Appendix B: Transistors

Transcription

1 Aedix B: Trasistors Of course, the trasistor is the most imortat semicoductor device ad has eabled essetially all of moder solid-state electroics. However, as a matter of history, electroics bega with vacuum tubes. As idicated reviously, i 1880 Thomas diso discovered that a two-elemet vacuum tube exhibits asymmetric coductio of curret, viz., the so-called diso effect. ve so, it was ot util the early twetieth cetury that the British hysicist, Joh Ambrose Flemig, discovered that the diso effect could be used to detect radio waves. Accordigly, Flemig develoed ad ateted a twoelemet vacuum tube, which came to be kow as the diode. Subsequetly, threeelemet vacuum tubes or triodes were develoed i the first decade of the twetieth cetury by Lee de Forest ad others. Most imortatly, triodes eabled develomet of the first true electroic amlifiers resultig i great imrovemet of telehoy as well as radio trasmitters ad receivers. Physically, a vacuum tube oerates by biasig two electrodes, covetioally called the filamet ad the late. oeratio, the filamet is heated to a high temerature such that electros ca easily be thermioically emitted ito the vacuum. Accordigly, if the late is ositive with resect to the filamet, electros are extracted ad curret flows. oversely, if the late is egative with resect to the filamet, curret does ot flow. This is essetially the diso effect. Moreover, as asserted above, a two-elemet tube is a diode; however, if a third electrode, called the grid (sice it is usually a wire mesh or scree) is istalled betwee the filamet ad late, a bias voltage laced o the grid ca modulate curret flow. t is this field effect that allows a triode to oerate as a electroic amlifier. As early as 195 it was recogized that field effect might also occur withi crystallie solids. deed, the first atet for a field effect trasistor (or FT) was filed i aada by Austria-Hugaria hysicist, Julius dgar Liliefeld. Subsequetly, i 1934 Germa hysicist, Oskar Heil, also ateted a field effect trasistor of differet desig. ve so, o ractical devices were ever built or tested. This is most likely a cosequece of the lack of high urity semicoductor materials at that time. The develomet of high quality semicoductor crystals was motivated by the eed durig the Secod World War for fast diodes, which were used i radars as a frequecy mixer elemet i microwave receivers. Vacuum tubes were foud to be too slow, but solid-state diodes fabricated from extremely ure germaium were foud to be suitable. After the war, Joh Bardee ad Walter Brattai workig uder William Shockley at Bell Telehoe Laboratories, succeeded i buildig the first oeratioal crystal triode, i.e., trasistor, i However, this was ot a FT, but rather a oit-cotact trasistor. Although commercialized by the Wester lectric omay, oit-cotact trasistors were ufortuately foud to be too fragile ad were soo relaced by the juctio trasistor iveted by Shockley i Subsequetly, the juctio field effect trasistor or JFT was also successfully fabricated. (Actually, Bardee, Brattai, Shockley, ad others were tryig to fabricate a JFT whe the oit-cotact trasistor was discovered sice the JFT more closely resembles Liliefeld s origial cocetio.) Solid-state electroics was domiated by juctio devices util the 1960 s, but these subsequetly have bee sulated by the metal-oxide-semicoductor field effect trasistor or MOSFT. The first ractical MOSFT was iveted ad ateted i 1959 by awo Kahg ad Marti M. (Joh) Atalla, agai, at Bell Telehoe Laboratories.

