Bipolar transistor with two different emitter portions having same type dopant of different concentrations for improved gain
View Patent ↗Insufficient gain in bipolar transistors ( 20 ) is improved by providing an alloyed (e.g., silicided) emitter contact ( 452 ) smaller than the overall emitter ( 42 ) area. The improved emitter ( 42 ) has a first emitter (FE) portion ( 42 - 1 ) of a first dopant concentration C FE , and a second emitter (SE) portion ( 42 - 2 ) of a second dopant concentration C SE . Preferably C SE ≧C FE . The SE portion ( 42 - 2 ) desirably comprises multiple sub-regions ( 45 i, 45 j, 45 k ) mixed with multiple sub-regions ( 47 m, 47 n, 47 p ) of the FE portion ( 42 - 1 ). A semiconductor-metal alloy or compound (e.g., a silicide) is desirably used for Ohmic contact ( 452 ) to the SE portion ( 42 - 2 ) but substantially not to the FE portion ( 42 - 1 ). Including the FE portion ( 42 - 1 ) electrically coupled to the SE portion ( 42 - 2 ) but not substantially contacting the emitter contact ( 452 ) on the SE portion ( 42 - 2 ) provides gain increases of as much as ˜278.
1. A bipolar transistor having a first surface, and comprising:
a base of a first conductivity type;
a collector of a second, opposite, conductivity type in contact with the base;
an emitter of the second conductivity type extending into the base at the first surface; and
a semiconductor-metal Ohmic emitter contact substantially on the emitter region;
wherein the FE portion has a FE dopant concentration C FE and the SE portion has a SE dopant concentration C SE of the same type dopant, and C SE ≧C FE .
2. The transistor of claim 1 , wherein C SE substantially equals C FE , and the SE portion is the portion of the emitter having thereon the Ohmic emitter contact.
3. The transistor of claim 1 , wherein the SE dopant concentration C SE and the FE dopant concentration C FE have a ratio C SE /C FE in a range of about C SE /C FE =1 to 10000.
4. The transistor of claim 1 , wherein the SE portion is laterally mixed with at least part of the FE portion.
5. The transistor of claim 1 , wherein a part of the FE portion is substantially laterally interspersed with the SE portion.
6. The transistor of claim 1 , wherein the FE portion comprises a plurality of first sub-emitters and the SE portion comprises a plurality of second sub-emitters that are at least in part adjacent to or mixed with the first sub-emitters or both.
7. The transistor of claim 6 , wherein the plurality of second sub-emitters are substantially electrically coupled in parallel.
8. The transistor of claim 1 , wherein the emitter comprises silicon and the semiconductor-metal Ohmic emitter contact comprises a silicide.
9. A semiconductor (SC) substrate, comprising:
an emitter, a base and a collector, wherein the emitter laterally has a total emitter area (TEA);
wherein the emitter has a first emitter (FE) portion laterally of a first emitter area (FEA) and a second emitter (SE) portion laterally of a second emitter area (SEA),
wherein the FE portion and the SE portion are electrically coupled to form the emitter; and
an Ohmic contact formed from a metal-SC compound or alloy substantially on the SE portion and not on the FE portion;
wherein the FE portion has a FE doping concentration C FE and the SE portion has a SE doping concentration C SE of the same type dopant and C SE ≧C FE .
10. The substrate of claim 9 , wherein the Ohmic contact has an Ohmic emitter contact area (ECA) less than the TEA.
11. The substrate of claim 9 , wherein the SE portion having thereon the Ohmic contact, comprises, multiple sub-emitters each with a part of the Ohmic contact thereon, said sub-emitters with the part of the Ohmic contact thereon being laterally intermingled with or adjacent to parts of the FE portion or both.