Introduction to Bipolar Junction Transistor ( BJT )

BJT has revolutionized the field of electronics. BJT is a main building block of all electronic systems. It is a three terminal device whose output current , voltage , power are  controlled by its input current. BJT has a very important property of raising the strength of a  weak signal. This property is known as amplification. Therefore transistor ( BJT ) is the   most widely used in the semiconductor devices.

 Bipolar Junction Transistor Construction:

A bipolar junction transistor has three regions known as emitter , base and collector.

1. Emitter : It is a region situated in one side of transistor, which supplies charge carries ( i.e. electrons and holes ) to the other two regions. The emitter is a heavily doped region.

2. Base : It is the middle region that forms two P-N junctions in the transistor. The base of transistor is thin , as compared to the emitter and is a lightly doped region.

3. Collector : It is region situated in the order side of transistor ( i.e. , the side opposite to the emitter ), which collects charge carriers ( i.e. electrons or holes ). The collector of a transistor is always larger than the emitter and base of a transistor. The doping level of the collector is intermediate between the heavy doping of emitter and light doping of the base.  

Unbiased Bipolar Junction Transistor : If the three terminals of transistor left open , is known as unbiased bipolar junction transistor. 

In the unbiased condition , the diffusion of free electrons across the junction produces two depletion layers as shown in above figure. The barrier potential voltage is 0.7V is for silicon and 0.3V for germanium transistor. It is observed that an emitter - base depletion layer penetrates slightly into the emitter, as it is heavily doped, whereas it penetrates deeply into base as it is lightly base region Similarly, the collector-base depletion layer penetrates more into base region and less into the collector region. Note that emitter-base depletion layer width is smaller than that of collector-base depletion layer. Unbiased transistor is never used in practical practice. 

Biased Bipolar Junction Transistor : The application of a suitable dc voltages across the transistor terminals, is called biasing. Each junction of a transistor may b forward biased or reverse-biased independently. Therefore , there are three different ways of biasing a transistor. 

Forward Active : In this mode , the emitter-base junction of a transistor is forward biased and collector-base junction is reverse biased. In a forward  active biasing the negative terminal of a battery is connected to N-side and positive terminal to P-side. The reverse biasing requires all the connections to be opposite to those for forward biasing.

Saturation : In this mode, both the emitter-base and collector-base junctions of a transistors are forward-biased. In this mode, transistor has a very large value of current . The transistor is operated in this mode, when it is used as a closed switch.

Cut-off : In this mode, both the emitter-base and collector-base junctions of a transistor are reverse biased . In this mode, transistor has practically zero current. The transistor is operated in this mode , when it is used as an open switch.

Remember that for amplifier , a transistor is used in forward active mode and for switching applications, transistor is used alternatively in saturation and cut-off region.

Modes of operations are the same for both NPN and PNP transistors. However, in PNP transistor biasing , the polarity of battery voltage are opposite to that of NPN transistor.  

Semiconductors

Semiconductors are of two types : 1. Intrinsic semiconductors   2.Extrinsic semiconductor 

Intrinsic Semiconductor : Pure form of semiconductor is known as Intrinsic ( Pure ) semiconductor. The silicon and germanium are two most widely used intrinsic semiconductors. The below figure represents the intrinsic structure of silicon atom. It has 4 valence electrons which makes it a tetravalent.

Silicon or Germanium Atom representation

The circle with +4 designation represents the central portion of the atom, including the nucleus and the electron except those in the outermost orbit. Red dot represents the valence electrons. These atoms are bonded together by a force which results from sharing of the neighboring atoms. This bonding force is known as Covalent bond.

The intrinsic semiconductor behaves as an insulator at absolute zero temperature ( 0 K ). As the temperature rises to room temperature(300K) some of the covalent bond breaks.

 

Intrinsic semiconductor with a broken covalent bond 

 

The energy required to break such a covalent bond is equal to the band gap energy (  Eg ). The vacancy of an incomplete covalent bond left behind the dislodged electron is known as a hole. A hole can serve as a carrier of electricity like the free electron. This combination of free electron and a hole is called as electron-hole pair.

Hole Conductivity Mechanism:

Let us imagine that the hole is created by an electron breaking away from the covalent bond. At this instant,  it is possible that an electron from neighbouring atom leaves its covalent bond to fill up the hole. However, this electron generates  a new hole in its initial position.When this occurs, it appears that a hole has moved in a direction opposite to that of the electron. This hole, in its new position may be filled by an electron from another covalent bond. Let consider above figure for more illustration,

Here red dot represents the electron and red empty circle represents hole. Consider the hole at point A, electron at this position had broken away from a covalent bond and moved anywhere in structure and is not shown in this figure. Now due to the thermal agitation, an electron at B may jump to position A.

This reestablishes the covalent bond at A. But creates a new hole at B. We can say that hole at A has moved to new position at B. Similarly, the hole may continue to move from B to C, C to D and D to E. This net displacement of hole from A to E constitutes the electric current.

The motion of hole, in one direction, actually means a transport of a negative charge by equal distance in opposite direction.   

