Transistor

Transistor

A transistor is a semiconductor device, commonly used as an amplifier or an electrically controlled switch. The transistor is the fundamental building block of the circuitry that governs the operation of computers, cellular phones, and all other modern electronics.

Because of its fast response and accuracy, the transistor may be used in a wide variety of digital and analog functions, including amplification, switching, voltage regulation, signal modulation, and oscillators. Transistors may be packaged individually or as part of an integrated circuit, which may hold a billion or more transistors in a very small area.

Introduction

Modern transistors are divided into two main categories: bipolar junction transistors (BJTs) and field effect transistors (FETs). Application of current in BJTs and voltage in FETs between the input and common terminals increases the conductivity between the common and output terminals, thereby controlling current flow between them. The transistor characteristics depend on their type. See Transistor models.

The term "transistor" originally referred to the point contact type, but these only saw very limited commercial application, being replaced by the much more practical bipolar junction types in the early 1950s. Ironically both the term "transistor" itself and the schematic symbol most widely used for it today are the ones that specifically referred to these long-obsolete devices. For a short time in the early 1960s, some manufacturers and publishers of electronics magazines started to replace these with symbols that more accurately depicted the different construction of the bipolar transistor, but this idea was soon abandoned.

In analog circuits, transistors are used in amplifiers, (direct current amplifiers, audio amplifiers, radio frequency amplifiers), and linear regulated power supplies. Transistors are also used in digital circuits where they function as electronic switches, but rarely as discrete devices, almost always being incorporated in monolithic Integrated Circuits. Digital circuits include logic gates, random access memory (RAM), microprocessors, and digital signal processors (DSPs).

Importance
The transistor is considered by many to be the greatest invention of the twentieth century. It is the key active component in practically all modern electronics. Its importance in today's society rests on its ability to be mass produced using a highly automated process (fabrication) that achieves vanishingly low per-transistor costs.

Although millions of individual (known as discrete) transistors are still used, the vast majority of transistors are fabricated into integrated circuits (often abbreviated as IC and also called microchips or simply chips) along with diodes, resistors, capacitors and other electronic components to produce complete electronic circuits. A logic gate consists of about twenty transistors whereas an advanced microprocessor, as of 2006, can use as many as 1.7 billion transistors (MOSFETs).

The transistor's low cost, flexibility and reliability have made it a universal device for non-mechanical tasks, such as digital computing. Transistorized circuits have replaced electromechanical devices for the control of appliances and machinery as well. It is often less expensive and more effective to use a standard microcontroller and write a computer program to carry out a control function than to design an equivalent mechanical control function.

Because of the low cost of transistors and hence digital computers, there is a trend to digitize information. With digital computers offering the ability to quickly find, sort and process digital information, more and more effort has been put into making information digital. As a result, today, much media data is delivered in digital form, finally being converted and presented in analog form by computers. Areas influenced by the Digital Revolution include television, radio, and newspapers.

Advantages of transistors over vacuum tubes

Prior to the development of transistors, vacuum (electron) tubes (or in the UK thermionic valves or just valves) were the main active components in electronic equipment. The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are:
 Small size and minimum weight, allowing the development of miniaturized electronic devices.
 Highly automated manufacturing processes, resulting in low per-unit cost.
 Lower possible operating voltages, making transistors suitable for small, battery-powered applications.
 No warm-up period required after power application.
 Lower power dissipation and generally greater energy efficiency.
 Higher reliability and greater physical ruggedness.
 Extremely long life. Transistorized devices produced more than 30 years ago are still in service.
 Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes.
 Ability to control very large currents, as much as several hundred amperes.
 Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications.

Types
Transistors are categorized by:
 Semiconductor material : germanium, silicon, gallium arsenide, silicon carbide, etc.
 Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
 Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
 Maximum power rating: low, medium, high
 Maximum operating frequency: low, medium, high, radio frequency (RF), microwave (The maximum effective frequency of a transistor is denoted by the term fT, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain).
 Application: switch, general purpose, audio, high voltage, super-beta, matched pair
 Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array, power modules
Thus, a particular transistor may be described as: silicon, surface mount, BJT, NPN, low power, high frequency switch.

