Semiconductor Electronics: Materials, Devices and Simple Circuits PDF

ls, Ch apt er- 14: Sem ico ndu cto r Electronics: Materia Dev ice s and SimRle Circuits... According to the Bohr at d. y of any of ,ts electrons is om,c mo el, in an isolated atom the energ...

ls, Ch apt er- 14: Sem ico ndu cto r Electronics: Materia Dev ice s and SimRle Circuits... According to the Bohr at d. y of any of ,ts electrons is om,c mo el, in an isolated atom the energ atoms come together to form a solid decided by the orbit in which it revolves. But when the th ons from neighbouring atoms ey are close to each other. So the outer orbits of electr fore the nature of electron motion in a woul d come very close or could even overlap. There solid is differ ent from that in an isolated atom. and no two electrons h~e ~~ yie Inside a crystal, each elec~ron has a unique position each electron will hav e~e~nt same patte rn of surrounding charges. Because of this, uous energy vari ati~ r~. nergy energ y level. T.!:!._ese differen_t e~ergy levels with contin levels of the val en~ ~s r called bands. The energy band whrch rncludes the energy ce band is cal le~ uct ion band( the\v alenc e band. The energy band above the valen R,_~ , which inclu de the free elect rons). c and ~ e r g y level in the valence The lowe st energ y level in the conduction band is_E 5 band is v. The gap betw een the top of the valen.e ~ d a~o ttom of the conduction band is called the energ y band g~E nerg y gap E ).~ V Ene r ban ds in conducto ductors and insulators a} Con duct ors or meta ls h e cond uctio n band is parti ally filled ~Ar' ·--- y emp ty( fig i) or the cond uctio n and and the balanced b 11 valance band s o ig i1. he resistance of such mate rials is low or the cond uctiv ity is Overlapping condu ction band E ff - Conduction J band (E9 :::: 0) C ~- -- -- -- -- ~;..;..;..;;. ;..:..;.;.~~ ~~~ Valence Valence 1 band ~.;.......;;........,..;.....;..;~..........---............ band (fl) (i) (a) 3 eV) exists. b) In semiconductors, a finite but small band gap (E g < e electrons Because of the small band gap, at room temperature, som energy gap and from valence band acquire enough energy to cross the iconductors is enter the conduction band. Hence, the resistance of sem not as high as that of the insulators. r r I " - E V c) Jn insulators, a large band gap Eg exists (E g > 3 eV). T~~c?r oSt, notpossible. ~ -~,., -; Ec~-_;_~...;_-x---- ✓~C:~!cuon b d R, ·' an , E9 > 3 e\7 Valence band semiconductor ~ 4,. NOT C ity of a semiconductor increase with temperature as , f':', mo an re electrons from the valence band gain energy, cross h e......Ygap and reach the conduction band.( Resistance decrease ~ "< crease in temperature). · ' x rcise 14.3 Carbon, silicon and germanium have four valence electrons each. These are characterised by valence and conduction bands separated by energy band gap respectively equal to (Eg)C, (E8)Si and (Eg)Ge. Which of the following statements is true? (a) (Eg}Si < (Eg)Ge < (Eg}C (b) (E 8)C < (E 8)Ge > (E8)Si (c) (E 8)C > (E8)Si > (Eg)Ge (d) (E 8)C = (E8 }Si = (E1 )Ge Intrinsic and extrinsic semiconductors 2  I semic ic onductors (pure):- As the electro ns in the va e b. they I nee an d gain energ y, 1) ntnns 55 , this proce.. e During they cross the energy gap and reach the conduction band. e oos1 t~- -ffectiv:,_::..r- leave a vacancy in the valence band. The vacancy with th ee electronic charge is called a hole. -- Q ~ -t (.; E(' i ~ 0 ~ ,._J r,,, - ~ (; ~ Et. ,,f :~.~r,. ::-·> n e J. Thus, known as ac erefore, s are mi no rity carriers. Th th e holes are the majority carriers and electron ype nd uc tors do pe d wi th triv alent im pu rity are called p-t extrinsic semico semiconductors.. T> thermally Fig shows energy bands of p-type semiconductor at generated electron-hole pair and 7 holes from acce m. as the emams neutra 1 NOTE:- 1. In n-type and p-type semiconductors, charge of additional charge carriers is just equal opposite to that of the ionised cores i ice.. ----- hermal equilibrium , 2. For both n-type and p-type semIcon Ine nh: ni2 I nd Ge depends upon 3. The difference in the resi E.-,.,__ the energy gap between.____ and valence bands. For C (diamond), Si and Ge th~ilfii[ are 5.4 eV, 1.1 eV and 0. 7 eV, respectively. Sn a element but it is a metal because the energy gap in its case is O Exercise 1~.. ~aihe statements given in ·Exercise 14.1 is true for p-type 28 3 Exa -_-pose a pure Si crystal has 5 x 10 atoms m-. It is doped by 1 c..::....:...:.... p one ration of pentavalent As. Calculate the number of electrons and 0 s. Jn th~t n; =1.5 x 10 16 m-3 ~ -n JUNCTI ON:-A P-N junction is a boundary between the p-type and the n-type, inside a semiconductor. p-n junctio n formati on:- There is a difference in the concentration of holes and electrons at the two sides of a junction. Two important processes that occur during the formation of a p-n junction are (i) diffusion and (ii) drift. (i)Diffusion:- The holes from the p-side diffuse to the n-side and the electrons from the n-side diffuse to the p-side. These give rise to a diffusion current across the junction....., {. \ ""' t~ I ~ ''~ 1 )~V\J\-,.. 