Extrinsic Semiconductors: To increase the conductivity of intrinsic semiconductor at room temperature, a certain amount of impurity atoms can be added to it, this resulting semiconductor is called as impure or extrinsic semiconductor. The purpose of adding impurity is to increase either the number of free electrons or holes in a semiconductor. Depending on the type of  impurity added there are two types: 1. N- Type Semiconductor 2. P- Type Semiconductor

N- Type Semiconductor 

N Type Semiconductor : 

The N type semiconductor are obtained by adding pentavalent impurity atoms. Pentavalent impurity added to the pure semiconductor displaces some of its atom. As Shown in N  type semiconductor figure , there is structure of silicon lattice containing an Antimony atom at is center.
Out of 5 valence electrons of Antimony , 4 electrons will form a covalent bond with 4 valence electrons of silicon atom. The 5th electron is an extra electron, and is loosely bound with antimony atom. This extra electron can be donated to  available as a carrier of the current. 

After donation , the impurity atom becomes a positively charged ion and is known as  a donor ion. Bounded by four covalent bonds in the crystal lattice and therefore it is immobile. Thus in N type semiconductor , the current flows due to the movement of electrons and holes but major part of conduction is due to electrons. That is why electrons are majority carriers and holes are minority carriers in N type semiconductor.

P type Semiconductor

P Type Semiconductor :

The P type semiconductor are obtained by adding trivalent impurity atoms by replacing few pure semiconductor atoms. The 3 valence electrons of Indium atom forms 3 covalent bonds by sharing one electron with the electrons of neighboring atoms. However , the fourth covalent bond is incomplete leading to form a hole. Now due to thermal motion, indium atom seeks its surrounding atoms to acquire the 4th electron to complete its covalent bond. After accepting , impurity atoms become negatively charged immobile ion. Thus in P type semiconductor , majority part of the current flows due to holes and electrons as minority carriers.

  

A.C. Circuit application of diode

Diodes are also used in AC circuits where it is desired to convert AC voltages to DC voltages.

Half Wave Rectifier : 

In the half-wave rectifier circuit, the diode is forward-biased during the positive half-cycle from 0 to 𝜋 of the input voltage Vin and so current flows through the diode and resistor where it develops an output voltage drop Vout = Vin - VD . For instance, if the maximum value of Vin  is 10 volts, the maximum value of Vout will be  Vout = 10 - 0.7 = 9.3 V The diode is reverse-biased during the negative half-cycle from 𝜋 to 2𝜋 so no current flows, and thus Vout = 0.

Full Wave Rectifier : 

We can see that during the positive half cycle conventional current flows from the voltage source to Point A, then to Point B, it goes through the resistor from Point B to Point C, and through Point D returns to the negative terminal of the voltage source. During the negative half cycle the lower terminal of the voltage source becomes the positive terminal, current flows from Point D to Point B, it goes through the resistor from Point B to Point C, and through Point A returns to the upper (now negative) terminal of the voltage source. We observe that during both the positive and negative half-cycles the current enters the right terminal of the resistor, and thus is the same for both half-cycles as shown in the output waveform. The output voltages from the half and full wave rectifiers are often called pulsating DC voltages.

Peak Rectifier Circuits :

Above figure shows the input Vin and output Vout waveforms and we observe that the value of is approximately equal to the peak of the input sinewave . We have assumed that the diode is ideal and thus as is applied and reaches its positive peak value, the voltage across the capacitor assumes the same value. However, when starts decreasing, the diode becomes reverse-biased and the voltage across the capacitor remains constant since there is no path to discharge.

Clipper Circuits : 

Clipper (or limiter) circuits consist of diodes, resistors, and sometimes DC sources to clip or limit the output to a certain level. Clipper circuits are used in applications where it is necessary to limit the input to another circuit so that the latter would not be damaged. The input-output characteristics of clipper circuits are typically those of the forward-biased and reverse-biased diode characteristics except that the output is clipped to a a certain level. As shown a clipper circuit and its input-output characteristics where the diode does not conduct for Vin < 0.6 V and therefore Vin = Vout. Diode conducts for Vin >= 0.7 V  and thus Vout = 0.7 V.

 DC Restorer Circuit : 

Here , Diode conducts only when Vin < 0 and thus the capacitor charges to the negative peak of the input which for this example is -5 V , and the capacitor voltage is Vc = 5 V with polarity as shown in above figure. We observe that there is no path for the capacitor to discharge and therefore Vc = 5V always. Since Vout = Vin + Vc , when Vin = -5 V, Vout = -5+5 = 0 V. and the output has shifted upwards to zero volts as shown in output waveform. When the input voltage is positive, Vin = 5 V and Vout = 5 + 5 = 10 V. 

Voltage Doubler Circuits :

DC restorer circuit consisting of the capacitor C1 and diode D1 and a peak rectifier consisting of C2 and diode D2 . During the positive half-cycle of the input waveform diode D1 conducts, the peak value Vp of the input Vin appears across the capacitor C1 of and thus VD1 = 0 . During the negative half-cycle of the input waveform diode D2 conducts, capacitor C2 is being charged to a voltage which is the sum of Vin and VC1 and thus the output voltage is Vout = Vin + VC1 = -Vin - Vin = 2Vin. Voltage triplers and voltage quadruplers can also be formed by adding more diodes and capacitor to a voltage doubler.

