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  • Bi ging K thut in t tng t

    1. CHT BN DN NGUYN CHT V CHT BN DN TP CHT

    1. Cht bn dn nguyn cht (cht bn dn thun)

    Hai cht bn dn thun in hnh l Si (14) v Ge (32), chng c c im chung l

    lp ngoi cng u c bn in t ho tr. Ta xt nguyn t Si trong mng tinh th .

    Khi vt liu Si c ch to thnh

    tinh th th t trng thi xp xp ln

    xn chng tr thnh trng thi hon

    ton trt t. Khi khong cch gia

    cc nguyn t cch u nhau.

    - Bn in t lp ngoi cng ca

    mt nguyn t khng nhng chu s

    rng buc vi ht nhn ca chnh

    nguyn t m cn lin kt vi bn

    nguyn t ng cnh n, hai nguyn t ng cnh nhau c mt cp in t gp

    chung.

    - Mi mt in t trong i gp chung va chuyn ng xung quanh ht nhn ca

    n va chuyn ng trn qu o ca in t gp chung. S lin kt ny c gi l

    lin kt ng ho tr.

    nhit xc nh, do chuyn ng nhit, mt s in t gp chung d dng

    tch khi mi lin kt vi ht nhn tr thnh cc in t t do, l ht dn in t.

    - Khi mt in t tch ra tr thnh in t t do th li mt lin kt b khuyt (l

    trng). Khi cc in t gp chung i k cnh d dng b ri vo l trng to

    thnh s di chuyn ca cc in t gp chung. S di chuyn ny ging nh s di

    chuyn ca cc in tch dng, l s di chuyn ca l trng.

    Nh vy, l trng cng l loi ht mang in. Khi t mt in trng ln vt

    liu bn dn th xut hin hai thnh phn dng in chy qua n: thnh phn dng in

    do cc in t t do chuyn ng c hng v thnh phn dng in l trng do in

    t gp chung dch lp l trng. in t t do mang in m, l trng mang in

    dng.

    Cc in t chuyn ng ngc chiu vi vc t cng in trng cn cc l

    trng th chuyn ng cng chiu to nn dng in trong cht bn dn.

    Si

    Si

    Si

    Si

    Si

    Si

    Si

    Si

    Si

    L trng

    in t t do

    Lin kt ng ho tr

  • Bi ging K thut in t tng t

    * Nh vy: Bn dn m dn xut c thc hin bng c hai loi ht mang in (in

    t t do v l trng) c s lng bng nhau c gi l cht bn dn thun (bn dn

    nguyn cht).

    2. Cht bn dn tp cht

    nng cao tnh dn in trong vt liu bn dn, ta thc hin pha thm tp cht

    vo cht bn dn nguyn cht, gi l cht bn dn tp.

    2.1. Cht bn dn tp loi P

    Ta pha thm tp cht l nhng nguyn t thuc nhm III trong bng tun hon

    (Ga, In) vo trong mng tinh th ca nguyn t Si. Khi trong mng tinh th, mt

    s nguyn t Ga s thay th v tr mt s nguyn t Si, ba in t ho tr ca Ga s

    tham gia vo ba mi lin kt vi ba nguyn t Si bn cnh, cn mi lin kt vi

    nguyn t Si th t b thiu mt in t c coi nh mt l trng. Cc mi lin kt b

    thiu mt in t ny d dng c lp y bi mt in t c bn ra t cc mi

    lin kt bn cnh b ph v, nh vy l trng c th di chuyn c, to thnh dng

    in.

    - Khi nhit tng ln s mi

    lin kt b ph v cng nhiu lm cho

    s lng in t t do v l trng

    tng. Nhng bn dn c pha thm

    cc tp cht thuc nhm III th s

    lng cc l trng bao gi cng ln

    hn s lng cc in t t do.

    * Nh vy:

    Vt liu bn dn m dn xut c thc hin ch yu bng cc l trng gi l cht

    bn dn tp loi P. L trng gi l ht dn in a s. in t t do l ht dn in

    thiu s.

