BASIC KNOWLEDGE - TRANSISTOR Transistors: the building blocks of modern electronics

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Transistors are one of the most important semiconductor devices that serve a variety of applications. There are many types of transistors like BJT, MOSFET, Phototransistor, Darlington Transistor, and Power Transistors. Here is a guide to understanding the construction and working of these transistors in detail.

What is a transistor?

Vacuum tubes dominated the electronics industry up to 1947. But just before the new year, two men Walter H. Brattain and John Bardeen demonstrated the amplification capability of a solid-state device at Bell Laboratories. Since that day, the device “transistor” became a building block of important components and the world of electronics has never looked back.

What does a transistor do?

A transistor has three terminals (sometimes four) - one input, one output, and a common terminal. Usually, one terminal of a transistor is a “control” that controls another terminal. Hence, most transistors are controllable devices. The transistor’s significance is evident from its inclusion in the IEEE list of historical achievements in the field of electronics. The basic function of a transistor is switching and amplification. Depending upon the range of operation, a transistor can be used as a switch that turns on and off. Such a solid-state switch uses transistors in cut-off and saturation regions. Another application of transistors is amplification. A transistor amplifier enhances the strength of a small and weak signal. The enhanced signal with improved voltage/current/power appears at the output of the transistor amplifier device.

What are transistors used for?

Transistors are used widely in consumer electronics like smartphones, computers, game consoles, computers, modems, and even wearable technology. We can say that transistors are everywhere⁠ — and in huge numbers. For example, an integrated circuit contains billions of transistors in its compact size. While the radiation-hardened transistors are embedded in satellite and aerospace applications.

Advantages of a transistor

• At just a few nanometers in size, they occupy very little space (especially when embedded in an integrated circuit).

• Moore’s law nearly doubles the number of transistors in an IC.

• Only a very low supply voltage is required to start the operation.

• High switching frequency.

• The minimal risk of overheating makes it suitable for power electronics applications.

• Transistors are mechanically strong due to being a solid-state circuit.

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Transistor Types

There are many different types of transistors (e. g. BJT, MOSFET, JFET - see more in figure 1) present in the market to serve a variety of applications. The main categories are junction transistors and field effect transistors. Junction transistors operate on the basis of the junctions formed while field effect transistors perform due to an electric field. Mostly every transistor falls into the category of either the junction or field-effect transistor.

Bipolar Junction Transistor

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device used for switching purposes. Most of the time, BJT is simply called a transistor because of its basic operation. Bipolar Junction Transistor is a current-controlled device because the output collector current I_C is an effect controlled by the input current.

There are three terminals of the BJT transistor:

• Emitter (E)

• Base (B)

• Collector (C)

Just as the name suggests, the Emitter region is responsible for “emitting” the charge carriers. Emitter is the source of injecting the majority of charge carriers into other regions. Similarly, the Collector region “collects” those charge carriers and proceeds further.

There are two configurations of BJT transistors: NPN and PNP

• NPN Transistor: A P-type region is embedded between two N-type regions.

• PNP Transistor: An N-type region is embedded between two P-type regions.

The arrow of the NPN transistor points out the direction of current flow from the P to N region (base to emitter). Similarly, the arrow in the PNP transistor denotes the direction of the current flow from N to P (emitter to base).

NPN is the most preferred configuration of BJT because it enables the electrons to flow the diffusion current. The mobility of electrons is higher than holes, making the transistor capable of fast switching operations.

BJT transistor construction

The emitter region is heavily doped, and the base and the collector region are lightly doped. The N⁺ emitter region denotes heavy doping while the N^_ collector indicates a lightly-doped region. The doping concentration of the emitter is about 10¹⁸/cm³. The area of the collector region is the biggest compared to the emitter and the base of the BJT transistor.

Two PN junctions form in the transistor, indicating its second name “Bijunction Transistor”. One junction forms between the emitter and base J_EB and another junction between the base and collector is J_CB.

Emitter and base always have opposite doping profiles. If the emitter is of N-type, the base must be of P-type and vice versa. It is done to keep an attractive force between the two regions to allow current flow. The width of the base region is kept thinner because electrons from the emitter may start recombination with holes. The electron-hole recombination must be reduced for the charges to move inside the semiconductor device. The electrons from the emitter must reach the collector, hence the base region is made thinner.

Again, the collector region has an opposite doping profile to the base for current flow. If the base is of P-type, the collector must be of N-type and vice versa. The collector is the largest in size compared to the emitter and base regions but it is lightly doped.

When BJT Transistor acts as an amplifier, it has three topologies:

• Common Emitter

• Common Base

• Common Collector (Emitter Follower)

NPN Common Emitter Amplifier is the most used BJT amplifier configuration because it provides high gain.