 

What is diode ?

 A diode is a two-terminal device that, roughly speaking, conducts electric current in one direction (from anode to cathode, called forward-biased) much better than the opposite direction (reverse-biased). A small positive voltage at the diode’s terminal biases the diode in the forward direction and can produce a large current. A negative voltage biases the diode in the reverse direction and produces little current even at large voltage values.  

                  

                  Forward-Bias Diode                                                             Reverse-Bias Diode

               Forward Characteristics                                                          Reverse Characteristics  

If we attach a voltage source to a junction diode with the plus (+) side of the voltage source connected to the P-type material and the minus (−) side to the N-type as shown in above Figure forward-biased PN junction is formed. Forward characteristics indicates that there is no diode current till the 0.7 V point is reached. It is due to the fact that external applied voltage is being opposed by junction voltage which is 0.7 V for silicon and 0.3 V for germanium. The applied voltage is should not increase beyond safe limit otherwise it will damage diode. Now, if we attach a voltage source to a junction diode with the minus (-) side of the voltage source connected to the P-type material and the plus (+) side to the N-type as shown in above Figure reverse-biased PN junction is formed. When a junction diode is reverse-biased, a very small current will flow and if the applied voltage exceeds a certain value (Vz) the diode will reach its avalanche or Zener region.

Different Types of Diode and Their Applications :

Zener Diode : The Zener diode is always connected as a reverse-biased diode. It is also called as voltage-reference ,voltage-regulator diode. It differs from rectifier diode that it is operated in reverse breakdown region. 

Applications : One important application of the Zener diode is in voltage regulation. A voltage regulator is a circuit or device that holds an output voltage constant during output load variations. It is also used as a fixed reference voltage in transistor biasing circuits. As a peak clipper or limiter in waveshaping circuits as it limits one side of a sinusoidal waveform to while limiting the other side to approximately zero,Protection against damage.

The Schottky Diode : A Schottky diode is a junction of a lightly doped -type semiconductor with a metal electrode such as gold , silver , platinum. Thus in Schottky Diode, there is metal to semiconductor junction rather than PN junction. When the diode is forward-biased, the electrons move from the N-type material to the metal and give up their energy quickly. Since there are no minority carriers (holes) the conduction stops quickly and changes to reverse-bias.  

Applications : As a switching device in digital computers, As low voltage rectifier because we have to substract only 0.25 V instead 0.7 V , As Schottky TTL logic circuits, To rectify high frequency signals.

The Tunnel Diode : The conventional junction diode uses semiconductor materials that are lightly doped with one impurity atom for ten-million semiconductor atoms. In a tunnel diode the semiconductor materials used in forming a junction are doped to the extent of one-thousand impurity atoms for ten-million semiconductor atoms. This gives rise to the region of negative resistance. Also  produces an extremely narrow depletion zone. It have low value of inductance and capacitance.

Application : As tunneling mechanism takes place at the speed of light , it is used as ultra-high speed switching device, As a logic memory storage device, Used in an oscillators and microwave amplifiers.

Varactor Diode : The varactor, or varicap, is a diode that behaves like a variable capacitor, with the PN junction functioning like the dielectric and plates of a common capacitor. A varactor diode uses a PN junction in reverse bias. When reverse bias voltage decreases, the depletion region widens. This increases dielectric thickness  which is in turn reduces capacitance. When reverse biars voltage increases , the depletion region narrows down, this increases dielectric thickness which is in turn reduces capacitance. 

Application : A voltage controlled capacitance is useful in tuning applications, varactor diode is also used in automatic frequency control device , As adjustable band-pass filter.

Step-Recovery Diode : Step-recovery diode employs a graded doping in which doping level reduced as the junction region approaches. It indicates that less charge carrier density at junction area. This type of doping affects the time required for the device to switch from on stage to off stage and vice-versa. In step-recovery diode, during the negative-half cycle, reverse current exists for short time and suddenly drops to zero. Since reverse recovery period is very sharp, it is called as step-recovery diode.

Application : In frequency multiplier , In ultra-high frequency switching applications.

PIN Diode : PIN diode consists of three semiconductor materials. P type material and N type material is separated by intrinsic material. The intrinsic region offers high resistance to the current flow through PIN diode. The capacitance between the P and N region decreases due to increased separation between N and P regions, it gives fast response time. When a PIN diode is forward biased, it acts like variable resistance. While in reverse biased mode, PIN diode acts as constant capacitance.

Application : As a d.c. controlled microwave switch , As modulator, In attenuater as its resistance can be controlled by the current.

Photodiode : Photodiode is a PN junction device , which operates in a reverse bias. Photodiode differs from a rectifier diode in a sense that its reverse current increases with the light intensity at PN junction. When there is no light incident light , reverse current almost is zero. Resistance of the photodiode is controlled by light intensity. Photodiode can be used to switch on the current at very fast rate.

Application : In Photodetection , In demodulation , In encoders etc.