Bipolar Junction Transistors Md. Rabiul Islam Dept. of Biomedical Engineering 8/3/2019 1

Lecture Overview  What is a Transistor?  History  Types  Characteristics  Applications 8/3/2019 2

What is a Transistor?  Semiconductors: ability to change from conductor to insulator  Can either allow current or prohibit current to flow  Useful as a switch, but also as an amplifier  Essential part of many technological advances 8/3/2019 3

What is a Transistor?  A transistor consists of two pn junctions formed by sandwiching either p-type or n-type semiconductor between a pair of opposite types.  Accordingly; there are two types of transistors, namely; I. n-p-n transistor II. p-n-p transistor  An n-p-n transistor is composed of two n- type semiconductors separated by a thin section of p-type.  a p-n-p transistor is formed by two p-sections separated by a thin section of n-type. 8/3/2019 4

A Brief History  Guglielmo Marconi invents radio in 1895  Problem: For long distance travel, signal must be amplified  Lee De Forest improves on Fleming’s original vacuum tube to amplify signals  Made use of third electrode  Too bulky for most applications 8/3/2019 5

The Transistor is Born  Bell Labs (1947): Bardeen, Brattain, and Shockley  Originally made of germanium  Current transistors made of doped silicon 8/3/2019 6

How Transistors Work  Doping: adding small amounts of other elements to create additional protons or electrons  P-Type: dopants lack a fourth valence electron (Boron, Aluminum)  N-Type: dopants have an additional (5 th ) valence electron (Phosphorus, Arsenic)  Importance: Current only flows from P to N 8/3/2019 7

Diodes and Bias  Diode: simple P-N junction.  Forward Bias: allows current to flow from P to N.  Reverse Bias: no current allowed to flow from N to P.  Breakdown Voltage: sufficient N to P voltage of a Zener Diode will allow for current to flow in this direction. 8/3/2019 8

 3 adjacent regions of doped Si ( each connected to a lead ):  Base. (thin layer,less doped).  Collector.  Emitter.  2 types of BJT:  npn.  pnp.  Most common: npn (focus on it). Bipolar Junction Transistor (BJT) npn bipolar junction transistor pnp bipolar junction transistor Developed by Shockley (1949) 8/3/2019 9

Naming the Transistor Terminals  A transistor (pnp or npn) has three sections of doped semiconductors.  The section on one side is the emitter and the section on the opposite side is the collector.  The middle section is called the base and forms two junctions between the emitter and collector. 8/3/

Naming the Transistor Terminals Emitter: The section on one side that supplies charge carriers (electrons or holes) is called the emitter. The emitter is always forward biased.r.t. base so that it can supply a large number of majority carriers. 8/3/

Naming the Transistor Terminals Collector: The section on the other side that collects the charges is called the collector. The collector is always reverse biased. Its function is to remove charges from its junction with the base. 8/3/

Naming the Transistor Terminals Base: The middle section which forms two pn-junctions between the emitter and collector is called the base. The base-emitter junction is forward biased, allowing low resistance for the emitter circuit. The base-collector junction is reverse biased and provides high resistance in the collector circuit. 8/3/

Some Facts about the Transistor Before discussing transistor action, it is important that the reader may keep in mind the following facts about the transistor :  The transistor has three regions, namely ; emitter, base and collector. The base is much thinner than the emitter while collector is wider than both. 8/3/

Some Facts about the Transistor  The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or holes) into the base.  The base is lightly doped and very thin ; it passes most of the emitter injected charge carriers to the collector.  The collector is moderately doped. 8/3/

Some Facts about the Transistor  The transistor has two pn junctions i.e. it is like two diodes. The junction between emitter and base may be called emitter-base diode or simply the emitter diode.  The junction between the base and collector may be called collector-base diode or simply collector diode. 8/3/

Some Facts about the Transistor  The emitter diode is always forward biased whereas collector diode is always reverse biased.  The resistance of emitter diode (forward biased) is very small as compared to collector diode (reverse biased). Therefore, forward bias applied to the emitter diode is generally very small whereas reverse bias on the collector diode is much higher. 8/3/

Transistor Circuit Configurations Basically, there are three types of circuit connections (called configurations) for operating a transistor. 1. common-base (CB), 2. common-emitter (CE), 3. common-collector (CC). The term ‘common’ is used to denote the electrode that is common to the input and output circuits. Because the common electrode is generally grounded, these modes of operation are frequently referred to as grounded- base, grounded-emitter and grounded-collector configurations. 8/3/

CB Configuration In this configuration, emitter current I E is the input current and collector current I C is the output current. The input signal is applied between the emitter and base whereas output is taken out from the collector and base. 8/3/

CB Configuration 8/3/

CB Configuration In common base configuration, emitter is the input terminal, collector is the output terminal and base terminal is connected as a common terminal for both input and output. That means the emitter terminal and common base terminal are known as input terminals whereas the collector terminal and common base terminal are known as output terminals. 8/3/

CB Configuration  The supply voltage between base and emitter is denoted by V BE while the supply voltage between collector and base is denoted by V CB.  In every configuration, the base-emitter junction J E is always forward biased and collector-base junction J C is always reverse biased. 8/3/

Current flow in common base amplifier  For the sake of understanding, let us consider NPN transistor in common base configuration.  The npn transistor is formed when a single p- type semiconductor layer is sandwiched between two n-type semiconductor layers. 8/3/

