Bipolar junction transistor having a high germanium concentration in a silicon-germanium layer and a method for forming the bipolar junction transistor
A method for forming a germanium-enriched region in a heterojunction bipolar transistor and a heterojunction bipolar transistor comprising a germanium-enriched region. A base having a silicon-germanium portion is formed over a collector. Thermal oxidation of the base causes a germanium-enriched region to form on a surface of the silicon-germanium portion subjected to the thermal oxidation. An emitter is formed overlying the germanium-enriched portion region. The germanium-enriched region imparts advantageous operating properties to the heterojunction bipolar transistor, including improved high-frequency/high-speed operation.
1. A heterojunction bipolar transistor comprising:
a collector;
a base disposed above the collector, the base comprising a silicon-germanium layer;
a germanium-enriched region proximate an upper surface of the base and within the silicon-germanium layer; and
an emitter overlying the germanium-enriched region.
2. The heterojunction bipolar transistor of claim 1 wherein the germanium-enriched region creates a band-gap differential between the emitter and the base.
3. The heterojunction bipolar transistor of claim 1 wherein carrier mobility is greater in the germanium-enriched region than in the base.
4. The heterojunction bipolar transistor of claim 1 wherein the germanium-enriched region comprises a strained germanium-enriched region.
5. The heterojunction bipolar transistor of claim 1 wherein a germanium concentration in the germanium-enriched region ranges from about 30 percent to about 75 percent.
6. The heterojunction bipolar transistor of claim 1 wherein a germanium concentration is greater in the germanium-enriched region than in the silicon-germanium layer.
7. The heterojunction bipolar transistor of claim 1 having a valence band offset of greater than about 0.21 eV.
8. The heterojunction bipolar transistor of claim 1 wherein the germanium-enriched region has a relatively low level of lattice defects.
9. The heterojunction bipolar transistor of claim 1 wherein the base comprises a graded doped silicon-germanium base or a stepped doped silicon-germanium base.
10. The heterojunction bipolar transistor of claim 1 wherein the base comprises a uniformly doped silicon-germanium base.
11. The heterojunction bipolar transistor of claim 1 wherein the germanium-enriched region is in contact with the emitter.
12. The heterojunction bipolar transistor of claim 1 wherein a concentration of germanium in the germanium-enriched region decreases abruptly from a concentration proximate the upper surface in a direction toward the collector.
13. A bipolar junction semiconductor comprising:
a silicon substrate;
a collector disposed in the substrate;
a base disposed overlying the collector, wherein the base comprises a silicon-germanium portion;
a germanium-enriched region formed proximate an upper surface of the base in the silicon-germanium portion, wherein a concentration of germanium in the germanium-enriched region is substantially greater than the concentration of germanium in the silicon-germanium portion; and
an emitter disposed overlying the germanium-enriched region
14. The bipolar junction semiconductor of claim 13 wherein the germanium-enriched region comprises a thermally oxidized enriched region.
15. The bipolar junction semiconductor of claim 13 wherein the germanium enriched region includes at least a 30% germanium concentration.