2 As the ame suggests, this device combies a MOS caacitor with a -juctio. Accordigly, MOSFT s are both structurally ad oeratioally differet from juctio trasistors. Withi this cotext, devices are made by fabricatig a isulatig layer o the surface of a semicoductor cotaiig a -juctio which defies the coductive chael. As for a simle MOS caacitor, a gate electrode is formed o the to of the isulator. Tyically, the semicoductor is crystallie silico ad the isulator is thermally oxidized silica. As oted reviously, for this material combiatio the desity of localized electro tras at the Si/SiO iterface ca be quite low. osequetly, wellmade silico MOSFT s are iheretly free from traig ad scatterig of carriers. Biolar Trasistors Both oit-cotact ad juctio trasistors are biolar, which meas that electros ad holes are majority carriers i differet arts of the device; hece, curret flowig through the device is carried by both electros ad holes. Physically, a biolar juctio trasistor or BJT cosists of two back-to-back -juctios that are i close roximity such that deletio regios iteract. Accordigly, there are two kids of biolar trasistors, viz., ad. These desigatios secify the arragemet of the iteractig juctios. A schematic reresetatio of a BJT is show i the followig figure: B Fig. 71: Schematic of a BJT Of course, a is idetical excet that door ad accetor doig is iverted. Here,, B, ad deote emitter, base, ad collector coectios. (These desigatios desced from the origial oit-cotact trasistor for which i articular the base was a slab of semicoductor, viz., germaium, to which emitter ad collector oit cotacts were made.) ocomitatly, the two juctios are called base-emitter ad base-collector juctios. ormal oeratio, for a trasistor the base-emitter juctio is forward biased (i.e., base is ositive with resect to the emitter) ad the base-collector juctio is reverse biased (i.e., base is egative with resect to the emitter). A BJT is a curret cotrolled device ad i ormal oeratio is govered by the equatios: qvb kt 0e 1

3 1 B (1 ) 1 1 Here, ad are idetified as commo base ad commo emitter curret gais, resectively. learly, these two arameters are ot ideedet ad, as such, are determied by the hysical characteristics of the trasistor structure. The value of is usually close to uity, e.g., to 0.998, which imlies that has a value betwee 49 ad 499. Withi this cotext, it is clear that is determied by the diode equatio for which V B is to be iterreted as the otetial differece across the base-emitter juctio. Tyically, this is just the diffusio otetial ad, hece, is 0.5 to 0.7 volts i ormal oeratio. (Obviously, 0 remais defied as reverse saturatio curret just as i the case of a simle diode.) For comleteess, if both juctios are forward biased, the trasistor is said to be saturated ad if both juctios are reversed biased the trasistor is said to be cut-off. saturatio, curret flowig through the device is large ad essetially ideedet of base curret or V B. oversely, i cut-off oly very small leakage currets flow. t is ossible to oerate the device i iverted mode i which the basecollector juctio is forward biased ad the base-emitter juctio is reverse biased. ricile if the device is exactly symmetric ormal ad iverted oeratio would have idetical characteristics; however, as is evidet from the figure, BJT s are geerally asymmetric ad iverted oeratio exhibits iferior characteristics. Uiolar Trasistors cotrast to biolar trasistors all kids of FT s are uiolar, which meas that curret is carried through the device either by electros, viz., -chael, or holes, viz., - chael. this regard, as asserted reviously FT s resemble a vacuum tube triode, which are also uiolar devices, i.e., curret is carried by electros. A schematic of a - chael JFT is show below: S G Fig. 7: Schematic of a -chael JFT

