Transistor

A transistor is a semiconductor device crucial for amplifying or switching electronic signals and electrical power. Constructed with three layers of semiconductor material, it allows the control of current or voltage flow between two layers by applying electric current or voltage to the third layer. Transistors are essential components in contemporary electronic devices, facilitating the operation of radios, computers, and intricate integrated circuits. Available in diverse types such as bipolar junction transistors (BJTs), field-effect transistors (FETs), and MOSFET  transistors, each variation is tailored for specific applications according to its distinct characteristics and operational mechanisms.

1.0Definition of Transistor

A junction transistor is a three-terminal semiconductor device with emitter, base, and collector terminals. It is structured with a thin layer of one type of doped semiconductor sandwiched between two thicker layers of a differently doped semiconductor.

2.0Transistor Applications

Transistors find extensive applications across diverse fields, owing to their capabilities in amplifying signals and switching currents. Some key uses include:

• Amplification
• Power Regulation
• Sensor Applications
• Lighting

3.0Types of Transistor: Two types of Junction Transistor are as given below

NPN Transistor

NPN transistor, categorized as a bipolar junction transistor (BJT), comprises three semiconductor layers. These layers consist of a thin P-type semiconductor layer (with positive majority charge carriers) sandwiched between two thicker N-type semiconductor layers (with negative majority charge carriers). In an NPN transistor, electrons primarily carry current in the N-type regions, while holes, the majority carriers in the P-type region, also play a role. The transistor functions by regulating the movement  of current between its emitter (electron source), base (current regulator), and collector (electron receiver).

PNP Transistor

A PNP transistor, a form of bipolar junction transistor (BJT), comprises three layers of semiconductor material. It features a thin layer of N-type semiconductor (where electrons are the dominant charge carriers) sandwiched between two layers of P-type semiconductor (where holes are the dominant charge carriers). 

4.0Symbols on NPN and PNP Transistor:

NPN Transistor

PNP Transistor

5.0Terminals of Transistor

• Emitter—The side of a transistor that is moderately large but heavily doped is called the emitter. It supplies the majority of charge carriers to the base region.
• Base- The middle region of a very thin and lightly doped transistor is called base.
• Collector—The other side of the transistor, which is doped slightly less than the emitter but has a thickness slightly more than that of the emitter, is called the collector. The collector gathers the majority of the charge carriers sent by the emitter.

Note: 

• The junction between the emitter and base is called the emitter-base junction, while the junction between the collector and the base is called the collector-base junction.
• The emitter-base junction is always forward-biased, whereas the collector-base junction is reverse-biased.

6.0Working of n-p-n and p-n-p Transistors

Working of n-p-n Transistor

The emitter-base junction is forward biased whereas the  collector-base junction is reverse biased. When an emitter-base junction is forward biased electrons(majority carriers) in the emitter are repelled by negative pole of the cell move towards base. The barrier potential of emitter-base junction decreases and the electrons enters the base. About 5% of these electrons combines with the holes in the base region resulting in small base current(I_B).The remaining 95% electrons enter the collector region because they are attracted towards the positive terminal of the battery. For each electron entering the positive terminal of the battery V_CB ,an electron from the negative terminal of the cell V_EB enters the emitter region. Thus continuous flow of electrons from emitter to collector through the base begins. The emitter current (I_E) is more than the collector current (I_C). The base current is the difference between I_E and I_C and is proportional to the number of electron-hole combinations in the base.

• IE=IB+IC and IC≃IE

Working of p-n-p Transistors

The emitter-base junction of the p-n-p transistor is forward biased whereas the collector-base junction is reverse biased. When an emitter-base junction is forward biased, holes (majority carriers) in the emitter (p-region) are repelled by the positive terminal of the cell V_EB and move towards the base and diffuse through the emitter-base junction. The emitter-base junction's barrier potential decreases, and holes enter the n-region (base). A small number of holes( 5%) combine with the electrons of the n-region, resulting in a small base current (I_B). The remaining holes (95%) enter the collector region because they are attracted towards the battery's negative terminal. These holes constitute the collector current (I_C ). The collector current(I_C) is slightly less than the emitter current(I_E). As one hole reaches the collector, it is neutralized by an electron from the battery's negative terminal. As soon as one electron and a hole get neutralized in the collector, a hole in the emitter is pushed towards the collector by the positive terminal of the cell V_EB. Thus, a continuous flow of holes from the emitter to the collector through the base begins.

• IE=IB+IC and IC≃IE

7.0Biasing of Transistor:

Transistor is a three-terminal device, so it can be connected in a circuit in three types of configurations

1. Common Base (CB) Configuration:

In the common base configuration of a transistor, the base terminal is grounded or connected to a low potential. The emitter is directly connected to the input signal source, and the collector is linked to the supply voltage through a load resistor(R_C). This configuration is designed to provide current gain and exhibits excellent high-frequency response. The input signal is applied to the emitter-base junction, which modulates the current flowing from the emitter to the collector. The output signal is then taken from the collector-base junction. The standard base configuration is frequently utilized in high-frequency amplifiers.

2. Common Emitter (CE) Configuration:

In the common emitter configuration of a transistor, the emitter is connected to the ground, the base is biased through a base resistor (R_B), and the collector is connected to the supply voltage through a load resistor (R_L). This configuration is designed to provide voltage gain and current amplification.  The input signal  fed to the emitter-base junction,, which controls the current flow from the emitter to the collector. The amplified output signal is then obtained from the collector-emitter junction. The common emitter configuration is widely used in amplifiers

3. Common Collector( CC) Configuration

In the common collector configuration of a transistor, also known as the emitter follower configuration, the collector is connected to ground (or a low potential), the emitter is connected to the supply voltage through a load resistor (R_C.), and the base is connected to the input signal source. This configuration is primarily designed to provide high current gain and low output impedance.The input signal is directed towards the base of the transistor, where it exerts control over the emitter current. The output signal is then taken from the emitter terminal, which follows the base signal but at a lower impedance. It acts as a buffer between high-impedance stages and low-impedance loads, maintaining signal integrity and providing impedance matching. The common collector configuration, or emitter follower, is commonly used in applications requiring high current amplification and low output impedance.

