❀ 半導(dao)體(ti)(t🌳i)的導(dao)電性能(neng)比導(dao)體(ti)(ti)差而比絕緣體(ti)(ti)強。實際上，半導(dao)體(ti)(ti)與(yu)導(dao)體(ti)(ti)、絕緣體(ti)(ti)的區別在不僅在于導(dao)電能(neng)力的不同，更重要的是半導(dao)體(ti)(ti)具有獨特的性能(neng)（特性）。 

1．在純(chun)凈的(de)半(ban)導體(ti)中適(shi)當地摻入(ru)一定種(🉐zhong)類的(de)極微(wei)量的(de)雜質，半(ban)導體(ti)的(de)導電性(xing)能就會(hui)成百萬倍的(de)增加—-這是半(ban)導體(ti)最顯著、最突出(chu)的(de)特(te)(te)性(xing)。例如(ru)，晶體(ti)管就是利用(yong)這種(zhong)特(te)(te)性(xing)制(zhi)成的(de🔜)。 

2．當(dang)環(huan)境(jing)溫(wen)度升高一(yi)些時，半導(dao)體(ti)的(de)(de)導(dao)電(dian)能(neng)力(li)(li)就顯著地增(zeng)加；當(dang)環(huan)境(jing)溫(wen)度下降(jia🐎ng)一(yi)些時，半導(dao)體(ti)的(de)(💖de)導(dao)電(dian)能(neng)力(li)(li)就顯著地下降(jiang)。這種(zhong)特(te)性稱(cheng)為“熱敏”，熱敏電(dian)阻(zu)就是利用半導(dao)體(ti)的(de)(de)這種(zhong)特(te)性制成的(de)(de)。 

3．當(dang)有光(guang)(guang)(guang)線照射(she)在某些半(ban)導(dao)(dao)體(ti)(ti)(ti)(ti)時，這些半(ban)導(dao)(dao)體(ti)(ti)(ti)(ti)就像導(dao)(dao)體(ti)(ti)(ti)(ti)一(yi)樣，導(dao)(dao)電能力很(hen)強；當(dang)沒有光(guang)(guang)(guang)線照射(she)時，這些半(ban)導(dao)(dao)體(ti)(ti)(ti)(ti)就像絕緣體(ti)(ti)(ti)﷽(ti)一(yi)樣不導(dao)(dao)電，這種特性稱為“光(guang)(guang)(guang)敏”。例如，用作自動化控制用的(de)(de)“光(guang)(guang)(guang)電二極管(guan)”、“光(guang)(guang)(guang)電三極管(guan)”和光(guang)(guang)(guang)敏電阻等(deng)，就是利用半(ban)導(dao)(dao)體(ti)(ti)(ti)(ti)的(de)(de)光(guang)(guang)(guang)敏特性制成的(de)(de)。  由此(ci)可見，溫(wen)度和光(guang)(guang)(guang)照對(dui)晶(jing)(jing)體(ti)(ti)(ti)(ti)管(guan)的(de)(de)影響(xiang)很(hen)大。因此(ci)，晶(jing)(jing)體(ti)(ti)(ti)(ti)管(guan)不能放(fang)在高(gao)溫(wen)和強烈的(de)(de)光(guang)(guang)(guang)照環(huan)境中。在晶(jing)(jing)體(ti)(ti)(ti)(ti)管(guan)表面涂上一(yi)層(ceng)黑(hei)漆也是為了防止光(guang)(guang)(guang)照對(dui)它的(de)(de)影響(xiang)。最后，明確一(yi)個基本(ben)概驗：所(suo)謂半(ban)導(dao)(dao)體(ti)(ti)(ti)(ti)材(cai)料，是一(yi)種晶(jing)(jing)體(ti)(ti)(ti)(ti)結構的(de)(de)材(cai)料，故“半(ban)導(dao)(dao)體(ti)(ti)(ti)(ti)”又(you)叫“晶(jing)(jing)體(ti)(ti)(ti)(ti)”。 

