6.25 The Tran­sis­tor

A sec­ond very im­por­tant semi­con­duc­tor de­vice be­sides the p-n diode is the tran­sis­tor. While the p-n diode al­lows cur­rents to be blocked in one di­rec­tion, the tran­sis­tor al­lows cur­rents to be reg­u­lated.

Fig­ure 6.35: Schematic of the op­er­a­tion of an n-p-n tran­sis­tor.

For ex­am­ple, an n-p-n tran­sis­tor al­lows the cur­rent of elec­trons through an n-type semi­con­duc­tor to be con­trolled. A schematic is shown in fig­ure 6.35. Elec­trons flow through the tran­sis­tor from one side, called the emit­ter, to the other side, called the col­lec­tor.

To con­trol this cur­rent, a very nar­row re­gion of p-type dop­ing is sand­wiched in be­tween the two sides of n-type dop­ing. This p-type re­gion is called the base. If the volt­age at the base is var­ied, it reg­u­lates the cur­rent be­tween emit­ter and col­lec­tor.

Of course, when used in a cir­cuit, elec­trodes are sol­dered to the emit­ter and col­lec­tor, and a third one to the base. The tran­sis­tor then al­lows the cur­rent be­tween the emit­ter and col­lec­tor elec­trodes to be con­trolled by the volt­age of the base elec­trode. At the same time, a well-de­signed tran­sis­tor will di­vert al­most none of the cur­rent be­ing reg­u­lated to the base elec­trode.

The tran­sis­tor works on the same prin­ci­ples as the p-n junc­tion of the pre­vi­ous sec­tion, with one twist. Con­sider first the flow of elec­trons through the de­vice, as shown in fig­ure 6.35. The junc­tion be­tween emit­ter and base is op­er­ated at a for­ward-bias volt­age dif­fer­ence. There­fore, the ma­jor­ity elec­trons of the n-type emit­ter pour through it in great num­bers. By the nor­mal logic, these elec­trons should pro­duce a cur­rent be­tween the emit­ter and base elec­trodes.

But here comes the twist. The p re­gion is made ex­tremely thin, much smaller than its trans­verse di­men­sions and even much smaller than the dif­fu­sion dis­tance of the elec­trons. Es­sen­tially all elec­trons that pour through the junc­tion blun­der into the sec­ond junc­tion, the one be­tween base and col­lec­tor. Now this sec­ond junc­tion is op­er­ated at a re­verse-bias volt­age. That pro­duces a strong elec­tric field that sweeps the elec­trons force­fully into the col­lec­tor. (Re­mem­ber that since the elec­trons are con­sid­ered to be mi­nor­ity car­ri­ers in the base, they get sweeped through the junc­tion by the elec­tric field rather than stopped by it.)

As a re­sult, vir­tu­ally all elec­trons leav­ing the emit­ter end up as an elec­tron flow to the col­lec­tor elec­trode in­stead of to the base one as they should have. The stu­pid­ity of these elec­trons ex­plains why the base volt­age can reg­u­late the cur­rent be­tween emit­ter and col­lec­tor with­out di­vert­ing much of it. Fur­ther, as seen for the p-n junc­tion, the amount of elec­trons pour­ing through the junc­tion from emit­ter to base varies very strongly with the base volt­age. Small volt­age changes at the base can there­fore dec­i­mate or ex­plode the elec­tron flow, and al­most all of it goes to the col­lec­tor.

There is one re­main­ing prob­lem, how­ever. The for­ward bias of the junc­tion be­tween emit­ter and base also means that the ma­jor­ity holes in the base pour through the junc­tion to­wards the emit­ter. And that is strictly a cur­rent be­tween the emit­ter and base elec­trodes. The holes can­not come from the col­lec­tor, as the col­lec­tor has vir­tu­ally none. The hole cur­rent is there­fore bad news. For­tu­nately, if you dope the p-type base only lightly, there are not that many ma­jor­ity holes, and vir­tu­ally all cur­rent through the emit­ter to base junc­tion will be car­ried by elec­trons.

A p-n-p tran­sis­tor works just like an n-p-n-one, but with holes tak­ing the place of elec­trons. There are other types of semi­con­duc­tor tran­sis­tors, but they use sim­i­lar ideas.

Key Points

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A tran­sis­tor al­lows cur­rent to be reg­u­lated.