Current flow in common base amplifier 8/3/

Current flow in common base amplifier  The base-emitter junction J E is forward biased by the supply voltage V BE while the collector-base junction J C is reverse biased by the supply voltage V CB.  Due to the forward bias voltage V BE, the free electrons (majority carriers) in the emitter region experience a repulsive force from the negative terminal of the battery similarly holes(majority carriers) in the base region experience a repulsive force from the positive terminal of the battery. 8/3/

Current flow in common base amplifier  As a result, free electrons start flowing from emitter to base similarly holes start flowing from base to emitter. Thus free electrons which are flowing from emitter to base and holes which are flowing from base to emitter conducts electric current.  The actual current is carried by free electrons which are flowing from emitter to base. However, we follow the conventional current direction which is from base to emitter. Thus electric current is produced at the base and emitter region. 8/3/

Current flow in common base amplifier 8/3/

Current flow in common base amplifier  The free electrons which are flowing from emitter to base will combine with the holes in the base region similarly the holes which are flowing from base to emitter will combine with the electrons in the emitter region.  From the previous figure, it is seen that the width of the base region is very thin. Therefore, only a small percentage of free electrons from emitter region will combine with the holes in the base region and the remaining large number of free electrons cross the base region and enters into the collector region. 8/3/

Current flow in common base amplifier  A large number of free electrons which entered into the collector region will experience an attractive force from the positive terminal of the battery.  Therefore, the free electrons in the collector region will flow towards the positive terminal of the battery.  Thus, electric current is produced in the collector region. 8/3/

Current flow in common base amplifier  The electric current produced at the collector region is primarily due to the free electrons from the emitter region similarly the electric current produced at the base region is also primarily due to the free electrons from emitter region. Therefore, the emitter current is greater than the base current and collector current. The emitter current is the sum of base current and collector current. I E = I B + I C  We know that emitter current is the input current and collector current is the output current. 8/3/

Current gain  The ratio of the collector current to the emitter current is called dc alpha ( α dc) of a transistor.  If we write a dc simply as α, then α = I C /I E 8/3/

CE Configuration 8/3/

CE Configuration  In common emitter configuration, the emitter terminal is grounded so the common emitter configuration is also known as grounded emitter configuration.  Sometimes common emitter configuration is also referred to as CE configuration, common emitter amplifier, or CE amplifier. The common emitter (CE) configuration is the most widely used transistor configuration. 8/3/

CE Configuration 8/3/

CE Configuration  The common emitter (CE) amplifiers are used when large current gain is needed.  The input signal is applied between the base and emitter terminals while the output signal is taken between the collector and emitter terminals.  Thus, the emitter terminal of a transistor is common for both input and output and hence it is named as common emitter configuration. 8/3/

CE Configuration  The supply voltage between base and emitter is denoted by V BE while the supply voltage between collector and emitter is denoted by V CE.  In common emitter (CE) configuration, input current or base current is denoted by I B and output current or collector current is denoted by I C.  The common emitter amplifier has medium input and output impedance levels. So the current gain and voltage gain of the common emitter amplifier is medium. However, the power gain is high. 8/3/

Current gain in CE The ratio of the d.c. collector current to dc base current is called dc beta ( β dc) or just β of the transistor.  ∴ β = –IC /–IB = IC /IB 8/3/

Relation between β and α β = I c /I B and α =I c/ I E  ∴ β / α =I E/ I B, we know, I E =I B +I C  β = α /1- α 8/3/

CC Configuration 8/3/

CC Configuration  In this configuration, the base terminal of the transistor serves as the input, the emitter terminal is the output and the collector terminal is common for both input and output. Hence, it is named as common collector configuration. The input is applied between the base and collector while the output is taken from the emitter and collector. transistor  In common collector configuration, the collector terminal is grounded so the common collector configuration is also known as grounded collector configuration. 8/3/

CC Configuration  In this configuration, the base terminal of the transistor serves as the input, the emitter terminal is the output and the collector terminal is common for both input and output. Hence, it is named as common collector configuration. The input is applied between the base and collector while the output is taken from the emitter and collector. transistor  In common collector configuration, the collector terminal is grounded so the common collector configuration is also known as grounded collector configuration. 8/3/

CC Configuration 8/3/

CC Configuration  The input supply voltage between base and collector is denoted by V BC while the output voltage between emitter and collector is denoted by V EC.  In this configuration, input current or base current is denoted by I B and output current or emitter current is denoted by I E.  The common collector amplifier has high input impedance and low output impedance. It has low voltage gain and high current gain. 8/3/

Current gain in CC Transistor action in the common collector is similar to the operation explained for the common base. It is based on the emitter-to-base current ratio called GAMMA ( γ ), because. the output is taken off the emitter. GAMMA ( γ )=I E /I B 8/3/

 1 thin layer of p-type, sandwiched between 2 layers of n-type.  N-type of emitter: more heavily doped than collector.  With V C >V B >V E :  Base-Emitter junction forward biased, Base-Collector reverse biased.  Electrons diffuse from Emitter to Base (from n to p).  There’s a depletion layer on the Base-Collector junction  no flow of e - allowed.  BUT the Base is thin and Emitter region is n + (heavily doped)  electrons have enough momentum to cross the Base into the Collector.  The small base current I B controls a large current I C BJT npn Transistor 8/3/

 Current Gain:  α is the fraction of electrons that diffuse across the narrow Base region  1- α is the fraction of electrons that recombine with holes in the Base region to create base current  The current Gain is expressed in terms of the β (beta) of the transistor (often called h fe by manufacturers).  β (beta) is Temperature and Voltage dependent.  It can vary a lot among transistors (common values for signal BJT: ). BJT characteristics 8/3/201946