P性(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)和N型(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)----前面講過，在純凈(jing)的(de)(de)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)中(zhong)(zhong)加入(ru)一(yi)♉定類(lei)型(xing)的(de)(de)微量雜(za)(za)質(zhi)，能使(shi)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)的(de)(de)導(dao)(dao)(dao)(dao)(dao)電(dian)能力(li)成(cheng)百(bai)萬倍(bei)的(de)(de)增加。加入(ru)了雜(za)(za)質(zhi)的(de)(de)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)可以分(fen)為兩(liang)種(zhong)類(lei)型(xing)：一(yi)種(zhong)雜(za)(za)質(zhi)加到(dao)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)中(zhong)(zhong)去(qu)后，在半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)中(zhong)(zhong)會產生大量的(de)(de)帶負(fu)電(dian)荷的(de)(de)自由電(dian)子(zi)，這種(zhong)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)叫(jiao)(jiao)做“N型(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)”（也叫(jiao)(jiao)“電(dian)子(zi)型(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)”）；另一(yi)種(zhong)雜(za)(za)質(zhi)加到(dao)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)中(zhong)(zhong)后，會產生大量帶正電(dian)荷的(de)(de)“空(kong)穴”，這種(zhong)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)叫(jiao)(jiao)“P型(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)”（也叫(jiao)(jiao)“空(kong)穴型(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)”）。例(li)如(ru)，在純凈(jing)的(de)(de)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)鍺(zang)中(zhong)(zhong)，加入(ru)微量的(de)(de)雜(za)(za)質(zhi)銻，就能形(xing)成(cheng)N型(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)。同樣，如(ru)果在純凈(jing)的(de)(de)鍺(zang)中(zhong)(zhong)，加入(ru)微量的(de)(de)雜(za)(za)質(zhi)銦，就形(xing)成(cheng)P型(xing)半(ban)(ban)(ban)(ban)導(dao)(dao)(dao)(dao)(dao)體(ti)(ti)。 

一個PN結構(gou)成晶體(ti)二極管(guan)----設法把(ba🍨)P型半導體(ti)（有大量(liang)(liang💟)的(de)帶正電(dian)荷(he)的(de)空穴）和(he)N型半導體(ti)（有大量(liang)(liang)的(de)帶負電(dian)荷(he)的(de)自由電(dian)子）結合在一起。

在P型(xing)半導體的N型(xing)半導體相結合(he)的地方，就會形成一個特殊的薄(bo)層，這個特殊的薄(bo)層就叫“PN結”。晶體二極(j🌠i)管(guan)實際(ji)上就是由一個PN結構成的。

例如，收音機中應用的(de)晶(jing)體二極管，其(qi)觸(chu)絲（即觸(chu)針）部分相當于P型半(ban)導體，N型鍺片就是(shi)N型半(ban)導體，他們之間的(de)接觸(chu)面就🦄是(shi)PN結。P端(duan)(duan)（或P端(duan)(duan)引出(chu)線）叫晶(jing)體二極管的(de)正(zheng)端(duan)(duan)（也稱正(zheng)極）。N端(duan)(duan)（或N端(duan)(duan)引出(chu)線）叫晶(jing)體二極管的(de)負(fu)端(duan)(duan)（也稱負(fu)極）。 

把(ba)正(zheng)(zheng)端(duan)連接電(dian)(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的(de)正(zheng)(zheng)極(ji)，把(ba)負端(duan)接電(dian)(ꩵdian)(dian)(dian)(dian)(dian)(dian)池(chi)的(de)負極(ji)，這是PN結(jie)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻值(zhi)就(jiu)小到只(zhi)有幾百歐姆(mu)了。因此，通過PN結(jie)的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)（I=U/R）就(jiu)很(hen)(hen)大。這樣(yang)的(de)連接方法(fa)（圖2a）叫(jiao)(jiao)“正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)連接”。正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)連接時，晶體(ti)(ti)管二(er)(er)(er)(er)極(ji)管（或(huo)PN結(jie)）兩(liang)端(duan)承受的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)叫(jiao)(jiao)“正(zheng)(zheng)向(xiang)(xi💙ang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)”；處在(zai)正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)下(xia)，二(er)(er)(er)(er)極(ji)管（或(huo)PN結(jie)）的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻叫(jiao)(jiao)“正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻”，在(zai)正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)下(xia)，通過二(er)(er)(er)(er)極(ji)管（或(huo)PN結(jie)）的(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)叫(jiao)(jiao)“正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)”。很(hen)(hen)明顯，因為晶體(ti)(ti)二(er)(er)(er)(er)極(ji)管的(de)正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻很(hen)(hen)小（幾百歐姆(mu)），在(zai)一(yi)定(ding)正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)下(xia)，正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)（I=U/R）就(jiu)會很(hen)(hen)大----這表明在(zai)正(zheng)(zheng)向(xiang)(xiang)(xiang)(xiang)(xiang)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)下(xia)，二(er)(er)(er)(er)極(ji)管（或(huo)PN結(jie)）具有像導體(ti)(ti)一(yi)樣(yang)的(de)導電(dian)(dian)(dian)(dian)(dian)(dian)(dian)本領。 

