Fabrication of self aligned base contacts for bipolar transistors

1. A method of forming a self aligned base contact for a transistor, the method comprising:

forming a collector layer on a substrate;

forming a base layer on the collector layer;

forming an emitter layer on the base layer;

forming a first metal on a first emitter area of the emitter layer of the transistor;

forming a second metal on a second emitter area of the emitter layer of the transistor, wherein the first metal and the second metal are not in direct physical contact;

etching the emitter layer to remove the emitter layer surrounding the first emitter area, wherein the first emitter area is masked by the first metal, and to remove the emitter layer surrounding the second emitter area, wherein the second emitter area is masked by the second metal, so that the base layer is exposed surrounding the first emitter area and the second emitter area; and

depositing base metal on the base layer, on the second metal, over sides of the second metal, and over sides of the second emitter area, so that the base metal on the base layer, the base metal on the second metal, the base metal over sides of the second metal, and the base metal over sides of the second emitter area are in contact with each other;

wherein the first emitter area is an emitter of the transistor and the first metal provides an emitter contact; and

wherein the base metal provides a base contact; and

wherein the base metal is not in direct physical contact with the first metal or the first emitter area.

2. The method of claim 1 wherein:

the steps of forming the first metal on the first emitter area of the emitter layer of the transistor and forming the second metal on the second emitter area of the emitter layer of the transistor are performed simultaneously.

3. The method of claim 1 further comprising:

etching the base layer and the collector layer of the transistor to form a mesa.

4. The method of claim 1 wherein the step of etching comprises a wet etch, a dry etch, or a combination of a wet and dry etch.

5. The method of claim 1 further comprising:

depositing a dielectric over the base layer, the base metal, the first metal on the first emitter area, and the second metal on the second emitter;

etching the dielectric to expose a top of the first metal and to expose a top of the base metal on the second metal;

depositing emitter contact metal on the first metal to form a transistor emitter contact; and

depositing base contact metal on the base metal to form a transistor base contact.

6. The method of claim 1 wherein the base metal completely covers the second metal.

7. The method of claim 1 wherein:

the base metal formed on the base layer at least partially surrounds the first emitter area; and

wherein the base metal is not in direct physical contact with the first emitter area.

8. The method of claim 7 wherein the base metal formed on the base layer is on three sides of the first emitter area.

9. The method of claim 1 wherein:

the base metal formed on the base layer completely surrounds the first emitter layer area; and

wherein the base metal is not in direct physical contact with the first emitter layer area.

10. The method of claim 1 wherein the transistor is a bipolar junction transistor, an InP heterojunction bipolar transistor (HBT), or an GaAs HBT.

11. The method of claim 1 wherein the second emitter area is the same as or smaller in size than the first emitter area.

12. The method of claim 1 wherein:

the first metal and the second metal are formed through an additive process comprising image reverse lithography, metal evaporation, a lift-off process or electro-plating, or a subtractive process comprising blanket metal deposition, etch mask definition by lithography, and an emitter metal etch.

13. The method of claim 1 wherein:

the base metal is formed through an additive process comprising image reverse lithography, metal evaporation, a lift-off process, or electro-plating, or a subtractive process comprising blanket metal deposition, etch mask definition by lithography, and a base metal etch.

14. The method of claim 1 wherein depositing base metal on the base and on the second metal further comprises forming a dielectric on sides of the first emitter area and the first metal prior to depositing the base metal.