The saturation and cutoff regions are useful in digital circuits, where the transistor acts as a high speed switch.

In the saturation region (rising part of the curve) both the emitter and collector are forward biased. In this region the transistor acts as closed switch with near zero voltage across it.

In the cutoff region (bottom curve with IB = 0) the emitter and the collector are reverse biased. The small collector current ICEO is caused by thermally produced minority carriers and by surface leakage current of the reverse collector junction. In this region the transistor acts as an open switch with near zero current through it.

Breakdown Region

At higher reverse collector voltages a breakdown occurs.

A transistor can operate safely in either the saturation region or the active region, but not in the breakdown region. The breakdown region is usually avoided because the risk of destroying the transistor is too high.

Collector Voltage & Power

According Kirchhoff's voltage law the sum of all voltages around a loop or closed path is equal to zero. When applied to the collector circuit of the figure, Kirchhoff's voltage law gives the important equation UCE = VCC - ICRC. The collector-emitter voltage equals the collector supply voltage minus the voltage across the collector resistor.

The transistor has a power dissipation of PC = ICUCE. The transistor power equals the collector-emitter voltage times the collector current. This power is what causes the junction temperature of the collector diode to increase.

Parameters - Current Gain

Current gain is one of the most important transistor parameters. Defined as the ratio of the collector current to the base current it is symbolised as βdc or hFE. For low power transistors, this is typically 100 to 300.The current gain has a wide variation with collector current, temperature change and transistor replacement.

Because of manufacturing tolerances, the current gain of a transistor may vary over as much as 3:1 range with transistor replacement of the same transistor type.

Ratings

Maximum ratings are the limits on the transistor currents, voltages, powers and other quantities.

These parameters normally represent a destructive level that a designer usually avoids under all operating conditions. For reliable device operation maximum ratings should not even be approached. Otherwise, the device may cease to function properly or its useful lifetime may be greatly shortened.

Maximum ratings for either type of transistor are given in data sheets.

Maximum Power

When power is dissipated within a device, its junction temperature Tj tends to rise. The higher the power, the higher the junction temperature. Transistors will burn out when the junction temperature excides Tjmax, which is between 150 and 200 oC.

The power dissipation must always be less than the max power PCmax. Otherwise the transistor will be destroyed. The maximum power that a transistor can handle depends on the temperature. It decreases when temperature increases

Heat Removal

The power rating can be increased if the internal heat is dissipated faster. When a heat sink (a mass of metal) is pushed on to the transistor case, heat radiates more quickly because of the increased surface area of the fins. In a power tab transistor a metal tab provides a path out of the transistor for heat.

Large power transistors have the collector connected to the case to remove heat as easy as possible. The transistor case is then fastened to the chassis or to a large heat sink with fins. The idea is that the heat will be transferred faster in the surrounding air, which increases the power rating of a transistor at the same ambient temperature.

Thermal Resistance

If a power of P watts is dissipated in a transistor, than the internal temperature of the device Tj will be Tj = P.Rth + Ta. Here Ta is the ambient temperature and Rth is the thermal resistance between the junction and the ambient temperature.

Rth indicates efficiency in removing heat from the transistor in units oK / W. Actually it consists of several resistances in series. Rjc is the resistance between the junction and the case, Rch - between the case and the heat sink and Rha - between the heat sink and the ambient temperature.. The less thermal resistance the higher power rating.

Maximum Collector Current

The maximum collector current of a bipolar transistor ICmax gives the maximum current a bipolar transistor can handle without exceeding its power rating PCmax.

Small-signal transistors dissipate 0.5 W or less. Power transistors can dissipate more than 100 W. The maximum collector current ICmax is related to the power rating as follows:

ICmax = PCmax/UCE, where UCE is the collector-emitter voltage.

Breakdown Ratings

For the bipolar transistors the following maximum ratings are given: BUBC, BUEB and BUCEO. These voltage ratings are reverse breakdown voltages. BUBC is the voltage between the collector and the base. BUEB is the voltage from the emitter to the base.

BUCEO represents the voltage from collector to emitter with the base open . For normal transistor operation voltages should always be less than the breakdown values.

Punchthrough

When the base is very thin and low doped another breakdown-like phenomenon called punchthrough is possible.

This occurs when the depletion layer on the base side of collector junction reaches all the way across the base and merges with the depletion layer at the emitter junction.

Thus the controlling action of the base on the collector current is lost. Any increase in UCE beyond the punchthrough value reduces the barrier at the emitter-base junction and enables a massive charge to flow. This causes the transistor destruction.

Transistor Testing

An ohmmeter can be used to check a PN junction either for an open circuit or a short circuit. A good diode has very high ratio of reverse to forward resistance. For testing bipolar transistors, the same diode test can be used for each PN junction.

The emitter-base junction for a normal transistor should have a low resistance in one direction and a high resistance in the other. If this fails the emitter diode is defective and the transistor can be thrown away. The collector junction is tested the same way.