Transistors, Physics tutorial


The troubles with vacuum tubes fuelled the search for alternative manners to make three terminal devices rather than by employing electrons in vacuum and researches and scientists similar start to believe how one might control electrons in solid materials, such as metals and semiconductors. In the year 1920, solid state two terminal devices were already being realized through point contact between a sharp metal tip and pieces of naturally occurring semiconductor crystal. Such point-contact diodes were employed to rectify signals and make simple AM radio receivers (that is, crystal radios). It was not till year 1947 though, when John Bardeen and Walter Brattain, working at Bell Telephone Laboratories, were trying to recognize the nature of the electrons at the interface between a metal and a semiconductor, that they recognized that by making two point contacts extremely close to one other, they could make a three terminal device - that was the point contact transistor.

Bell Telephone Company instantly understands the potential power of this new technology that sparked off huge research effort in solid state electronics. Bardeen and Brattain get the Nobel Prize in Physics, in the year 1956, altogether with William Shockley, for their researches on semiconductors and their innovation of the transistor effect. Shockley had invented a so-called junction transistor that was built on thin slices of various kinds of semiconductor material pressed altogether, was easier to understand theoretically and could be produced more consistently.

The transistor is a 3-terminal, solid state electronic device in which it is possible to control the electric current or voltage between two of the terminals by applying an electric current or voltage to the third terminal. Such three terminal features of the transistor are what make it possible to build an amplifier for the electrical signals, such as the one in radios and televisions.

By the three-terminal transistor it is possible to build an electric switch that can be controlled by the other electrical switch and by cascading these switches (that is, switches which control switches that control switches and so on) it is possible to build up extremely complicated logic circuits.

Nowadays, these logic circuits are extremely compact and a silicon chip can have 1,000,000 transistors per square centimeter. Such switches can alternate between the on and the off states every 0.000000001 seconds and are the heart of nowadays personal computer and most of the other electronic appliances seen on a daily basis.

Types of Transistors:

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Transistors can usually be categorized into two. The first categorization is the Bipolar Junction Transistors often termed to as BJT whereas the second is the Field Effect Transistors termed to as FET, Bipolar Junction Transistors fall into two kinds that have slight differences in how they are utilized in a circuit. These are NPN PNP transistors. We are familiar that a bipolar transistor has terminals labeled base, collector and emitter, and that a small current at the base terminal (which is, flowing from the base to the emitter) can control or switch a much bigger current among the collector and emitter terminals.

Now for a field-effect transistor, the terminals are labeled gate, source and drain and a voltage at the gate can manage a current between sources and the drain.

Bipolar junction transistor:

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Bipolar transistors are so named as they conduct via utilizing both the majority and minority carriers. The bipolar junction transistor (BJT), the first kind of transistor to be mass-produced, is a grouping of two junction diodes and is made of either a thin layer of p-type semiconductor sandwiched between two n-type semiconductors (that is, an n-p-n transistor), or a thin layer of n-type semiconductor sandwiched between two p-type semiconductors (that is, a p-n-p transistor). This construction generates two p-n junctions: a base-emitter junction and a base-collector junction, separated through a thin area of semiconductor termed as the base region (that is, two junction diodes wired altogether devoid of sharing an intervening semiconducting area will not make a transistor).

The BJT consists of three terminals, corresponding to the three layers of semiconductor - an emitter, a base and a collector. It is helpful in amplifiers because the currents at the emitter and collector are controllable through a relatively small base current.  In an NPN transistor operating in the active area, the emitter-base junction is forward biased (that is, electrons and holes recombine at the junction), and electrons are injected to the base area or region. As the base is narrow, most of such electrons will diffuse into the reverse-biased (that is, electrons and holes are formed at, and move away from the junction) base-collector junction and be removed into the collector; maybe one-hundredth of the electrons will recombine in the base that is the dominant method in the base current. By controlling the number of electrons which can leave the base, the number of electrons entering the collector can be managed. Collector current is just about β (that is, common-emitter current gain) times the base current. This is usually more than 100 for small-signal transistors however can be smaller in transistors designed for the high-power applications.

Dissimilar to FET, the BJT is a low-input-impedance device. As well, the base-emitter voltage (Vbe) raised the base-emitter current and therefore the collector-emitter current (Ice) rises exponentially according to the Shockley diode model and the Ebers-Moll model. Due to this exponential relationship, the BJT consists of a higher transconductance than the FET.

Bipolar transistors can be building to conduct by exposure to light, as absorption of photons in the base region produces a photocurrent which acts as a base current; the collector current is around β times the photocurrent. Devices designed for this aim encompass a transparent window in the package and are termed as phototransistors.

Field-effect transistor:

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The field-effect transistor (FET), at times termed as a unipolar transistor, employs either electrons (that is, in N-channel FET) or holes (that is, in P-channel FET) for conduction. The four terminals of the FET are named source, gate, drain and body (substrate). In most of the FETs, the body is joined to the source within the package, and this will be supposed for the given explanation.

In FETs, the drain-to-source current flows through a conducting channel which joins the source region to the drain region. The conductivity is varied through the electric field which is generated when a voltage is applied between the gate and source terminals; therefore the current flowing between the drain and source is regulated by the voltage applied between the gate and source. As the gate-source voltage (Vgs) is increased, the drain-source current (Ids) rises exponentially for Vgs beneath threshold, and then at a roughly quadratic rate [Ids ∝ (Vgs - VT)2] (in this, VT is the threshold voltage at which the drain current starts) in the space-charge-limited region above threshold. A quadratic behavior is not viewed in modern devices, for illustration, at 65 nm technology node. 