5  I n ~. ~. n-side to the p-side, an ionised don.9r i:. I~ when a hole diffuses fr~.21e p- (ii)When an electron diffuses from ::ile Similarly, behind on the n-s1de, ~h'.ch is ,mm. left behind on the p-side, As ~e~ ul!, near side to the n-side, an ion!ze~ accep_tdor tf harges and has onlK:immobile ions is. t")~ ' - the junction, a region which is devo1 o re~ c n. " -~. formed. This region is termed as the depletion regio · th ion and oositive char ges 1n the reg _ - ,.,.;... h (iii)The accumulation of negative charges in e p- ~o~ s ~he Junct~on whic n-region due to diffusion sets up a potential diffe ren~ barner(Eu1lt 1n potential Vo). opposes further diffusion. This is called potential (iv)Due to the potential barrier, an electric fl ec ~m n-region to p:regi?n vv-,;::,v, J- Cc~ ,_. ~ , , \.._ -.. t ~j Due t o th'1s f' I an ~~n on develops across t he junction. p-side of the junction moves to n-side n- side of the junction moves to p-side. This motion of minority charge carf t electric field is called drift. Thus a drift current, which is opposite t tt1'."'1~R:>n of the diffusion current, starts. Initially, diffusion current i arg rift current is small. As the diffusion process continues, the depl a the junction extend, thus increasing the electric field t. This process continues until the diffusion curre nt equals strength and hen the drift curr junction is f n junction under equilibrium there is no net current. join it to e take one slab of p-type semiconductor and physically emiconductor to get p-n junction? ion to n-region Exerc1 1.4 In an unbiased p-n junction, holes diffuse from the p-reg (b} they move across the junction by because (a) free electrons in then-region attract them. the potential difference. (c) hole concentration in p-re gion is more as compared to n- region. (d) All the above. diode is basically a SEM ICO NDU CTO R DIO DE: - A semiconductor the application of an external p-n junction with metallic contacts provided at the ends for voltage. It Is a two terminal device. 6  o-----i~,,/ r·"·~t o.. d' d The direction of Fig shows the symbol of p-n Junction 10 e. d.. h h diode is under forwar arrow indicates the conventional d1rect1on of current (w en t e bias). ~-n junction diode under forward bias:- When an externa_, ~oltage. isl -.. R, nnected to the pos1t1ve term1na applied across a semiconductor diode such that p-side 15 co b' s:sd 'd to be forward ,ase of the battery and n-side to the negative term1na 1 it 1s sa, q, ~~ I I I p: I 11 I I I 9 The direction of the applied voltage (V) is op.,!H'MU.L o t e otential barrier Vo. The width of depletion layer decreases. If the applied v , the current will be small. Due to the applied voltage, electr cross the depletion region and reach p- side (where they are minority carr ~,~ ly, holes from p-side cross the junction and reach the n-side (where th ·n carries). This process under forward bias is known as minority carrier injectio th junction, the minority carrier concentration increases. Due to this concentr , the injected electrons on p-side diffuse from the junction edge of -s other end of p-side. Similar process happens in n-side also. This motion o rriers on either side nt. e total diode forward current is sum of hole diffusion current and urrent due to electron diffusion. The magnitude of this current is usually in.ction diode under reverse bias:- When an external voltage (V) is applied across the diode such that n-side is positive and p-side is negative, it is said to be reverse biased.... P N r 4 The direction of applied voltage is same as the direction of barrier potential. Width of depletion region increase. Diffusion current decrease. The direction of the electric field across the junction is such that minority carriers in each side will be swept to its majority zone. This drift of carriers gives rise to current. The drift current is of the order of a few µA. 1-V characteristics in forward and reverse bias :- a) Forward bias Vol tn1eter(V) ---!'~----------, p n Milliammeter (mA) } Switch J...----------- (a) b)Reverse bias p n Microamn1eter (µA) Svlitch (b).,,~'- ~ ·1· n diode. ' h c) -I c ara cteristics of a s1 ,co 8  / (111/\l 100 ~o 60 40 20 20 100 80 60 40 o10.4n-0.6nr0.8o.1.0nr v(\' ) ~~ b~ +T.20.2 10 20 30 1 (µA) (c) slowly, till the voltage across the diod e In forward bias, the current first i crosses a certain value. After t ac ·stic voltage, the diode curr ent increases ~.. :- e in the forward bias voltage. Ttiis volt age is significantly even for a ve ("'0.2V for germanium diod e and -o. 7 V for called the threshold vo" 8~ cut voltage silicon diode). ~ '-.J to a small The dynami ~fi ned 1~ as as the ratio of small change in voltage /1V change in (NµA) and almost e e in reverse bias, the current Is very small constant with change In bias. called reverse saturation curren~ ft!s of the applied voltage. Even a small NOTE:- The diode reverse current Is independent from one side of the junction t~ the voltage Is sufficient to sweep the minority carriers t ",_,....,.,. ·---~- other side of the Junction. Junetton.  The current under reverse bias is essentially voltage independ~nt upto a critical reverse ~ias voltage, known as breakdown voltage (Vbr ). When V = Vbr, the diode reverse current increases sharply. Example 14.4 The V-1 characteristic of a silicon diode is shown in the Fig. 14.17. Calculate the resistance of the diode at (a) ID= 15 mA and (b) VD =-10 V. / (n1A) 30 20 , - - SIiicon -~~. 1................:. lO ·-- - : - o -~...._,_ _ _,,.. lµA _o.5 o.s -v(V) _. _ __ , -.-. : : ~~....;. -- - ,.,... -.. ,__ --- - --· ~.. --;..:._;f_-..?,._:,.,_-_;_____ ; __ barrier. (b) reduces the majority carrie c re~ro. (c) lowers the potential barrier. (d) None of the above. APPLICATION OF JU DIODE AS A RECTIFIER Rectifier converts AC t