Semiconductor diode
Ideal diode conduct current in only one direction. Forward resistance of ideal diode is zero. Reverse bias resistance value is infinity. Barrier potential is generated by diffusion of electrons. PN diode can be made by alloying technique, diffusion technique, vapour deposition technique (epitaxial growth), rate growth technique
Thickness of depletion region is 1 micron. Silicon has higher temperature and current capacity hence used in switching operation. Depletion layer width depends on doping concentration on both sides of junction. Inside the junction of diode is satisfied with poisson equation. Outside the junction of diode is satisfied with laplace equation
Forward current is in milli ampere range and reverse current in micro ampere range. Cannot measure potential across diode using a voltmeter, only show zero reading. Reverse current depends on doping concentration. Reverse saturation current doubles for every 10 degree rise in temperature ( 7% rise per degree). In case of silicon reverse current in range of nano ampere and for germanium it is micro amperes. Less number of electron hole pair is generated in silicon compared to germanium
Below the cutting voltage/threshold voltage/offset voltage/break point current is very low, after that it increases. Typical static forward resistance of a diode is 80 ohm and reverse resistance is 500M. Dynamic resistance depends upon the operating voltage of the diode. Transition/space charge/depletion region capacitance exist in reverse bias condition. Transition capacitance is about 20 pf without external bias and its normal range from 5 to 200 pf. If value of transition capacitance is high then that diode have very high power. This type capacitance appears across varactor, vari caps or volta caps. Diffusion or storage capacitance exist in forward biased junction. Diffusion capacitance is greater than depletion region capacitance. Diffusion capacitance range is 10 to 1000pf. Large value of Diffusion capacitance means large anode current
Reverse recovery current depends on forward field current. Under small signal operation of a diode, its junction resistance predominates. Reverse current of a silicon diode is highly temperature sensitive. In switching diode fabrication aluminium is added to reduce the mean life time of carriers. Binistor is bistable negative resistance device. Spacistor generates upto 10GHz frequancies by injecting carriers into space charge. Silicon diode is not used in low voltage rectifier because of high breakdown voltage. External pass resistor is used in a voltage regulator to protect from short circuit. Varistor is used for line filtering to eliminate spikes and dips it is called transient supervisor. Photo diode is used to charge auxiliary storage battery
Solar cell, LAD, tunnel diode and laser diode are forward biased. By piece wise linear model is the diode model circuit which considers threshold voltage, average resistance and switch. Conductors have only one valance electron. Voltage multiplier converts AC to DC, where DC output have high amplitude than C
Random motion of charge carriers in semiconductors due to thermal agitation is called diffusion. Creation of free electron by zener effect is also called high field emission. Diode is a nonlinear device because its current and voltage are not directly proportional. Charge of hole is equal to that of a proton. Sum of the resistance of p and n region is called bulk resistance and it is less than 1 ohm. As doping increases bulk resistance decreases
Time taken by the diode to operate in reverse bias from forward bias is called reverse recovery time (turn off time of forward bias diode), Heavily doped diodes have low resistance. Gallium arsenide, aluminium arsenide and gallium phosphide are secondary semiconductor. An effect occurs in entire bulk semiconductor rather than a portion is called bulk effect. Point contact diode is called hot carrier diode with catwhisker as anode
Clamper is also called DC restorter. Annihilation of holes and electrons are called recombination. Breakdown mechanisms in PN junction diode are threshold and knee effects. Work function is the amount of additional energy required to emit an electron from a metal
Forward resistance of a diode is in the order of few ohms. Ideal stability factor of a diode is unity. Nuclei with common number of protons but different number of neutrons are called isotopes. Anotron is a cold cathode glow discharge diode having a copper anode and a large cathode of sodium. Saturation current in a diode is called scale current which is proportional to cross sectional area. Junction diode is rated by its maximum current and PIV. When a transistor is fully saturated then collector current is at its maximum. Second approximation of diode is series connection of switch with barrier potential. Third approximation of diode is series connection of switch with battery and resistance
Parallel combination of diode can withstand large leakage current. PIV is the maximum reverse voltage applied to a diode before break point. In emitter follower, output and input signals have same phase
Varactor is reverse biased. Varactor diode is fabricated with ICs of III-V. Varactor diode is called as voltage controlled capacitor, vericap, voltacap and epicap, Schottky diode is a metal semiconductor diode used for rectification and also used in digital computers. Gunn diode has no depletion region and operates with hot carriers. Gunn diode is a negative conductance device, works on the principle that electron in one region of a conduction band to another. Gunn diodes are also known as transferred electron devices. PIN diode is used in high frequency application, attenutors and phase shifting devices
In PNP voltage divider bias we should use a negative power supply. Sum of neutrons and protons are called atomic mass of an atom. Maximum forward current of a junction diode is limited by junction temperature. DIAC is a three layer device. Darlington pair increases overall beta
Break down diodes
Break down diodes works in reverse biased condition, It have adequate power dissipation capabilities, It is also called avalanche break down or zener diode. Zener diode works on the principle of diffusion of charge carriers. Silicon ICs are best suited for zener diode. Upper limit of current depends on power dissipation rating of the diode
It have a process called avalanche multiplication, in which thermally generated electrons breaks the bond of other electrons and diode in this state is called avalanche break down voltage. If this thermally generated electron don’t have sufficient energy to break the bond then the presence of strong electric field will break the bond this is called zener breakdown. This zener effect occurs at voltage below 6V, above 6 is avalanche multiplication. In case of zener effect it is negative temperature coefficient and in case of avalanche multiplication it is positive temperature coefficient
Tunnel diode
Tunnel diode works in forward bias. It is a voltage controlled negative resistance, It is also called esaki diode. here width of depletion region is reduced by heavy doping ( width is inversely proportional to square root of impurity concentration) [ width reduced from 5 microns to 100 A, this thickness is only one fifth of the wavelength of visible light]. Tunnel diode is useful in pulse and digital circuitry. It is a very high speed switch, relaxation oscillator, microwave oscillator, logic memory device.
Advantages – low power, low noise and ease of operation. Disadvantage – voltage range is less than 1V and no isolation