Methods of forming a bipolar transistor having a collector with a doping spike
This disclosure relates to methods of forming bipolar transistors, such as heterojunction bipolar transistors. The methods may include forming a sub-collector over a substrate, forming a first portion of a collector over the sub-collector and doping a second portion of the collector to form a doping spike. The method may further include forming a third portion of the collector over the doping spike and forming a base of the bipolar transistor over the third portion of the collector.
1. A method of forming a bipolar transistor comprising:
forming a sub-collector over a substrate;
forming a first portion of a collector over the sub-collector, the first portion having a varying doping concentration across a cross section of the first portion;
doping a second portion of the collector to form a doping spike;
forming a third portion of the collector over the doping spike; and
forming a base of the bipolar transistor over the third portion of the collector.
2. The method of claim 1 wherein the bipolar transistor is a single heterojunction bipolar transistor.
3. The method of claim 2 wherein the doping spike extends substantially an entire length of the collector in a direction substantially parallel to a base-collector interface.
4. The method of claim 3 further comprising:
doping a fourth portion of the collector disposed between the doping spike and the subcollector, the fourth portion comprising a substantially uniform doping and extending substantially an entire length of the collector in the direction substantially parallel to the base-collector interface.
5. The method of claim 3 wherein the doping spike is disposed within 0.5 μm from the base-collector interface.
6. The method of claim 3 wherein the collector further includes a grading that begins at a point farther from the base-collector interface than the doping spike.
7. The method of claim 1 wherein the thickness of the doping spike is substantially equal to 150 Å.
8. The method of claim 1 wherein the thickness of the doping spike is substantially equal to 100 Å.
9. The method of claim 1 wherein the doping spike is disposed at a location that results in approximately a local maximum of BV CEO of the bipolar transistor.
10. The method of claim 1 wherein the bipolar transistor is a GaAs transistor.
11. The method of claim 1 wherein the bipolar transistor is a SiGe transistor.
12. The method of claim 1 wherein the collector has a collector thickness from a base-collector interface to a collector-subcollector interface, and the doping spike is located within ⅓ of the collector thickness from the base-collector interface.