4 learly, with the excetio of exteral coectios this figure is essetially idetical to revious figure of a trasistor ad, thus, also cosists of back-to-back juctios; however, oeratio is quite differet. ( ractice, the dee -tye regio is reverse biased ad, as such, serves to cofie carriers, viz., holes, to the chael.) Of course, i aalogy to a BJT a -chael JFT corresods to iversio of door ad accetor doig. As a matter of covetio, S, G, ad deote source, gate, ad drai coectios. ocomitatly, if the gate is relatively ubiased ad if the source is more ositive tha the drai, curret ca be exected to flow through the chael betwee source ad drai. However, if the gate is biased ositively with resect to the chael, i.e., the gate-chael juctio is reverse biased, the the deletio regio exads ad reduces effective coductivity of the chael. deed, if the chael is sufficietly thi, the deletio regio ca exted all the way across the chael, thus, effectively ichig it off so that very little curret flows. this case, the JFT is said to be off. geeral, this kid of oeratio is characteristic of a deletio mode FT. learly, if the gate-chael juctio is forward biased, a udesirably large curret flows through the gate ito the chael. Therefore, JFT s are geerally ot oerated i this mode. However, if the gate is isulated as i a MOSFT, oeratio i ehacemet mode is ossible. Such a device is illustrated as follows: S G Fig. 73: Schematic of a ehacemet mode -chael MOSFT Here, the light gray regio is a isulator, tyically formed of thermal oxide, although for very small devices other tyes of dielectrics may be used. Obviously, as asserted reviously, a MOSFT is the combiatio of a MOS caacitor ad -juctios. ay case, if the surface is accumulated or deleted uder the MOS structure, very little curret, i.e., oly leakage curret, ca flow betwee the source ad drai. However, as the surface becomes iverted the surface becomes -tye ad substatial curret ca flow. deed, i strog iversio the device becomes saturated i aalogy to a BJT. Physically, source-drai curret, is determied by the bias voltage alied to the gate. Therefore, a FT is a voltage cotrolled device. the case that the chael is accumulated or deleted, is aroximated by the exressio: 0 e q( V GS Vt ) kt

5 Here, V GS is the otetial differece betwee gate ad source coectios ad V t is threshold voltage. ractice, V t is determied by device structure ad broadly corresods to the otetial associated with miimum caacitace of the MOS structure. ocomitatly, the ideality factor,, corresods to the formula: 1 s ox Of course, ox are s are oxide ad deletio caacitaces er uit area defied just as for a simle MOS caacitor. Naturally, the recedig exressio is characteristic of cut-off ad corresods to V GS < V t. Accordigly, i ormal oeratio, i.e., V GS > V t ad V S < (V GS V t ), ca be rereseted as follows: ox w ( V l GS V ) V t S V S As might be exected, V S is the otetial differece betwee drai ad source coectios, is effective carrier mobility, ad w ad l are surface width ad legth dimesios of the chael. Naturally, i saturatio, i.e., V GS > V t ad V S (V GS V t ), becomes ideedet of V S, hece: ox w ( V l GS Vt ) This exressio does ot accout for aomalies ad is alicable oly to ideal devices for which l is much greater tha w. For comleteess, a deletio mode MOSFT is illustrated by the followig figure, thus: S G Fig. 74: Schematic of a deletio mode -chael MOSFT learly, oeratio of this device ca be exected to be similar to a -chael JFT, that is to say, if the gate is sufficietly egative with resect to the chael, the semicoductor surface is deleted ad the chael becomes iched off.

6 State-of-the-Art evices coclusio, trasistor structure has rogressed far beyod the simle ideal structures described reviously. The curret state-of-the-art is embodied by devices with chael legths of 3 to m or less. The former is still of laar desig, but icludes modificatios such as a stressed chael ad high-k gate dielectric. The reaso for usig stressed materials is to icrease carrier mobility. deed, it has log bee kow that stress causes slittig of the bad structure, which is characterized by sub-bads i which carriers have lower effective mass ad, thus, higher mobility. As a ractical matter, stress ca be iduced by eitaxial deositio of SiGe alloy o a ure silico substrate. this case, the SiGe alloy has a slightly larger lattice arameter tha ure silico, which results i biaxial stress i the deosited layer. additio, stress may be modified ad cotrolled by deositio of itrisically stressed dielectric layers over the chael. As oted elsewhere, high-k gate dielectric is eeded to reduce gate to chael leakage curret which is iheret i ordiary thermal silica. Reductio of chael legth to m or less requires eve more radical modificatio of trasistor structure. this case, a three dimesioal FiFT structure has bee itroduced. Such a structure is characterized by a thi blade or fi of silico, which is almost comletely isolated from the substrate. The gate ca the wra aroud the fi o three sides, a so-called tri-gate structure, thus, resultig i more effective cotrol of the chael.