    2.2. Cht bn dn tp loi N

    Ta pha thm cc nguyn t thuc nhm V trong bng tun hon (As, P) vo

    trong cu trc mng tinh th ca nguyn t Si. Khi mt s nguyn t P s thay th

    mt s v tr nguyn t Si trong mng tinh th.

    Si

    Si

    Si

    Si

    Ga

    a Si

    Si

    Si

    Si

    L trng

    in t t do

    Cp in t t do-l trng

    +3

  • Bi ging K thut in t tng t

    Nguyn t P c nm in t ho

    tr, bn trong nm in t ho tr s

    tham gia vo bn mi lin kt vi bn

    nguyn t Si ng xung quanh n, cn

    in t ho tr th nm khng tham

    gia vo mi lin kt no m chu s

    rng buc rt yu vi ht nhn, chng

    d dng tch khi mi lin kt vi ht

    nhn tr thnh cc in t t do v

    s tham gia vo vic vn chuyn dng

    in.

    Khi nhit tng ln, s mi lin kt b ph v cng tng sinh ra nhiu cp in t

    t do - l trng. Nhng cht bn dn pha thm tp cht thuc nhm V th s lng

    cc in t t do bao gi cng ln hn s lng cc l trng.

    * Nh vy, loi bn dn m dn xut c thc hin ch yu bng cc in t t

    do gi l cht bn dn tp loi N. in t t do l ht dn a s, l trng l ht dn

    thiu s.

    2. TIP GIP P - N. TNH CHT CHNH LU CA IT BN DN

    1. Tip gip P-N khi cha c in trng ngoi

    Khi cho hai khi bn dn P v N tip xc

    cng ngh vi nhau, gia hai khi bn dn hnh

    thnh mt mt tip xc P-N, do s chnh lch v

    nng ht dn gia hai khi s xy ra s

    khuych tn. Cc l trng khi P s khuych

    tn sang khi N v cc in t t khi N s

    khuych tn sang khi P.

    Kt qu lm cho b mt gn lp tip gip ca khi P ngho i v in tch dng v

    giu ln v in tch m. B mt gn lp tip gip ca khi N mt in tch m v nhn

    thm l trng nn tch in dng. Nu s chnh lch v nng cc loi ht mang

    in hai khi ny cng ln th s khuch tn din ra cng mnh.

    P N - - - -

    + + + + Et.xc Ik.tn

    Itri

    Mt tip xc

    Ut.xc

    l0

    U

    Si

    Si

    Si

    Si

    P

    Si

    Si

    Si

    Si

    in t t do +5

  • Bi ging K thut in t tng t

    Kt qu: Hai bn mt tip gip hnh thnh nn in trng vng tip xc Etx c

    chiu hng t khi N sang khi P. in trng tip xc ny cn tr s khuych tn

    ca cc ht mang in a s t khi ny sang khi kia. Khi Etx cn bng vi lc

    khuych tn th trng thi cn bng ng xy ra. Khi vng in tch khng gian

    khng tng na, vng ny gi l vng ngho kit (vng thiu vng ht dn in) l

    chuyn tip P-N bao gm cc ion khng di chuyn c. Khi cn bng ng, c bao

    nhiu ht dn in khuych tn t khi ny sang khi kia th cng by nhiu ht dn

    c chuyn tr li qua mt tip xc, chng bng nhau v tr s nhng ngc chiu

    nhau nn chng trit tiu nhau, kt qu dng in qua tip xc P-N bng 0.

    Kt lun: Khng c dng in chy qua lp tip gip P N khi cha c in trng

    ngoi.

    2. Tip gip P- N khi c in trng ngoi

    2.1. Trng hp phn cc thun

    t in p mt chiu vo tip gip P-N sao cho cc dng ni vo khi P, cc

    m ni vo khi N. in p ny to ra mt in trng ngoi Eng c chiu hng t

    khi P sang khi N. Khi in trng ngoi Eng c chiu ngc vi in trng vng

    tip xc Etx nn in trng tng vng tip xc gim.

    E = Etx Eng gim. Khi b rng

    vng ngho gim lm cho s khuych tn

    din ra d dng. Cc ht mang in a s d

    dng khuych tn t khi ny sang khi kia.

