Presentation on theme: "Diodes and Transistors. Diodes Diodes are the semiconductor pn junction devices. They are formed by creating p-type and n-type semiconductors in a single."— Presentation transcript:
Diodes and Transistors
Diodes Diodes are the semiconductor pn junction devices. They are formed by creating p-type and n-type semiconductors in a single Si/Ge (mostly) crystal. They are unidirectional devices. Every diode requires a minimum potential across it to allow a significant current to flow through it. This voltage is called as cut-in voltage. The VI relationship for a normal diode is exponential and is given by, I d =I 0 (e (Vd/ηVt) -1)
where, I d = current flowing through diode I 0 = Reverse saturation current V d = Voltage across diode η = Material constant (actually it depends on doping level & manufacturing process also and varies between 1 to 2) The reverse voltage at which the pn junction is damaged called as breakdown voltage (here diode may get permanently open or close) Diode doesn’t follow its current equation in breakdown region
Transistors Transistor is a solid state device made up of Silicon or Germanium. The name is derived from “Transfer Resistor” There are two main types of transistors, – Bipolar Junction transistor (BJT) NPN PNP – Field Effect Transistor (FET) JFET MOSFET
Bipolar Junction transistors These are constructed on a single Si / Ge crystal by doping 3 p-type and n-type impurities alternately. The doping sequence can be n-p-n p-n-p It has two pn junctions and 3 doped regions Transistor can’t be constructed by adding two diodes back to back, because it will have 4 doped regions. The three regions in transistors are Emitter Base Collector
Symbols NPN transistor PNP Transistor
Continued… The three regions are called as emitter (extreme left), base (middle) and collector (extreme right) Input can be given between any two terminals and output can be taken from any two terminals keeping one terminal common to both input and output. Depending upon the terminal which is kept common to both i/p and o/p, it can be configured in three ways. – Common Base (CB) config. – Common Emitter (CE) config. – Common Collector (CC) config.
Common Base Configuration In this configuration, base is kept common between input and output. Input is given between base & emitter and output is taken between base & collector. This configuration is also known as Grounded base configuration. The output equation can be given as, V CB = V 0 = V CC – I C R L
Common Collector Configuration In this configuration, collector is kept common between input and output. The output is obtained by connecting a load resistance (R L ) in emitter This configuration is also known as emitter follower (because voltage across the emitter resistor follows the input voltage at base) or buffer The output equation can be given as, V 0 = I E R L
Common Emitter Configuration In this configuration, emitter is kept common between input and output. Input is given between base & emitter and output is taken between emitter & collector. The output equation can be given as, V CE = V 0 = V CC – I C R L
Comparison Parameter Common Base (BC) Common Emitter (CE) Common Collector (CC) Phase ShiftZero180 0 Zero Current Gain>1High Voltage gainHigh >1 Power gainModerateHighLow to Moderate Input ImpedanceLowModerateHigh Output ResistanceHighModerateLow
Common Emitter Static Characteristics Input Characteristics This is plotted for I B Vs. V BE keeping V CE constant. Firstly V CE is kept constant and then V BE is slowly increased and corresponding values of I B are recorded. This is done for different values of V CE. Output Characteristics This is plotted for I C Vs. V CE keeping I B constant. Firstly I B is kept constant and then V CE is slowly increased and corresponding values of I C are recorded. This is done for different values of I B.