Transistor Biasing
The proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage of signal is known as transistor biasing.

The basic purpose of transistor biasing is to keep the base-emitter junction properly forward biased and collector-base junction properly reverse biased during the application of signal. This can be achieved with a bias battery or associating a circuit with a transistor. The latter method is more efficient and is frequently employed. The circuit which provides transistor biasing is known as biasing circuit. It may be noted that transistor biasing is very essential for the proper operation of transistor in any circuit.

Essentials of a Transistor Biasing Circuit
Transistor biasing is required for faithful amplification. The biasing network associated with the transistor should meet the following requirements:
 It should ensure proper zero signal collector current.
 It should ensure that VCE does not fall below 0.5V for Ge transistors and 1V for silicon transistors at any instant.
 It should ensure the stabilisation of operating point.

Stabilisation
The collector current in a transistor changes rapidly when:
i. The temperature changes,
ii. the transistor is replaced by another of the same type. This is due to the inherent variations of transistor parameters.

When the temperature changes or the transistor is replaced, the operating point (i.e. zero signal IC and VCE) also changes. However, for faithful amplification, it is essential that operating point remains fixed. This necessitates to make the operating point independent of these variations. This is known as stabilisation.

The process of making operating point independent of temperature Changes or variations in transistor parameters is known as stabilisation.

Once stabilisation is done, the zero signal IC. and VCE become independent of temperature variations or replacement of transistor i.e. the operating point is fixed. A good biasing circuit always ensures the stabilisation of operating point.

Stability Factor
It is desirable and necessary to keep IC constant in the face of variations of ICBO (sometimes represented as ICO). The extent to which a biasing circuit is successful in achieving this goal is measured by stability factor S. It is defined as under:
The rate of change of collector current IC w.r.t. the collector leakage current *ICO at constant  and IB is called stability factor i.e.
Stability factor, S = at constant IB and 
The stability factor indicates the change in collector current IC due to the change in collector leakage current ICO. Thus a stability factor 50 of a circuit means that IC changes 50 times as much as any change in ICO. In order to achieve greater thermal stability, it is desirable to have as low stability factor as possible. The ideal value of S is 1 but it is never possible to achieve it in practice. Experience shows that values of S exceeding 25 result in unsatisfactory performance.
The general expression of stability factor lot a C.E. configuration can be obtained as under:
IC = IB + (+1) ICO
** Differentiating above expression w.r.t IC, we get,

1=  +(+1)
1=  +
S=
Need for stabilisation.
Stabilisation of the operating point is necessary due to the following reasons:-
(i) Temperature dependence of IC (ii) Individual variations (iii) Thermal runway.
(i) Temperature dependence of IC.
The collector current IC is given by;
IC=IB+ICEO = IB+(+1)ICBO
The collector leakage current ICBO is greatly influenced (especially in germanium transistor) by temperature changes. A rise of 10°C doubles the collector leakage current which may be as high as 0.2 mA for low powered germanium transistors. As biasing conditions in such transistors are generally so set that zero signal IC = 1mA, therefore, the change in IC due to temperature variations cannot be tolerated. This necessitates to stabilise the operating point i.e. to hold IC constant in spite of temperature variations.
(ii) Individual variations.
The value of  and VBE are not exactly the same for any two transistors even of the same type. Further, VBE itself decreases when temperature increases. When a transistor is replaced by another of the same type, these variations change the operating point. This necessitates to stabilise the operating point i.e. to hold IC constant irrespective of individual variations in transistor parameters.
(iii) Thermal runway.
The collector current for a CE configuration is given by ;
IC =IB + ( + 1) ICBO......(i)
The collector leakage current ICBO is strongly dependent on temperature. The flow of collector current produces heat within the transistor. This raises the transistor temperature and if no stabilisation is done, the collector leakage current ICBO also increases. It is clear from e.q. (i) that if ICBO increases, the collector current IC increases by ( + 1 ) ICBO. The increased IC will raise the temperature of the transistor, which in turn will cause ICBO to increase. This effect is cummulative and in a matter of seconds, the collector current may become very large, causing the transistor to bum out.
The self-destruction of an unstahilised transistor is known as thermal runway.
In order to avoid thermal runway and consequent destruction of transistor, it is very essential that operating point is stabilised i.e. IC is kept constant. In practice, this is done by causing IB to decrease automatically with temperature increase by circuit modification. Then decrease in IB will compensate for the increase in (+ 1 ) ICBO, keeping IC nearly constant. In fact. this is what is always aimed at while building and designing a biasing circuit.