Field Effect Transistor

A FET (Field Effect Transistor) is one of the widely used transistors in industries. These are named “Field Effect” because the electric field is responsible for the transistor to operation. While in junction transistors, junctions are responsible for the operation. There are many FETs but MOSFET is one of the most used transistors in the world.

MOSFET

A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a four-terminal semiconductor device used in integrated circuits. The small area of the MOSFET makes it suitable for VLSI (very large scale integration), and ULSI (ultra large scale integration).

MOSFET is a type of field effect transistor with four terminals - gate, drain, source, and body. The gate terminal is isolated from the device through an insulating silicon oxide layer. The addition of insulation silicon oxide makes the gate current I_g zero.

According to ohm’s law, the following equation applies: Voltage = Current x Resistance (V = i x R).
Resistance is Impedance (R = I)
Voltage = Current x Impedance
V = i x I
I = v/i
The input impedance of MOSFET is very high (infinite).
I = ∞

MOSFET is a voltage-controlled device because its output voltage is a function of the input voltage V_GS. MOSFET enables the gate terminal to control the current between the drain and the source. The channel width can be controlled by controlling the gate voltage of the device.

When MOSFET Transistor acts as an amplifier, it has three topologies:

• Common Source

• Common Gate

• Common Drain (Source Follower)

N-channel Enhancement Mode Common Source Amplifier is the most used MOSFET amplifier configuration analogous to NPN CE BJT.

BASIC KNOWLEDGE - MOSFET VS. BJT

What’s the difference between MOSFET and BJT?

Special Transistors

Phototransistor

A phototransistor is a special device that converts light energy into a photocurrent with high gain. The current produced by the action of incident photons is termed “photocurrent”. A phototransistor can either be a bipolar junction transistor or a field effect transistor. It is a combination of a photodiode and transistor to provide a high gain and amplify the signal.

The symbol indicates that the base region in the phototransistor is unbiased. Instead of biasing the base of the phototransistor, photons are projected to enable the current flow.

The area of the base region in the phototransistor is slightly increased to allow the projection of a large number of photons. It is because more photons would produce more current. The base is unbiased while the collector is connected to the positive terminal of the external supply. The base terminal is either made light-sensitive or a lens is placed at the base-collector junction. When there is no external bias on the base terminal, a small leakage current termed dark current flows through the transistor due to minority charge carriers. The emitter-base junction is forward biased and the collector-base junction is reverse-biased. As soon as the light is projected on the base terminal, photocurrent flows through the transistor.

Darlington Transistor

A Darlington Transistor is a pair of BJTs. The compound device performs double amplification to provide a very large current gain. The transistor Q1 amplifies the input current and the transistor Q2 amplifies it again.

A Darlington transistor pair behaves as a single transistor that provides a current gain of 10,000+. It can either be an NPN BJT pair or PNP pair. Only a small amount of base current drives the transistor to handle high-switching applications.

How does a transistor work?

BJT Working

When there is no external supply to the BJT transistor or at Thermal Equilibrium, the electrons in the N-type doped emitter have sufficient thermal energy to enter the P-type doped base region. As soon as electrons enter the lightly doped base region, electron-hole recombination starts. Diffusion current flows in a non-uniform concentration of semiconductor due to movement of charge carriers from a higher doping concentration to a lower doping concentration region. When electrons travel from N to P, they neutralize holes. Similarly, holes travel from P to N and neutralize electrons. When the acceptor impurity accepts an electron, the hole vanishes and it forms a negative ion. The donor impurity donates an electron and becomes a positive ion.

The neutralization of charges leads to the formation of negative ions on the P-side base and positive ions on the N-side collector. The region that contains ions but is devoid of any charge carriers (electrons or holes) is known as the depletion layer. It forms around the J_CB junction, extending more into the collector region because of its larger size and concentration of charge carriers. The depletion layer is an insulating region that does not allow any current to flow through the transistor. There is another depletion region formed around J_EB junction. However, it has negligible width because the area of the base is smaller with light doping.

In the depletion region, an electric field is generated from positive ions to the negative ions, i.e. from N-side to P-side. According to the figure, the electric field opposes the diffusion tendency of holes and electrons by applying force in the opposite direction to their movement. The lowest voltage at which the opposing electric field exists in a transistor is known as barrier potential. When a transistor is given sufficient input voltage, it exceeds the barrier potential and allows the current to flow. The voltage at which the depletion region vanishes and the current starts flowing in a transistor is known as cut-in voltage.

Cut-in voltage is sometimes referred barrier potential or knee voltage, denoted by V_<Gamma>
Cut-in voltage of Silicon V = 0.7 V
Cut-in voltage of Germanium V = 0.3 V

Working of NPN transistor explained

The emitter-base junction J_EB is forward biased and the collector-base junction J_CB is reverse-biased. The resistances RC and RE are there to limit the high current flow inside the transistor.

