Diodes and Diode Applications | Electronics PPT
Diodes and Diode Applications
Topics Covered in this ppt
The PN Junction Diode
Volt-Ampere Characteristic Curve
- Semiconductors conduct less than metal conductors but more than insulators.
- Some common semiconductor materials are silicon (Si), germanium (Ge), and carbon (C).
- Silicon is the most widely used semiconductor material in the electronics industry.
- Almost all diodes, transistors, and ICs manufactured today are made from silicon.
- Intrinsic semiconductors are semiconductors in their purest form.
- Extrinsic semiconductors are semiconductors with other atoms mixed in.
- These other atoms are called impurity atoms.
- The process of adding impurity atoms is called doping.
- Thermal energy is the main cause for the creation of an electron-hole pair, as shown in Fig. 27-3.
- As temperature increases, more thermally generated electron-hole pairs are created.
- In Fig. 27-3, the hole acts like a positive charge because it attracts a free electron passing through the crystal.
- Fig. 27-4 shows the doping of a silicon crystal with a pentavalent impurity.
- Arsenic (As) is shown in this figure, but other pentavalent impurities such as antimony (Sb) or phosphorous (P) could also be used.
- Fig. 27-5 shows the doping of a silicon crystal with a trivalent impurity.
- Aluminum (Al) is shown in this figure, but other trivalent impurities such as boron (B) or gallium (Ga) could also be used.
- One of the valence electrons in the pentavalent impurity atom in Fig. 27-4 is not needed in the covalent bond structure and can float through the material as a free electron.
- One more valence electron is needed at the location of each trivalent atom in the crystal to obtain the maximum electrical stability as shown in Fig. 27-5.
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. 27-6 (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. 27-6 (b).
- Fig. 27-7 (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. 27-7 b).
- The term bias is defined as a control voltage or current.
- Forward-biasing a diode allows current to flow easily through the diode.
- Fig. 27-8 (a) illustrates a PN junction that is forward-biased.
- Fig. 27-8 (b) shows the schematic symbol of a diode with the voltage source, V, connected to provide forward bias.
- Fig. 27-9 illustrates a reverse-biased PN-junction.
- Fig. 27-9 (a) shows how an external voltage pulls majority current carriers away from the PN junction.
- This widens the depletion zone.
- Fig. 27-9 (b) shows a schematic symbol showing how a diode is reverse-biased with the external voltage, V.
- Diodes Have Polarity (They must be installed correctly.)
Volt-Ampere Characteristic Curve
- Figure 27-10 (next slide) is a graph of diode current versus diode voltage for a silicon diode.
- The graph includes the diode current for both forward- and reverse-bias voltages.
- The upper right quadrant of the graph represents the forward-bias condition.
- Beyond 0.6 V of forward Bias the diode current increases sharply.
- The lower left quadrant of the graph represents the reverse-bias condition.
- Only a small current flows until breakdown is reached.
- Three different approximations can be used when analyzing diode circuits.
- The one used depends on the desired accuracy of your circuit calculations.
- These approximations are referred to as
- The first approximation
- The second approximation
- The third approximation
- The first approximation treats a forward-biased diode like a closed switch with a voltage drop of zero volts, as shown in Fig. 27-11.
The second approximation treats a forward-biased diode like an ideal diode in series with a battery, as shown in Fig. 27-12 (a).
- The third approximation of a diode includes the bulk resistance, rB.
- The bulk resistance, rB is the resistance of the p and n materials.
- The third approximation of a forward-biased diode is shown in Fig. 27-13 (a).
- Diode ratings include maximum ratings and electrical characteristics.
- Typical ratings are
- Breakdown Voltage Rating, VBR
- Average Forward-Current rating, IO
- Maximum Forward-Surge Current Rating, IFSM
- Maximum Reverse Current, IR
- A circuit that converts the ac power-line voltage to the required dc value is called a power supply.
- The most important components in power supplies are rectifier diodes, which convert ac line voltage to dc voltage.
- Diodes are able to produce a dc output voltage because they are unidirectional devices allowing current to flow through them in only one direction.
- The circuit is shown in Fig. 27-15 (a) is called a half-wave rectifier.
- When the top of the transformer secondary voltage is positive, D1 is forward-biased, producing current flow in the load.
- When the top of the secondary is negative, D1 is reverse-biased and acts like an open switch. This results in zero current in the load, RL.
- The output voltage is a series of positive pulses, as shown in the next slide, Fig. 27-15 (c).
- The circuit is shown in Fig. 27-17 (a) is called a full-wave rectifier.
- When the top of the secondary is positive, D1 is forward-biased, causing current to flow in the load, RL.
- When the top of the secondary is negative, D2 is forward-biased, causing current to flow in the load, RL.
- The combined output voltage produced by D1 and D2 are shown in Fig. 27-17 (f) on the next slide.
Full-wave bridge rectifier
- The circuit is shown in Fig. 27-19 (a) is called a full-wave bridge rectifier.
- When the top of the secondary is positive, diodes D2 and D3 are forward-biased. producing current flow in the load, RL.
- When the top of the secondary is negative, D1and D4 are forward-biased, producing current flow in the load, RL.
- Figure 27-21 (a) shows a half-wave rectifier with its output filtered by the capacitor, C.
- Usually, the filter capacitors used in this application are large electrolytic capacitors with values larger than 100 μF.
- Notice the time before to in Fig. 27-21 (b).
- During this time, the capacitor voltage follows the positive-going secondary voltage.
- At time t0, the voltage across C reaches its peak positive value.
- Output ripple voltage of the half-wave rectifier is illustrated.
- Fig. 27-22 (a) shows a full-wave rectifier with its output filtered by the capacitor, C.
- When the top of the secondary is positive, D1 conducts and charges C.
- When the bottom of the secondary is positive, D2 conducts and recharges C.
- Besides rectification, a semiconductor diode has many other useful applications.
- Semiconductor diodes can be manufactured to regulate voltage or emit different colors of light.
- Examples of two special purpose diodes are
- Light-emitting diode
- Zener diode
- A light-emitting diode (LED) is a diode that emits a certain color light when forward-biased.
- The color of light emitted by an LED is determined by the type of material used in doping.
- A schematic symbol of an LED is shown in Fig. 27-23.
- A zener diode is a special diode that has been optimized for operation in the breakdown region.
- Voltage regulation is the most common application of a zener diode.
- Fig. 27-25 shows the schematic symbol for a zener diode.