DIODE APPLICATION By: Ashwini T.P Sr.Assistant Professor Mechatronics Department

Contents PN junction diode. V-I Characteristics. Junction Diode models. Junction Diode as a switch. Circuit applications of diode. Smoothing circuits. Zener diode voltage regulator.

LESSON PLAN: UNIT-1 SL.N O Planned hrs TOPICDATE engaged Remark s 1.11Introduction to semiconductor material 1.22PN junction diode & VI characteristics 1.33Junction diode as switch 1.44Diode application 1.55Diode application conti. 1.66Clipping circuit 1.77Clamping circuit 1.88Smoothing circuits 1.99Zener diode voltage regulator Discussion on previous paper

The Silicon Atom Nucleus: 14 protons 14 neutrons 10 core electrons: 1s 2 2s 2 2p valence electrons The 4 valence electrons are responsible for forming covalent bonds

Two-dimensional Picture of Si note: each line ( —) represents a valence electron covalent bond At T=0 Kelvin, all of the valence electrons are participating in covalent bonds There are no “free” electrons, therefore no current can flow in the silicon  INSULATOR Si

Silicon at Room Temperature - - For T>0 K, the silicon atoms vibrate in the lattice. This is what we humans sense as “heat.” Occasionally, the vibrations cause a covalent bond to break and a valence electron is free to move about the silicon. = free electron

Some important facts The number of electrons = the number of holes – that is, n = p in pure silicon – this is called intrinsic material Very few electrons/holes at room temperature – n=1.5x10 10 per cm 3 – This is known as intrinsic carrier density of a semiconductor.

Important Facts (cont.) Band Gap: energy required to break a covalent bond and free an electron – E g = 0.66 eV (germanium) – E g = 1.12 eV (silicon) – E g = 3.36 eV (gallium nitride) Metals have E g = 0 – very large number of free electrons  high conductance Insulators have E g > 5 eV – almost NO free electrons  zero conductance

* Energy Diagrams – Insulator, Semiconductor, and Conductor the energy diagram for the three types of solids 9

Doping Intentionally adding impurities to a semiconductor to create more free electrons OR more holes (extrinsic material) n-type material – more electrons than holes (n>p) p-type material – more holes than electrons (p>n)

Periodic Table of Elements Relevant Columns: III IV V

n-type silicon add atoms from column V of the periodic table Si P - Column V elements have 5 valence electrons Four of the electrons form covalent bonds with Si, but the 5 th electron is unpaired. Because the 5 th electron is weakly bound, it almost always breaks away from the P atom This is now a free electron.

VERY IMPORTANT POINT Si P+P+ - The phosphorus atom has donated an electron to the semiconductor (Column V atoms are called donors) The phosphorus is missing one of its electrons, so it has a positive charge (+1) The phosphorus ion is bound to the silicon, so this +1 charge can’t move! The number of electrons is equal to the number of phosphours atoms: n = N d

p-type silicon add atoms from column III of the periodic table Si B Column III elements have 3 valence electrons that form covalent bonds with Si, but the 4 th electron is needed. This 4 th electron is taken from the nearby Si=Si bond

VERY IMPORTANT POINT Si B-B- + The boron atom has accepted an electron from the semiconductor (Column III atoms are called acceptors) The boron has one extra electron, so it has a negative charge (-1) The boron ion is bound to the silicon, so this -1 charge can’t move! The number of holes is equal to the number of boron atoms: p = N a

Drift current The process in which charged particles move because of an electric field is called drift. Charged particles within a semiconductor move with an average velocity proportional to the electric field. Drift current is proportional to the carrier velocity

Diffusion current Due to thermally induced random motion, mobile particles tend to move from a region of high concentration to a region of low concentration. – Analogy: ink droplet in water Current flow due to mobile charge diffusion is proportional to the carrier concentration gradient In equilibrium, diffusion current (I D ) is balanced by drift current (I S ). So, there is no net current flow.

The PN Junction Diode  A popular semiconductor device called a diode is made by joining p- and n-type semiconductor materials, as shown in Fig. (a).  The doped regions meet to form a p-n junction.  Diodes are unidirectional devices that allow current to flow in one direction.  The schematic symbol for a diode is shown in Fig. (b).