At forward bias, the applied voltage enables the charge carriers to overcome the opposing electric field of the depletion region. When the applied input voltage V_BE exceeds the barrier potential V_Gamma, the depletion region vanishes, and the charge carriers can easily cross the junctions. There is no opposition to the diffusion of electrons from N to P and holes from P to N. The majority charge carrier electrons from the emitter enter the lightly doped base region. The emitter current IE flows opposite the direction of the flow of electrons. In the base region, only 2-5 % of all the electrons combine with the holes, and the rest 95-98% move toward the collector. The majority charge carrier holes generate a small base current I_B into the transistor. The reverse biasing potential is kept high to support the flow of electrons into the collector. Due to the attractive force of the positive terminal of the battery, the electrons get injected into the collector region. This process of transfer of electrons from the base to the collector is termed injection of electrons. A small reverse leakage current flows from collector to base due to thermally generated minority charge carriers. The small reverse leakage current adds to the collector current I_C in the opposite direction to the movement of electrons.

Characteristics of an NPN Transistor is the graph of collector current I_C at the Y-axis against the collector-to-emitter voltage V_CE at the X-axis for different values of the base current I_B.

Below the barrier potential, the transistor is in the cut-off region. Beyond the barrier potential, the value of the collector current I_C increases for increasing V_CE. But collector current I_C attains a constant value for increasing values of V_CE.

Working of PNP transistor explained

The emitter-base junction J_EB is forward biased and the collector-base junction J_CB is reverse-biased. The resistances R_C, and R_E are there to prevent high current flow inside the transistor.

At forward bias, the applied voltage enables the charge carriers to overcome the opposing electric field of the depletion region. When the applied input voltage V_BE exceeds the barrier potential V_Gamma, the depletion region vanishes, and the charge carriers can easily cross the junctions. There is no opposition to the diffusion of holes from P to N. The majority charge carrier holes from the emitter enter the lightly doped base region. The emitter current I_E flows in the same direction as the flow of holes. In the base region, only 2-5 % of all the holes combine with the electrons, and the rest 95-98% move toward the collector. Electrons are the majority charge carrier in the base region. A very small base current I_B flows into the transistor. Due to the attractive force of the negative terminal of the battery, the holes get injected into the collector region. A small reverse leakage current flows from the base to the collector due to thermally generated minority charge carrier electrons moving from the collector to the base in the opposite direction. The small reverse leakage current adds to the collector current I_C in the same direction to the flow of holes. The leakage current due to minority charge carriers is heavily dependent upon the temperature.

Characteristics of a PNP Transistor explain its working in the cut-off region, active region, and saturation region. It is a graph of collector current I_C at the Y-axis vs base-to-collector voltage V_BC at X-axis for different values of emitter current I_E.

How does a MOSFET work?

Working of N-channel enhancement-mode MOSFET

Silicon Wafer is doped with a trivalent impurity to form a P-type substrate. The p-type substrate is called the body region of MOSFET. In the body region, two N-type wells are created with uniform doping. One of the N-well acts as a source and the other drain. Both source and drain are symmetrical, and interchangeable because of uniform doping. Two depletion regions of the same width are formed around the N-type wells due to the junction between the source and drain with the P-type body region. The metal contacts are added for the gate, source, drain, and body regions. A thin layer of silicon dioxide insulates the gate terminal of MOSFET below its metal contact. The gate-to-source voltage is represented by V_GS because the source and body are at the same potential and internally connected. The insulating silicon dioxide layer acts as the dielectric medium between the gate metal contact and the body region. The arrangement behaves like a parallel gate capacitance formed by the positive gate metal contact and the body region.

MOSFET modes

Case 1: v_GS=0

No channel formation takes place between the drain and the source. Hence, no current flows through MOSFET.

Case 2: V_GS<0

Accumulation mode

Electrons are present over the gate terminal and holes accumulate under the gate region. An electric field establishes in the direction of the gate.

Case 3: V_GS> 0, V_GS < V_Threshold

Depletion mode

V_Threshold is the minimum voltage at which a channel starts to form inside the MOSFET. The positive charges will build up over the gate plate and negative charges will accumulate below the plate. The positively charged holes are pushed below due to the repulsive force. An electric field establishes in the direction of the body. Since V_GS < V_Threshold, no channel formation takes place, and no current flows through the transistor. But the process of depletion starts below the gate region at this voltage.

