Faculty of Degree Engineering Department of Electronic& communication Engineering - 11 Subject: EDC ( ) Topic: Diode Fundamentals Prepared By: Hiral Kubavat ( ) Pethani Krunal ( ) Guided By: Prof. N.Y.Chavda

Outline  Introduction  Formation of the p–n Junction  Energy Band Diagrams  Concepts of Junction Potential  Modes of the p–n Junction  Derivation of the I–V Characteristics of a p–n Junction Diode  Linear Piecewise Models  Breakdown Diode  Special Types of p–n Junction Semiconductor Diodes  Applications of Diode

INTRODUCTION  The origin of a wide range of electronic devices being used can be traced back to a simple device, the p–n junction diode.  The p–n junction diode is formed when a p-type semiconductor impurity is doped on one side and an n-type impurity is doped on the other side of a single crystal.  All the macro effects of electronic devices, i.e., wave shaping, amplifying or regenerative effects, are based on the events occurring at the junction of the p–n device.  Most modern devices are a modification or amalgamation of p–n devices in various forms.  Prior to the era of semiconductor diodes, vacuum tubes were being extensively used. These were bulky, costly and took more time to start conducting because of the thermo-ionic emission.  The semiconductor diodes and the allied junction devices solved all these problems.

FORMATION OF THE p–n JUNCTION  When donor impurities are introduced into one side and acceptors into the other side of a single crystal semiconductor through various sophisticated microelectronic device-fabricating techniques, a p–n junction is formed.  The presence of a concentration gradient between two materials in such intimate contact results in a diffusion of carriers that tends to neutralize this gradient. This process is known as the diffusion process.  The nature of the p–n junction so formed may, in general, be of two types:  A step-graded junction:- In a step-graded semiconductor junction, the impurity density in the semiconductor is constant.  A linearly-graded junction:- In a linearly-graded junction, the impurity density varies linearly with distance away from the junction. A semiconductor p–n junction

ENERGY BAND DIAGRAMS  The discussion in this section is based on the realistic assumption that a junction is made up of uniformly doped p-type and n-type crystals forming a step-graded junction.  The p–n Junction at Thermal Equilibrium p-type and n-type semiconductors just before contact From the discussion of the law of mass action, the carrier concentrations on either side away from the junction are given by: (where p n is the hole concentration in n-type semiconductors, n p is the electron concentration in p-type semiconductors; n n and pp are the electron and hole concentrations in n- and p-type semiconductors respectively.)

CONCEPTS OF JUNCTION POTENTIAL  Space-charge Region  The non-uniform concentration of holes and electrons at the junction gives rise to a diffusive flow of carriers.  Since the electron density is higher in the n-type crystal than in the p- type crystal, electrons flow from the n-type to the p-type and simultaneously, due to reversibility, the holes flow from the p-type to the n-type.  The result of this migration of carriers is that the region near the junction of the n-type is left with a net positive charge (only ionized donor atoms) while that of the p-type is left with a net negative charge (only ionized acceptor atoms).  This diffusive mechanism of migration of the carriers across the junction creates a region devoid of free carriers, and this region is called the space-charge region, the depletion region or the transition region.

 The electrons diffusing from the n-type to the p-type leave behind uncompensated donor ions in the n-type semiconductor, and the holes leave behind uncompensated acceptors in the p-type semiconductors.  This causes the development of a region of positive space charge near the n-side of the junction and negative space charge near the p-side. The resulting electric field is directed from positive charge towards negative charge.  Thus, E 0 is in the direction opposite to that of the diffusion current for each type of carrier.  Therefore, the field creates a drift component of current from n to p, opposing the diffusion component of the current.  Since no net current can flow across the junction at equilibrium, the current density due to the drift of carriers in the E 0 field must exactly cancel the current density due to diffusion of carriers.  Moreover, since there can be no net build-up of electrons or holes on either side as a function of time, the drift and diffusion current densities must cancel for each type of carrier. CONCEPTS OF JUNCTION POTENTIAL

BREAKDOWN DIODE  Breakdown diodes are p–n junction diodes operated in the reverse-bias mode.  This breakdown occurs at a critical reverse-bias voltage (V br ). At this critical voltage the reverse current through the diode increases sharply, and relatively large currents flow with little increase in voltage.  These diodes are designed with sufficient power-dissipation capabilities to work in the breakdown region. The following two mechanisms can cause reverse breakdown in a junction diode. Reverse-biased p–n junction Reverse breakdown in a p–n junction

BREAKDOWN DIODE  Comparison between Zener and avalanche breakdown The I–V characteristics comparison between Zener and avalanche breakdown

SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES  Tunnel Diode  The tunnel diode is a negative-resistance semiconductor p–n junction diode. The negative resistance is created by the tunnel effect of the electrons in the p–n junction as already discussed in the section of Zener diode. Tunnel diode under zero bias equilibrium Small reverse bias

 The doping of both the p- and n-type regions of the tunnel diode is very high—impurity concentration of to atoms/cm 3 are used (which means both n-type and p-type semiconductors having parabolic energy bands are highly degenerate)—and the depletion layer barrier at the junction is very thin, in the order of cm. SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES Small forward bias Increased forward bias Increased forward bias condition where the current begins to increase again

SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES Small-signal model of the tunnel diode. (Typical values for these parameters for a tunnel diode of peak current I P 10 mA are –R n – 30 Ω, R s 1 Ω, Ls 5 nH and capacitance C 20 pF respectively) I–V characteristics of a tunnel diode Symbol of tunnel diode

SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES  Light-emitting Diode  Charge carriers recombination takes place at the p–n junction as electron crosses from the n-side and recombines with holes on the p- side.  When the junction is forward-biased the free electron is in the conduction band and is at a higher energy level than the hole located at valence band.  The recombination process involves radiation of energy in the form of photons. If the semiconductor material is translucent, the light will be emitted and the junction becomes a light source, i.e., a light-emitting diode (LED). LEDs are p–n junctions that can emit spontaneous radiation in ultraviolet, visible, or infrared regions.  Advantages of LEDs 1. Low operating voltage, current and power consumption make LEDs compatible with electronic drive circuits. 2. LEDs exhibit high resistance to mechanical shock and vibration and allow them to be used in severe environment conditions. 3. LEDs ensure a longer operating life line, thereby improving the overall reliability and lowering the maintenance costs of equipment.

SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES 4. LEDs have low inherent noise levels and also high immunity to externally generated noise. 5. LEDs exhibit linearity of radiant power output with forward current over a wide range.  Limitations of LEDs 1. Temperature dependence of radiant output power and wavelength. 2. Sensitivity to damages by over voltage or over current. 3. Theoretical overall efficiency is not achieved except in special cooled or pulsed conditions. (a) Schematic showing the basic process of absorption (b) emission The symbol of an LED

SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES  Photovoltaic Diode  The photovoltaic diode or solar cell is an important technological device for overcoming energy problems.  It is also known as solar energy converter; it is basically a p–n junction diode which converts solar energy into electrical energy.  The energy reaching the earth’s surface from the sun is primarily electromagnetic radiation, which covers a spectral range of 0.2 to 0.3 micrometre.  The conversion of this energy into electrical energy is called photoelectric effect.  Construction and working principle  A photovoltaic diode essentially consists of a silicon p–n junction diode usually packaged with a glass window on the top.  Surface layer of the p-material is made extremely thin so that the incident light (photons) can penetrate and reach the p–n junction easily.

SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES  When these photons collide with the valence electrons, they impart in them sufficient energy so that they gain enough energy to leave the parent atoms. In this way, free electrons and holes are generated on both sides of the junction. Consequently, their flow constitutes a current (minority current).  This current is directly proportional to the illumination (lumen/m 2 or mW/m 2 ).  This, in general depends on the size of the surface being illuminated. The open circuit voltage V oc is a function of illumination.  Consequently, power output of a solar cell depends on the level of sunlight illumination. Power cells are also available in the form of a flat strip so as to cover sufficiently large surface areas. Structure of a solar cell

SPECIAL TYPES OF p–n JUNCTION SEMICONDUCTOR DIODES I–V characteristics of an illuminated solar cell showing the point of maximum power Top finger contact with anti- reflecting coating  Current–voltage characteristics  It is seen that the curve passes through the fourth quadrant and hence the device can deliver power from the curve.  The power delivered by the device can be maximized by maximizing the area under the curve or by maximizing the product (I sc V oc ). By properly choosing the load resistor, output power can be achieved. In the absence of light, thermally generated minority carriers across the junction constitute the reverse saturation current.

APPLICATIONS OF DIODE  Radio Demodulation:- In demodulation of amplitude modulated (AM) radio broadcasts diodes are used. The crystal diodes rectify the AM signal, leaving a signal whose average amplitude is the desired audio signal. The average value is obtained by using a simple filter and the signal is fed into an audio transducer, which generates sound.  Power Conversion:- In the Cockcroft–Walton voltage multiplier, which converts ac into very high dc voltages, diodes are used. Full-wave rectifiers are made using diodes, to convert alternating current electricity into direct current.  Over-voltage Protection:- Diodes are used to conduct damaging high voltages away from sensitive electronic devices by putting them in reverse- biased condition under normal circumstances. When the voltage rises from normal range, the diodes become forward-biased (conducting). In stepper motor, H-bridge motor controller and relay circuit’s diodes are used to de- energize coils rapidly without damaging voltage spikes that would otherwise occur. These are called a fly-back diodes.  Logic Gates:- AND and OR logic gates are constructed using diodes in combination with other components. This is called diode logic.

 Ionizing Radiation Detectors  In addition to light, energetic radiation also excites semiconductor diodes.  A single particle of radiation, having very high electron volts of energy, generates many charge carrier pairs, as its energy is transmitted in the semiconductor material.  If the depletion layer is large enough to catch the whole energy or to stop a heavy particle, an accurate measurement of the particle’s energy is possible.  These semiconductor radiation detectors require efficient charge collection and low leakage current. They are cooled by liquid nitrogen. Common materials are Ge and Si.  Temperature Measuring:- The forward voltage drop across the diode depends on temperature. A diode can be used as a temperature measuring device. This temperature dependence follows from the Shockley ideal diode equation and is typically around -2.2 mV per degree Celsius.  Charge-coupled Devices:- Arrays of photodiode, integrated with readout circuitry are used in digital cameras and similar units. APPLICATIONS OF DIODE