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P-N JUNCTION

Introduction

• History of p-n Junction• In November 16, 1904 first vacuum tube was invented

by Sir John Ambrose Fleming and it is called the Fleming valve, the first thermionic valve. There was no existence of p-n junction in electronics field. In October 20, 1906 Triode Tube had been developed by Dr. Lee de Forest. A conceptual figure of vacuum diode is shown below.

• Here the vacuum tube works mostly like modern diode. But its size is larger. It consists of a vacuum container with cathode and anode inside. This cathode and anode are connected across a high voltage source. Generally it works on principle of thermo ionic emission. This cathode is heated by filament an hence electron get emitted from cathode towards anode. So it is also known as thermionic tube. Current only flow from the anode to cathode i.e. unidirectional flow. The V-I characteristics of a vacuum tube is shown below.

How does vacuum tube diode work?

• Filament creates heat to the cathode to emit electrons. Beam of electrons flows from cathode to anode through the space between cathode and anode. The voltage difference is created across the cathode and anode by applying high voltage across their terminal. The replacement of the electrons in the electrodes is happened by this voltage source. Under reverse bias this vacuum tube does not work or it does not have any breakdown. This vacuum tube was the basic component of electronics throughout the first half of the twentieth century. It was available and common in the circuit of radio, television, radar, sound reinforcement, sound recording system, telephone , analog and digital computers, and industrial process control.

• Gradually p-n junction semiconductor has come in the market and vacuum tubes got replaced by them. But till today somewhere vacuum tubes are being used widely. These fields for application of the vacuum tubes are in • Atomic Clocks • Audio Systems • Car Dashboards • Cellular Telephone Satellites • Computer Monitors • DVD Players & Recorders • Electromagnetic Testing • Electron Microscopes • Gas Discharge Systems • Gas Lasers • Guitar Amplifiers • Ham Radio • High-speed Circuit Switching • Industrial Heating • Ion Microscopes • Ion Propulsion Systems • Lasers • LCD Computer Displays • Lighting • Microwave Systems • Microwave Ovens • Military Systems • Mobile Phone, Bluetooth & Wi-Fi Microwave Components • Musical Instrument Amplifiers • Particle Accelerators • Photomultiplier Tubes • Plasma Panel Displays • Plasma Propulsion Systems • Professional Audio Equipment • Radar Systems • Radio Communications • Radio Stations • Recording Studios • Solar Collectors • Sonar Systems • Strobe Lights • Satellite Ground Stations • Semiconductor Vacuum Electronic Systems • TV Stations • Vacuum Electron Devices • Vacuum Panel Displays

• Types of Vacuum Diodes

• The vacuum diodes are classified as 1. frequency range wise (audio, radio, microwave) 2. power rating wise (small signal, audio power) 3. cathode/filament type wise (indirectly heated, directly heated) 4. application wise (receiving tubes, transmitting tubes, amplifying or switching) 5. specialized parameters wise (long life, very low micro phonic sensitivity and low noise audio amplification) 6. specialized functions wise (light or radiation detectors, video imaging tubes)

• After the vacuum tubes, p-n junction semiconductor came in the market. The circuit gets lighter and more compact. The p-n junction semiconductor made of either Silicon or Germanium material which has four numbers of electrons in the valence band. From this valence band the electron transit to the conduction band penetration the energy gap of one electron volt approximately. Generally pure Silicon or Germanium has no extra electron available in their crystal structure. But applying of thermal energy to this crystal some bonds break and some electrons get available in the conduction band. But current is very small or in the order of microampere.

• This pure semiconductor is called intrinsic semiconductor. But some impurities are added to the pure semiconductor material like Al, P etc. Boron has three electrons in valence band. So one Boron atom holds four Silicon atoms with one bond with one electron. This deficiency of one electron in this bond is called as hole. After adding Boron to the intrinsic material this semiconductor gets abundance of holes in its lattice structure. This semiconductor is called extrinsic semiconductor. Due to abundance of holes it is known as positive type or p-type semiconductor.

• Phosphorus has five electrons in valence band. So one phosphorus atom holds four Silicon atoms. But one electron becomes extra. After adding phosphorus to the intrinsic material this semiconductor gets abundance of electrons in its lattice structure. This semiconductor is called extrinsic semiconductor. Due to abundance of electrons it is known as negative - type or n - type semiconductor.

• A p-n junction is formed by placing p-type and n-type semiconductor substrate side by side. It has homo junction between p-type and n-type.

• When p-type and n-type semiconductor comes to contact, some interesting cases arise. • The region of p-type is enriched of holes and the region of n-type is

enriched of electrons.• Now Electrons and holes come into action to diffuse from zone of high

concentration toward zone of low concentration, i.e. electrons travel from the n-region to the p-region and ionized donor atoms are left in this region. • In the p-region of the p- type substrate the electrons recombine with the

abundant holes. Again, holes come to diffuse from the p-region into the n-region. Hence negatively charged ionized acceptor atoms are left in the p-region.• Next, at the contact region in n - type semiconductor the holes which

come from p - type semiconductor recombine with the mobile electrons and at the contact region in p - type semiconductor electrons come from n - type semiconductor recombine with holes. This kind of diffusion process will be continuing up to the charge balance in two regions.