For low noise at narrow bandwidth the higher input resistance of the FET is beneficial.

FETs are categorized into two families: Junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more generally termed as a metal-oxide-semiconductor FET (MOSFET), reflecting its original construction from layers of metal (that is, the gate), oxide (that is, the insulation) and semiconductor.

Dissimilar to IGFETs, the JFET gate makes a PN diode having the channel that lies between the source and drain. Functionally, this forms the N-channel JFET the solid state equivalent of the vacuum tube triode that, likewise, makes a diode between its grid and cathode. As well, devices operate in the depletion mode, they both encompass high input impedance and they both conduct current beneath the control of an input voltage.

Metal-semiconductor FETs (MESFETs) are the JFETs in which the reverse biased PN junction is substituted through a metal-semiconductor Schottky-junction. These and the HEMTs (that is, high electron mobility transistors, or HFETs), in which the two-dimensional electron gas having extremely high carrier mobility is employed for charge transport, are particularly appropriate for use at extremely high frequencies (that is, microwave frequencies; several GHz).

Dissimilar to bipolar transistors, FETs don't inherently amplify a photocurrent. Nonetheless, there are ways to utilize them, particularly JFETs, as light-sensitive devices, through exploiting the photocurrents in channel - gate or channel - body junctions.

FETs are further categorized into depletion-mode and enhancement-mode kinds, based on whether the channel is turned on or off by zero gate-to-source voltage. For enhancement mode, the channel is off at zero bias, and a gate potential can 'enhance' the conduction. For depletion mode, the channel is on at zero bias and a gate potential (that is, of the opposite polarity) can 'deplete' the channel, decreasing the conduction. For either mode, a more positive gate voltage corresponds to the higher current for N-channel devices and a lower current for P-channel devices. Almost all JFETs are depletion-mode as the diode junctions would forward bias and conduct when they were enhancement mode devices; most of the IGFETs are enhancement-mode kinds.

Transistor Operation:

Transistor is basically a semiconductor device employed to amplify and switch electronic signals. This is made up of a solid piece of semiconductor material, by at least three terminals for connection to the external circuit. A current or voltage applied to one pair of the transistor's terminals changes the current flowing via the other pair of terminals. As the controlled (output) power can be much more than the controlling (input) power, the transistor gives amplification of a signal. Nowadays, some transistors are packaged separately, however numerous are found embedded in the integrated circuits.

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The operation of a transistor is hard to describe and understand in terms of its internal structure. This is more helpful to make use of this functional mode l that presupposes that the base-emitter junction behaves similar to a diode. 

A base current IB flows only if the voltage VBE across the base-emitter junction is 0.7V or more for Silicon transistors.

The small base current IB controls the big collector current IC

IC = hFE × IB (unless the transistor is completely saturated) 

hFE is the direct current gain and is usually 100

The collector-emitter resistance RCE is controlled through the base current IB:

IB = 0   RCE = infinity; transistor off 

IB small   RCE reduced; transistor partly on

IB increased   RCE = 0; transistor completely on (saturated)

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This simple circuit represented above is the Emitter, Base and Collector that are the labels of a bipolar transistor. The value of a transistor is as a result of its capability to employ a small signal applied between one pair of its terminals to control a much bigger signal at the other pair of terminals. This is termed as gain and it can be a current or voltage gain. A transistor can control its output in proportion to the input signal that means it acts as the amplifier. 

Otherwise, you can make use of the transistor to turn current on or off in a circuit whereby it acts as an electrically controlled switch.

Transistor as an amplifier:

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We will make use of this transistor amplifier circuit in the common-emitter configuration to explain the transistor as an amplifier.

The common-emitter amplifier is enviable where a small change in the input voltage (Vin) changes the small current through the base of the transistor and the transistor's current amplification combined with the properties of the circuit mean that small swings in Vin produce large changes in Vout.

Different configurations of single transistor amplifier are possible, by some providing current gain, some voltage gain and some both and from mobile phones to televisions; vast numbers of products comprise amplifiers for sound reproduction, radio transmission and signal processing.

Advantages and Limitations of Transistors:

Advantages of Transistors:

The key benefits that have allowed transistors to substitute their vacuum tube predecessors in most of the applications:

1) Small size and minimal weight permitting the growth of miniaturized electronic devices.

2) Highly automated manufacturing methods, resultant in the low per-unit cost.

3) Lower possible operating voltages, forming transistors appropriate for small, battery-powered applications.

4) No warm-up period for cathode heaters needed after power application.

5) Lower power dissipation and usually higher energy efficiency.

6) High consistency and greater physical ruggedness.

7) Very long life. A few transistorized devices have been in service for more than 50 years.

8) Complementary devices accessible, facilitating the design of complementary-symmetry circuits, somewhat not possible by vacuum tubes.

9) Tactlessness to mechanical shock and vibration, therefore avoiding the dilemma of micro phonics in the audio applications.

Limitations of Transistors:

1) Silicon transistors don't operate at voltages higher than around 1,000 volts (that is, SiC devices can be operated as high as 3,000 volts). In contrary, electron tubes have been build up which can be operated at tens of thousands of volts.

2) High power, high frequency operation, like which employed in over-the-air television broadcasting, is better accomplished in electron tubes due to enhanced electron mobility in the vacuum.

3) Silicon transistors are much more susceptible than electron tubes to an electromagnetic pulse produced through a high-altitude nuclear explosion.

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