Dopant distribution inside a pn junction p>>nn>>p excess electrons diffuse to the p-type region excess holes diffuse to the n-type region

The unbiased PN Junction Diode  Fig. (a) shows a p-n junction with free electrons on the n side and holes on the p side.  The free electrons are represented as dash (-) marks and the holes are represented as small circles (○).  The important effect here is that when a free electron leaves the n side and falls into a hole on the p side, two ions are created; a positive ion on the n side and a negative ion on the p side (see Fig. b).

n~0, and donor ions are exposed Dopant distribution inside a pn junction excess electrons diffuse to the p-type region excess holes diffuse to the n-type region DEPLETION REGION: + p~0, and acceptor ions are exposed p>>nn>>p

The PN Junction Steady State 1 P n NaNd Metallurgical Junction Space Charge Region ionized acceptors ionized donors E-Field ++__ h+ drift h+ diffusion e- diffusion e- drift ==

Voltage in a pn junction p>>nn>>p x charge,  (x) xx electric field, E(x) voltage, V(x) + ~0.7 volts (for Si)

The Biased PN Junction Pn + _ Applied Electric Field Metal Contact “Ohmic Contact” (Rs~0) + _ V applied I The pn junction is considered biased when an external voltage is applied. There are two types of biasing: Forward bias and Reverse bias.

Biasing the PN-Junction * Forward Bias: dc voltage positive terminal connected to the p region and negative to the n region. It is the condition that permits current through the pn-junction of a diode. 28 Decrease the built-in potential, lower the barrier height. Increase the number of carriers able to diffuse across the barrier Diffusion current increases Drift current remains the same. The drift current is essentially constant, as it is dependent on temperature. Current flows from p to n

Reverse Bias Dc voltage negative terminal connected to the p region and positive to the n region. external voltage pulls majority current carriers away from the pn junction Depletion region widens until its potential difference equals the bias voltage, majority-carrier current ceases. Increase the built-in potential, increase the barrier height. Decrease the number of carriers able to diffuse across the barrier. Diffusion current decreases. Drift current remains the same Almost no current flows. Reverse leakage current, I S, is the drift current, flowing from N to P.

V-I CHARACTERISTICS

Low current region: High current region: Reverse bias: Breakdown region:

Ideal diode characteristics

Diode current equation

Junction diode models A device model represents the physical operation of the device over a limited range of variables to a certain degree of accuracy. The device model consists of a ideal circuit elements.

Ideal diode

Dc model

Large signal model

Reverse biased model

Small signal model At low frequency At high frequency

Diode as a switch

Switching response of a diode Reverse recovery time: If the conducting diode is suddenly reverse biased at t = t1, the diode current does not fall immediately from IF to zero but it becomes zero after a time, trr, called reverse recovery time. Storage time (ts ): Time required for the minority carriers to return to their majority carrier state. Transition time (tt ): Time taken to charge stray capacitance Reverse recovery time = Storage time + Transition Time trr = ts + tt

Diode parameter and temperature dependence Reverse saturation current doubles for every 10 degree C rise in temp. Diode terminal voltage V D and Vγ decreases at the rate of 2.2mV/degree C for constant I A

Circuit application of diode Rectifiers: Rectifiers convert alternating current to direct current. They are useful in the design of dc power supplies required for all electronic components to work. Half-wave Rectifier: The process of removing one-half of the input signal, to establish a dc level is called half-wave rectification. Half-wave rectifier converts only one half of cycle of ac signal in to dc.

working of half-wave rectifier can be understood under two regions: Conduction region (0 → T/2) Non-conduction region (T/2 → T)

Output Waveform of HWR

Full-wave rectifiers: A full-wave rectifier converts the whole of the input waveform to one of the constant polarity (positive or negative) at its output Bridge rectifier

Half wave rectifier with centre tapped transformer

Clipping and clamping circuits Clippers: A clipper is a circuit that is used to eliminate a portion of a input signal without distorting the remaining part of the applied waveform. Parallel clipper

Series clipper

Symmetrical square wave generator

Clampers: A clamper is a network that shifts a waveform to a different dc level without changing the appearance of the applied signal. They are also called as dc restorers or dc inserters. Negative Clamper

Positive clampers

Smoothing circuits

Zener diode as voltage

Assignment questions