• Then a narrow region on both sides of the junction is created where no charge carriers (electrons or holes) are there. This region is called the depletion layer. • In p - type or n - type region, just after creation of

depletion region, it contains only holes in p-type and electron in the n-type semiconductor.• The depletion layer depends on the impurity level or

doping level in both type of semiconductor. It is inversely proportional to the doping level.• Now, as a whole joint of two layers looks like a depletion

region in the middle portion with two electric fields at the both end. These electric field points to p-type from the n-type region.• This depletion layer in the middle portion creates build-in-

potential or contact potential with respect to two regions.

• No net current flows through this depletion region. This depletion layer is also known as potential barrier.

• Symbol of p - n Junction Diode• A p - n junction is nothing but a diode hence an p-n

junction can also be refereed as p-n junction diode.

• Arrowed portion is called anode or positive terminal and bar portion is called cathode or negative terminal. Biased p-n Junction• Forward Bias of p-n Junction• When the p-type end of a p-n junction is connected to

the positive end of a battery and negative end of this junction is connected to the negative of this battery this biasing is called as the forward biasing.

At this biasing condition, the positive potency always repels the holes of the connected p-region. Similarly the negative voltage repels the electrons from the n-type region. Now both the major carriers i.e. the electrons and the holes penetrate to the depletion region and arrive their opposite region. Hence current flows from the positive region to the negative region. When battery voltage is applied across the junction in the forward bias, a current will flow continuously through this junction.

IS is Saturation Current (10-9 to 10-18 A) VT is Volt-equivalent temperature (= 26 mV at room temperature) n is Emission coefficient (1 ≤ n ≤ 2 for Si ICs) Actually this expression is approximated. Reverse Bias of p-n JunctionWhen a p-n junction is connected across a battery in such a manner that its n-type region is connected to the positive potency of the battery and the p-type region is connected to the negative potency of the battery. Now the holes are engulfed by the negative potency of the battery leaving behind negative static ions in the region and the electrons are engulfed by the positive potency of the battery leaving behind positive static ions in the region . Ultimately the depletion region at the p-n junction covers total p and n region of the diode. Hence no current will flow through this diode.

iD drops to zero value or very small value. iD can be written as i0.

IS is Saturation Current (10-9 to 10-18 A) VT is Volt-equivalent temperature (= 26 mV at room temperature) n is Emission coefficient (1 ≤ n ≤ 2 for Si ICs) Actually this expression is approximated.

• General Specification of p-n Junction• A p-n junction is specified in four manners. Forward voltage

drop (VF) : Is the forward biasing junction level voltage (0.3V for Germanium and 0.7V for Silicon Diode )• Average forward current (IF): It is the forward biased current due

to the drift electron flow or the majority carriers. If the average forward current exceeds its value the diode gets over heated and may be damaged.• Peak reverse voltage (VR) : It is the maximum reverse voltage

across the diode at it reverse biased condition. Over this reverse voltage diode will go for breakdown due to its minority carriers.• Maximum power dissipation (P) : It is the product of the forward

current and the forward voltage.

V-I Characteristics of A P-N Junction

• In the forward bias, the operational region is in the first quadrant. The threshold voltage for Germanium is 0.3V and for Silicon is 0.7V. Beyond this threshold voltage the graph goes upward in a non linear manner. This graph is for the dynamic Resistance of the junction in the forward bias.• In the reverse bias the voltage increases in the reverse direction

across the p-n junction, but no current due to the majority carriers, only a very small leakage current flows. But at a certain reverse voltage p-n junction breaks in conduction. It is only due to the minority carriers. This amount of voltage is sufficient for these minority carriers to break the depletion region. At this situation sharp current will flow through this junction. This breakdown of voltage is of two types. (a) Avalanche breakdown: it is not properly sharp, rather inclined linear graph i.e. after break down small increase in reverse voltage causes more sharp current gradually. (b) Zener Breakdown: this breakdown is sharp and no need to increase reverse bias voltage to get more current, because current flows sharply.

• Resistances of p-n Junction• Dynamic Resistance of p - n Junction• From V-I characteristics of a p-n junction, it is clear

that graph is not linear. The forward biased p-n junction resistance is rd ohm it is called AC resistance or dynamic resistance. It is equivalent to slope of voltage – current of the PN junction.

• Average AC Resistance of p - n Junction• Average AC resistance is determined by the straight

line that is drawn linking the intersection of the minimum and maximum values of external input voltage.

• Some important terms related to p-n Junction Transition Capacitance of p-n Junction• When depletion region exist in the common junction around, the

diode acts as a capacitor. Here the depletion region is the dielectric and two regions (p-type and n-type) at both ends act as the charged plates of a capacitor. As the depletion layer decreases the capacitance value goes down. Diffusion Capacitance of p-n Junction• It the capacitance of the diode in forward biased condition and it is

defined as the ratio of transiting charge created to the differential change in voltage. When the current through the junction increases the diffusion capacitance also increases. Along with this increase in current, the forward biased resistance also decreases. This diffusion capacitance is somewhat greater than the Transition capacitance. Storage Time of p-n Junction• It is the time taken by the electrons to move from n-type region to p-

type region and p-type region to n-type region by applying simultaneous forward and reverse bias voltage during switching.

• Transition Time of p-n Junction• It is the time taken by the current to decrease to reverse

leakage current. This transition time can be determined by geometry of P-N junction and concentration of the doping level.

• Reverse Recovery Time of p-n Junction• It is sum of the storage time and transition time. It is

the time for diode to raise applied current to get 10% of the constant state value from the reverse leakage