Part III: Introduction To Electronic Devices

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Beverley Carroll
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1 Ir ), [ r Part III: Introduction To Electronic Devices Objectives: 1. To understand the electrical conduction in Semiconductor materials. 2. To study the characteristics of PN junctions. 3. To model the PN junction (Diode) as a circuit element. 4. To analyze circuits comprising diodes. I. Electrical conduction in Semiconductors:. Semiconductors are materials that have electrical properties falling between those of conducting and insulating materials... When a thermal energy is supplied to the lattice structure of the silicon material, some electrons break their bonds with the lattice and become free electrons leaving a positive hole in their place. =~~~~:4= " " "!l = ~:=; I' I' II " Both free electrons and holes contribute to the conduction process. Electrons are negative charges, and holes are positive charges.. To control the number of free carriers (electrons and/or holes) inside a semiconductor and thus. achieve better conduction, semiconductors go through doping process.. Doping means adding some impurities to the structure of the semiconductor.. When a material is doped with extra holes, the material becomes a positive type (ptype). semiconductor. When extra electrons are added the material becomes a negative type (Ntype). Ntype, The majority carriers are electrons and the minority are holes. Ptype, The majority carriers are holes and the minority are electrons. II. The P.N Junction (Diode) ~~~lle~}(1) G> Q ~ 'P; $ <ilf'i GIG). The PN junction is an Ntype material brought in contact with a ptype material.. At the contact surface and the surrounding area, the electrons and the holes combine to neutralize this zone and almost no carriers will be found in this area. This area is referred to as depletion region.. A contact potential appears across this region and in Silicon PN junction this potential has a value of O. 7V. This potential is called the offset voltage (V y).

2 2 III. IV characteristics of PN junction (Diode Characteristics).. "0 + "0 ~ 1<' N ~ ~. e.\jtx""se (..b Oyd.,\j 0 Co~itM CO'1'\~JC;~iO V\ ~4.. ~ l, "". When the diode is forward biased (fig.!), conduction starts at V D = V y, where V D is the voltage across the diode (AnodeCathode) voltage. The diode behaves like a switch in the "on" position.. When the diode is reverse biased (fig 2), very small current (order of na) flows which is neglected and the diode considered to be a switch in its "off' position.. Therefore, the diode is either conducting (forward biased) or not conducting (reverse biased). IV. The Diode as a circuit element When a diode is placed in a circuit this diode can be represented by one of the following two models: a) Ideal diode model: In this model the diode is considered to be a switch. This switch is turned on when a positive voltage appears across the diode and switched off when a negative voltage appears across the diode. The diode therefore has the following characteristics:'. V D = 0 (Forward) ID = 0 (Reverse)! ~I ~ ~ ~ ~ """ J(... '\= OytADra (j'.'2t)/e)"s.~

3 .3' b) Offset model: In this model a d.c source of 0.7 V is added to the diode ideal model in the forward bias mode to representhe contact voltage across the depletion region. A K It o'lv k A t'. "'1'. ;.,11" ~ ~c> ~~'(d ~e.\/c.v's (' V. Analysis of Diode Circuits V.1 Determination of the diode conduction state: 1. Assume a state for each diode you have in the circuit. (Either "on" or "off'). You need 2N states for N diodes. 2. Analyze the to determine the current through the diodes assumed to be "on" and the voltage across the diodes assumed to be "off'. 3. Check to see if the result is consistent with the assumed state for each diode. ) Current must flow in the forward direction for the diodes assumed to be on. ) Voltage across the diodes assumed to be "off' must be negative (reverse biased). 4. If the results are consistenthe analysis is finished, otherwise return to step 1 and choose different states. VI. Examples 1. The diode in the given circuit is ideal. Determine whether the diode is conducting. First assume no conduction (Reverse Biased S~ \0"" Case) V, = 2 )( 12.. = 8V 1< Y : IOIfl == \\\1 10 +s Vz = t' V ' VO/,= v, ~ = 3Y VOl is negative. Therefore the diode is reverse ~ \ ~ ~ CD I,,) biased. Now let's assume the opposite assumption..sfll IotA. S; (Forward biased) IotA 't,u 11'1 y, 12.,'Z. L iii:: ov. IIV; o.5a ~ ""D/'::' ~o I';') ~:Jl...,...!:t + ';"s ' "'l D ~"" V ~ /0 I~ 1)\1 \ F _&.1? Since io cannot flow in this direction, the ~..~YJ'.(. VWQ/\Ol. assumption is incorrect and the diode is not conducting.

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Lecture 3: Diodes. Amplitude Modulation. Diode Detection.

Whites, EE 322 Lecture 3 Page 1 of 10 Lecture 3: Diodes. Amplitude Modulation. Diode Detection. Diodes are the fourth basic discrete component listed in Lecture 2. These and transistors are both nonlinear