Methods of Transistor Biasing
In the transistor amplifier circuits drawn so far biasing was done with the aid of a battery VBB which was separate from the battery VCC used in the output circuit. However, in the interest of simplicity and economy, it is desirable that transistor circuit should have a single source of supply-the one in the output circuit (i.e. VCC). The following are the most commonly used methods of obtaining transistor biasing from one source of supply (i.e. VCC):
(i) Base resistor method (ii) Biasing with feedback resistor
(iii) Voltage-divider bias.
In all these methods, the same basic principle is employed i.e. required value of base current (and hence IC) is obtained from VCC in the zero signal conditions. The value of collector load RC is selected keeping in view that VCE should not fall below 0.5V for germanium transistors and 1V for silicon transistors.
Base Resistor Method
In this method, a high resistance RB (several hundred K) is connected between the base and +ve end of supply for npn transistor and between base and negative end of supply for pnp tran¬sistor. Here, the required zero signal base current is provided by VCC and it flows through RB. It is because now base is positive w.r.t emitter i.e. base emitter junction is forward biased. The required value of zero signal base current IB (and hence IC = IB) can be made to flow by selecting the proper value of base resistor RB.

Circuit analysis.
It is required to find the value of RB so that required collector current flows in the zero signal conditions. Let IC be the required zero signal collector current.
IB =
Considering the closed circuit ABENA and applying Kirchhoff’s voltage law, we get, VCC = IB RB + VBE
or IB RB.= VCC - VBE
RB=
As VCC and IB are known and VBE can be seen from the transistor manual, therefore, value of RB can be readily found from exp. (i).
Since VBE is generally quite small as compared to VCC. the former can be neglected with little error. It then follows from exp. (i) that
RB=
It may be noted that VCC is a fixed known quantity and IB is chosen at some suitable value. Hence, RB can always be found directly, and for this reason, this method is sometimes called fixed-bias method.
Stability factor
Stability factor,S = S=
In fixed-bias method of biasing, IB is independent of IC so that dIB/dIC = 0. Putting the value of dIB/dIC = 0 in the above expression, we have,
Stability factor,S = + 1
Thus the stability factor in a fixed bias is ( + 1). This means that IC changes ( + 1) times as much as any change in ICO. For instance, if  = 100, then S = 101 which means that IC increases 101 times faster than ICO. Due to the large value of S in a fixed bias, it has poor thermal stability.
Advantages
i. This biasing circuit is very simple as only one resistance RB is required.
ii. Biasing conditions can easily be set and the calculations are simple.
iii. There is no loading of the source by the biasing circuit since no resistor is employed across base-emitter junction.
Disadvantages
i. This method provides poor stabilisation. It is because there is no means to stop a self-increase in collector current due to temperature rise and individual variations. For example, if  increases due to transistor replacement, then IC also increases by the same factor as IB is constant.
ii. The stability factor is very high. Therefore, there are strong chances of thermal runway. Due to these disadvantages, this method of biasing is rarely employed.