Case 4: V_GS > 0, V_GS > V_Threshold

Inversion mode

Due to the accumulation of excessive electrons near the gate region, channel formation starts to take place. The concentration of electrons is much more than the majority charge carrier holes. The region near the gate plate becomes less “P-type” and exhibits the character of “N-type”. The process when V_GS increases beyond V_Threshold to turn the P-region into an N-type is called inversion. The minimum value of V_GS at which inversion takes place is termed V_Threshold. A conductive N-type channel forms between the source and the drain as shown in the figure below. The electrons move from source to drain and current I_D flows from drain to source.

Transfer Characteristics of a enhancement mode MOSFET is a graph of I_D on the Y-axis vs V_GS on the X-axis.

The graph explains that when applied voltage V_GS crosses V_Threshold, enhancement mode MOSFET turns on.

Working of N-channel depletion-mode MOSFET

An n-channel is constructed between the source and drain through doping. It saves ON-time of the transistor, compared to enhancement configuration.

The Depletion-type MOSFET symbol signifies the presence of a conducting channel.

The channel width varies depending on the potential difference between the gate and the drain.

Case 1:V_DS= 0, V_GS= 0

Channel is already present between the drain and the source. But there is no electric field between the drain and the source and I_D does not flow.

Case 2:V_DS> 0, V_GS= 0

The electric field establishes from the drain to the source. The electrons flow from source to drain and current flows from drain to source.

Case 3: V_DS> 0, V_GS< 0

Since negative voltage is near the drain, the electric field is also higher near the drain. The electrons will move downwards/away from the drain. The width of the channel will reduce but the width of the depletion layer will increase near the drain region. The resistance will increase, and current and conductance will decrease. Less current I_D flows from drain to source.

Transfer Characteristics of a depletion mode MOSFET is a graph of I_D on the Y-axis vs V_GS on the X-axis.

Depletion-type MOSFET operates when gate-to-source voltage V_GS is negative and positive both.

Drain characteristics of a MOSFET Transistor

Drain characteristics of a MOSFET is a graph of I_D on the X-axis vs V_DS on the Y-axis. The two modes- enhancement and depletion are depicted in the graph for values of V_GS from negative to positive.

Power Transistors

Power transistors are semiconductor devices that can withstand high voltage and current without damage. An additional N^_ drift layer is added in power transistors to maintain a high-power rating. Some of the popular power transistors are Power MOSFET, IGBT, Power BJT, etc.

Power BJT

A Power BJT provides higher current handling capacity than low-power BJT in ON-state. The vertically oriented structure puts an additional N- layer between the base and the collector.

The doping of the emitter region is increased up to 10¹⁹/cm³. The collector region is also heavily doped with the same concentration while the base is lightly doped. The device is only operated in the cut-off and saturation region for switching purposes in high-power applications.

A power transistor operates in two types of saturation modes- hard saturation and quasi saturation. Hard saturation is an operating mode where the transistor operates in the saturation region for all values of operating voltages, currents, resistances, and other conditions. It offers low ON-state resistance and less power loss. Or we can say that inductance abruptly drops when the value of the current becomes equal to the saturation current. Quasi saturation region is a mode of operation where the power transistor can be quickly turned on and off i.e. small values of rise and fall times. The secondary saturation region exists because of the additional N^_ drift region.

We can observe from the characteristic curve that the base current remains constant in the active region for increasing collector-to-emitter voltage.

Power MOSFET

A Power MOSFET is constructed in a vertical channel structure where source and drain terminals are on the opposite side of the Silicon. The vertical fabrication allows the Power MOSFET to have a high-current rating and withstand large power. Just like its low-power version, the Power MOSFET is a voltage-controlled high-power active semiconductor device. The switching frequency of a Power MOSFET is more than 100 kHz. A Power MOSFET has three terminals- gate, drain, and source.

The source and drain are doped heavily with N-type impurities. The body region has p-type light doping. An additional N^_ drift layer is added between the p-type body and the N⁺ drain layer. A parasitic BJT forms inside the structure due to N^_PN⁺ layers. The PN junction forms a body diode inside the Power MOSFET structure.

A positive gate voltage V_GS is applied to enable channel formation in a Power MOSFET. The presence of the Silicon Dioxide insulating layer accumulates positive charges at the metal plate and negative charges under the gate. Electron concentration is more than hole concentration under the gate and near the body region.

A channel formation takes place to enable the current flow. The channel extends from the N⁺ near drain to the N^_ drift layer and toward the body region and N⁺ source. Upon applying V_DS, the electrons move from source to drain, and current flows from drain to source in the conducting channel. The small body diode current flows from source to drain. Both currents make Power MOSFET a bidirectional device.

Characteristics of a Power MOSFET is a graph of drain current I_D on the Y-axis against drain-to-source voltage V_DS at the X-axis for different values of gate-to-emitter voltage V_GS.

The characteristic graph explains that drain current I_D obtains a constant value for further increase in drain-to-source voltage V_DS.

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