反過來，如果把P端(duan)接到(dao)電(dian)(dian)(dian)池的負極，N端(duan)接到(dao)電(dian)(dian)(dian)池的正極（見(jian)圖2b）。這(zhe)時PN結(jie)(jie)的電(dian)(dian)(dian)阻很大（大到(dao)幾(ji)百(bai)(bai)千毆(ou)），電(dian)(dian)(dian)流（I=U/R）幾(ji)乎不(bu)能(neng)通(tong)(tong)(tong)過二(er)極管，或者說(shuo)通(tong)(tong)(tong)過的電(dian)(dian)(dian)流很微弱。這(zhe)樣的連接方法(fa)叫“反向(xiang)(xiang)(xiang)連接”。反向(xiang)(xiang)(xiang)連接時，晶(jing)體管二(er)極管（或PN結(jie)(jie)）兩端(duan)承受的電(dian)(dian)(dian)壓(ya)叫“反向(xiang)(xiang)(xiang)電(dian)(dian)(dian)壓(ya)”；處在反向(xiang)(xiang)(xiang)電(dian)(dian)(dian)壓(ya)下，二(er)極管（或PN結(jie)(jie)）的電(dian)(dian)(dian)阻叫“反向(xiang)(xiang)(xiang)電(dian)(dian)(dian)阻”，在反向(xiang)(xiang)(xiang)電(dian)(dian𝐆)(dian)壓(ya)下，通(tong)(tong)(tong)過二(er)極管（或PN結(jie)(jie)）的電(dian)(dian)(dian)流叫“反向(xiang)(xiang)(xiang)電(dian)(dian)(dian)流”。顯然，因為晶(jing)體二(er)極管的正向(xiang)(xiang)(xiang)電(dian)(dian)(dian)阻很大（幾(ji)百(bai)(bai)千歐姆），在一(yi)定的反向(xiang)(xiang)(xiang)電(dian)(dian)(dian)壓(ya)下，正向(xiang)(xiang)(xiang)電(dian)(dian)(dian)流（I=U/R）就會很小，甚至可(ke)以忽略不(bu)計(ji)，----這(zhe)表明(ming)在一(yi)定的反向(xiang)(xiang)(xiang)電(dian)(dian)(dian)壓(ya)下，二(er)極管（或PN結(jie)(jie)）幾(ji)乎不(bu)導電(dian)(dian)(dian)。 

上敘實驗說明(ming)這樣一個結論：晶體二(er💃)極管（或(huo)PN結）具有單向(xiang)導(dao)電特性。 

晶(jing)體二極管(guan)ꦫ用(yong)字母“D”代(dai)表(biao)，在電(dian)路中(zhong)常用(yong)圖3的符(fu)號表(biao)示，即表(biao)示電(dian)流(liu)（正電(dian)荷）只能順(shun)著箭頭(tou)方(fang)向(xiang)(xiang)流(🌌liu)動，而不能逆著箭頭(tou)方(fang)向(xiang)(xiang)流(liu)動。

利用二(er)極(ji)管(guan)(guan)的(de)單向導電性可(ke)以用來整流(liu)（將交流(liu)電變(bian)成(cheng)(cheng)直(zhi)流(liu)電）和(he)檢波（從高(gao)(gao)(gao)頻(pin)(pin)(pin)或(huo)中頻(pin)(pin)(pin)電信號取出(chu)音頻(pin)(pin)(pin)信號）以及變(bian)頻(pin)(pin)(pin)（如(ru)把高(gao)(gao)(gao)頻(pin)(pin)(pin)變(bian)成(cheng)(cheng)固定的(de)中頻(pin)(pin)(pin)465千周(zhou)）等。  PN結的(de)極(ji)間(jian)🍌電容(rong)----PN結的(de)P型(xing)和(he)N型(xing)ꦅ兩快(kuai)半導體(ti)之間(jian)構成(cheng)(cheng)一(yi)(yi)個電容(rong)量很小的(de)電容(rong)，叫做(zuo)“極(ji)間(jian)電容(rong)”。由于電容(rong)抗隨頻(pin)(pin)(pin)率的(de)增高(gao)(gao)(gao)而減小。所以，PN結工(gong)作(zuo)于高(gao)(gao)(gao)頻(pin)(pin)(pin)時，高(gao)(gao)(gao)頻(pin)(pin)(pin)信號容(rong)易被極(ji)間(jian)電容(rong)或(huo)反饋而影(ying)響(xiang)PN結的(de)工(gong)作(zuo)。但在(zai)直(zhi)流(liu)或(huo)低(di)頻(pin)(pin)(pin)下工(gong)作(zuo)時，極(ji)間(jian)電容(rong)對(dui)直(zhi)流(liu)和(he)低(di)頻(pin)(pin)(pin)的(de)阻抗很大，故(gu)一(yi)(yi)般不(bu)會影(ying)響(xiang)PN結的(de)工(gong)作(zuo)性能。PN結的(de)面積(ji)越(yue)大，極(ji)間(jian)電容(rong)量越(yue)大，影(ying)響(xiang)也約(yue)大，這(zhe)就是(shi)面接觸(chu)型(xing)二(er)極(ji)管(guan)(guan)（如(ru)整流(liu)二(er)極(ji)管(guan)(guan)）和(he)低(di)頻(pin)(pin)(pin)三極(ji)管(guan)(guan)不(bu)能用于高(gao)(gao)(gao)頻(pin)(pin)(pin)工(gong)作(zuo)的(de)原因。