8.0Transistor as an Amplifier 

An amplifier is a device designed to boost the amplitude of the input signal.

• The input signal is applied across base-emitter circuit(input circuit).The input circuit is forward biased using a battery of e.m.f = V_BB Volts, the amplified output signal is taken across the load resistance in the collector-emitter circuit(output circuit).The output circuit is reverse biased using a battery of e.m.f = V_CE Volt.
• According to Kirchhof’s law, I_E = I_B + I_C…………..(1)
• When a current (I_C) flows through load resistance (R_L) ,the output voltage drop across R_L

                                             V_O = V_CE - I_C R_L ......................(2)

During positive half cycle of input signal ,the forward bias of emitter-base junction increases. Due to increased forward bias, emitter current (I_E) increases according to equation (1),collector current (I_C) increases, Voltage drop across R_L increases, according to equation (2) output voltage(V_O) decreases. Since collector is connected to the positive terminal of the battery (V_CE) so decrease in V_O means that the collector voltage becomes less positive. So amplified negative signal is obtained across the output.

In a common-emitter amplifier, the input and output signals are out of phase; there is a phase difference of $\pi$ radian between them.

9.0Transistor I-V Characteristics

Input Characteristics-It is the variation of base current(I_B) with the base emitter(V_BE) voltage keeping collector emitter voltage(V_CE) to be fixed.

Output Characteristics-It is the variation of collector current (I_C) with collector emitter voltage (V_CE) keeping the base current(I_B) fixed.

Applying KVL for input circuit we get,V_BB = I_BR_B + V_BE (∴ V_BB = V_i)

                 V_i = I_BR_B + V_BE…………..(1)

 Applying KVL for  output circuit we get,

         V_CC = I_CR_C + V_CE……………..(2)          (∴ V_CE = V_o (output voltage)

         V_CC = I_CR_C + V_o

         V_o = V_CC - I_CR_C              (∴ I_C = 0)    

          V_o = V_CC

Transistor not working if input voltage is low.

• For V_i < 0.6 V, the transistor is in the cut off region, it will not conduct and I_C = 0, so from eqn.2 , we have V₀ = V_CC.
• For V_I > 0.6V and < 1V the transistor is in the active region I_C will increase linearly so V₀ will be decreasing due to increase in I_CR_C.
• For V_i > 1V the transistor has the maximum possible current I_CR_C will be maximum and V₀ will be least, it is the saturation state of the transistor
• When base input voltage V_BB = V_i  is very low, the transistor is not forward biased then I_C = 0, hence V_o = V_CC, transistor is in cut off mode.(OFF)
• When base input voltage  V_BB is made high ,transistor is forward biased current I_C flows through R_C and V_CE = 0 transistor comes in saturation state (ON)

Case 2- If  V_i > 0.7V Transistor turns ON

• If V_i is High V₀ is Low
• If V_i is Low V₀ is High
• If V_i is High Transistor switch ON
• If V_i is Low   Transistor switch OFF
• In the Active state, Collector Current is times the base current .
• If the Transistor is Cut-off, there is no base current, so there is no collector or emitter current.
• In Saturation the collector and emitter  are shorted together ,in this transistor behaves as a switch.

10.0Transistor Formula

1. Current gain- It is The ratio of a small change in collector current to the corresponding change in base current remains constant at a fixed collector-emitter junction voltage.

${\beta }_{a.c}=\frac{\Delta I_c}{\Delta I_B}\left(V_{CE}=\text{ Constant }\right)$

2. Voltage gain-It is the ratio of small change in output voltage to the small change in input voltage when transistors act in the middle of the active region.

$A_V=\frac{\Delta V_0}{\Delta V_I}$

$V_0=V_{CC}-I_C{R}_L\Rightarrow \Delta V_0=-I_C{R}_L$

$V_i=V_{BE}+I_B{R}_B$

$\left|A_V\right|=\frac{\Delta I_C}{\Delta I_B}\times \frac{R_L}{R_B}$

$\left|A_V\right|={\beta }_{a.c}\times \text{ Resistance gain }$

3. Resistance gain - it is defined as the ratio of output resistance to the input resistance.

Resistance gain = $\frac{R_0}{R_i}\simeq \frac{R_L}{R_B}$

4. Power gain-It is the ratio of small change in output power to the small change in the input power.

$\text{ Power gain }=\frac{\Delta P_0}{\Delta P_i}={\beta }_{a,c}^{2}x\text{ Resistance gain }$

11.0Solved Questions on Transistors

Q-1 For a common emitter transistor amplifier current gain is 72.Calculate the base current for which emitter current is 8.9 mA.

Sol.  $\beta$  = 72 , I_E = 8.9mA 

$\beta =\frac{I_C}{I_o}\Rightarrow I_C=\beta I_B=72I_B$

I_E = I_C + I_B

8.9 = 72 I_B + I_B

$I_B=\frac{8.9}{73}=0.12\mathrm{mA}$

Q-2.What is the change in the collector current in a transistor with an AC current gain of 150, when there is a 100 microampere change in the base current?

Sol.   $\beta =\frac{\Delta I_C}{\Delta I_B}\Rightarrow \Delta I_C=\beta \Delta I_B=150\times 100\times 10^{-6}=15\times 10^{-3}=15\mathrm{mA}$