    Do mt ht mang in a s ln nn dng

    khuych tn Ikt ln, dng in ny gi l

    dng in thun Ith. Ta ni tip gip P-N

    thng.

    Trong :

    l0: B rng vng ngho khi cha c in trng ngoi

    l0: B rng vng ngho khi phn cc thun

    Do s lng ht dn thiu s t, nn dng in tri dt rt nh, Itr 0. in tr

    tip gip P- N trong trng hp ny gi l in tr thun, c gi tr nh Rth 0.

    P N - - - -

    + + + + Et.xc

    l0

    Ut.xc

    Ut.xc - Ungoi

    U

    Engoi

    (+) (-)

    IThun

    l0

    Ungoi

  • Bi ging K thut in t tng t

    2.2. Trng hp phn cc ngc

    t cc dng vo khi N, cc m vo

    khi P. Khi Eng cng chiu vi Etx nn

    in trng tng vng tip xc tng, do

    b rng vng ngho tng, n ngn cn cc ht

    dn a s khuch tn t khi ny sang khi

    kia, do vy dng khuch tn coi Ikt = 0. Dng

    in tri c gi tr nh do s ht dn thiu s

    rt t, Itr = 0, nn dng in qua tip gip P-N

    khi phn cc ngc c gi tr bng 0.

    Ta ni tip gip P-N b kho, trong trng hp ny tip gip P-N coi nh mt

    in tr c gi tr v cng ln gi l in tr ngc, Rng .

    * Nh vy: Tip gip P-N ch c tc dng dn in theo mt chiu (t khi P

    sang khi N) khi c phn cc thun. Tnh cht ny gi l tnh cht van hay tnh cht

    chnh lu, l tnh cht chnh lu ca it bn dn.

    3. it bn dn.

    3.1. Cu to, k hiu.

    it thc cht l mt tip gip P-N. in cc ni vi khi P c gi l Ant (k

    hiu l A), in cc ni vi khi N gi l Katt (k hiu l K), ton b cu trc trn

    c bc trong mt lp v bng kim loi hay bng nha.

    * Nguyn l lm vic: Chnh l cc hin tng vt l xy ra tip gip P-N trong

    cc trng hp: cha phn cc, phn cc thun v phn cc ngc xt trn.

    3.2. c tuyn V-A.

    c tuyn V-A c chia lm 3 vng:

    + Vng : ng vi trng hp phn cc

    thun. Khi tng UAK , lc u dng tng t t,

    sau khi UAK > U0 (thng U0 = (0,60,7)V nu

    it c ch to t vt liu Silic,

    P N Ant Katt

    Hnh a: Cu to

    A K

    Hnh b: K hiu

    Ungoi

    P N - - - -

    + + + + Et.xc

    l0

    Ut.xc+ Ungoi

    Ut.xc

    U

    Engoi

    (-) (+)

    l0

    Ingc

    UAK

    )1( TAKu

    ngbhth eIi

    0

    Ungc Ung.max

    Uthu

    n

    IA

    Ingc

  • Bi ging K thut in t tng t

    U0 = (0,20,3)V nu it c ch to t vt liu Gecmani) th dng in tng theo

    in p vi quy lut ca hm s m.

    + Vng : Tng ng vi trng hp phn cc ngc vi gi tr dng in

    ngc ing c gi tr nh (ing Ibha).

    + Vng : Gi l vng nh thng, tng ng Ung > Ung.max (Unh thng).

    Dng in ngc tng ln t ngt, dng in ny s ph hng it (v vy

    bo v it th ch cho chng lm vic di in p: U = (0,7 0,8).Uz , Uz l in p

    nh thng) trong khi in p gia Ant v Katt khng i tnh cht van ca

    it b ph hng. Tn ti hai dng nh thng: do nhit cao v in trng mnh

    lm cho cc ht dn chuyn ng nhanh, gy va p v gy nn hin tng ion ho do

    va chm lm cho qu trnh to thnh ht dn t, dn n dng in tng n