Biasing with Feedback Resistor
In this method, one end of RB is connected to the base and the other end to the collector. Here, the required zero signal base current is determined not by VCC but by the collector-base voltage VCB. It is clear that VCB forward biases the base-emitter junction and hence base current IB flows through RB. This causes the zero signal collector current to flow in the circuit.

Circuit analysis. The required value of RB needed to give the zero signal current IC can be determined as follows.
VCC = *IC RC + IB RB + VBE
RB =
=
Alternatively
VCE = VBE + VCB
VCB = VCE – VBE
RB = = where IB =
It can be shown mathematically that stability factor S for this method of biasing is less than ( + 1) i.e.
Stability factor, S < ( + 1 ) Therefore this method provides better thermal stability than the fixed bias. Advantages i. It is a simple method as it requires only one resistance RB. ii. This circuit provides some stabilisation of the operating point as discussed below: VCE = VBE+VCB Suppose the temperature increases. This will increase collector leakage current and hence the total collector current. But as soon as collector current increases, VCE decreases due to greater drop across RC. The result is that VCB decreases i.e. lesser voltage is available across RB. Hence the base current IB decreases. The smaller IB tends to decrease the collector current to original value. Disadvantages i. The circuit does not provide good stabilisation because stability factor is fairly high, though it is lesser than that of fixed bias. Therefore, the operating point does change. although to lesser extent, due to temperature variations and other effects. ii. This circuit provides a negative feedback which reduces the gain of the amplifier as explained hereafter. During the positive half-cycle of the signal, the collector current increases. The increased collector current would result in greater voltage drop across RC. This will reduce the base current and hence collector current. Voltage Divider Bias Method This is the most widely used method of providing biasing and stabilisation to a transistor. In this method, two resistances R1 and R2 are connected across the supply voltage VCC and provide biasing. The emitter resistance RE provides stabilisation. The name “voltage divider” comes from the voltage divider formed by R1 and R2. The voltage drop across R2 forward biases the base-emitter junction. This causes the base current and hence collector current flow in the zero signal conditions. Circuit analysis. Suppose that the current flowing through resistance R1 is I1. As base current IB is very small, therefore, it can be assumed with reasonable accuracy that current flowing through R2 is also I1. (i) Collector current IC: I1 = Voltage across resistance R2, V2 = Applying Kirchhoff’s voltage law to the base circuit. V2 = VBE+VE V2 = VBE+IERE IE = IE = IC IC = It is clear from exp. (i) above that IC does not at all depend upon . Though IC depends upon VBE but in practice V2 >> VBE so that IC is practically independent of VBE. Thus IC in this circuit is almost independent of transistor parameters and hence good stabilisation is ensured. It is due to this reason that potential divider bias has become universal method for providing transistor biasing.
(ii) Collector-emitter voltage VCE.
Applying Kirchhoff’s voltage law to the collector side,
VCC = ICRC +VCE +IERE
= ICRC +VCE +ICRE
= IC (RC+RE)+VCE
VCE = VCC-IC(RC+RE)
Stabillsalion In this circuit, excellent stabilisation is provided by RE. Consideration of eq. (i) reveals this fact.
V2 = VBE+ICRE
Suppose the collector current IC increases due to rise in temperature. This will cause the voltage drop across emitter resistance RE to increase. As voltage drop across R2 (i.e. V2) is *independent of IC, therefore, VBE decreases. This in turn causes IB to decrease. The reduced value of IB tends to restore IC to the original value.
Stability factor. It can be shown mathematically that stability factor of the circuit is given by:
Stability factor, S =
= (+1) x where RT =
If the ratio RT/RE is very small, then RT/RE can be neglected as compared to 1 and the stability factor becomes:
Stability factor= (+ 1) x = 1
This is the smallest possible value of S and leads to the maximum possible thermal stability. Due to design **considerations, RT/RE has a value that cannot be neglected as compared to 1. In actual practice, the circuit may have stability factor around 10.

This Projects Contains a lot of chemical equations and diagrams, so you can download the transistor.doc from the below given link.
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