Heterojunction bipolar transistors for improved radio frequency (RF) performance
The present disclosure relates to heterojunction bipolar transistors for improved radio frequency (RF) performance. In this regard, a heterojunction bipolar transistor includes a base, an emitter, and a collector. The base is formed over the collector such that a base-collector junction is formed between the base and the collector. The base-collector junction is configured to become forward-biased at a first turn-on voltage. The emitter is formed over the base such that a base-emitter junction is formed between the base and the emitter. The base-emitter junction is configured to become forward-biased at a second turn-on voltage, as opposed to the first turn-on voltage. Notably, the second turn-on voltage is lower than the first turn-on voltage.
1. A heterojunction bipolar transistor comprising:
a collector;
a base formed over the collector such that a base-collector junction is formed between the base and the collector, wherein the base-collector junction is configured to become forward-biased at a first turn-on voltage; and
an emitter formed over the base such that a base-emitter junction is formed between the base and the emitter, wherein the base-emitter junction is configured to become forward-biased at a second turn-on voltage, which is lower than the first turn-on voltage.
2. The heterojunction bipolar transistor of claim 1 wherein the first turn-on voltage is between 0.1 volts and 0.4 volts higher than the second turn-on voltage.
3. The heterojunction bipolar transistor of claim 1 wherein the first turn-on voltage is above 1.3 volts.
4. The heterojunction bipolar transistor of claim 1 configured as a double heterojunction bipolar transistor.
5. The heterojunction bipolar transistor of claim 1 wherein a composition gradient of the base is graded such that a bandgap corresponding to a portion of the base near the base-collector junction is wider than a bandgap corresponding to a portion of the base near the base-emitter junction.
6. The heterojunction bipolar transistor of claim 1 wherein the collector comprises:
a first collector level formed over a subcollector; and
a second collector level formed over the first collector level wherein a doping concentration of the second collector level is graded such that a bandgap corresponding to a portion of the second collector level near the base-collector junction is wider than a bandgap corresponding to a portion of the base near the base-emitter junction.
7. The heterojunction bipolar transistor of claim 6 wherein the second collector level comprises:
a first layer formed over the first collector level;
a second layer formed over the first layer; and
a third layer formed over the second layer.
8. The heterojunction bipolar transistor of claim 7 wherein:
the first layer has a doping concentration between 1e15 cm −3 and 1e16 cm −3 ;
the second layer has a doping concentration between 1e18 cm −3 and 1e20 cm −3 ; and
the third layer has a doping concentration between 1e15 cm −3 and 1e16 cm −3 .
9. The heterojunction bipolar transistor of claim 8 further comprising:
an emitter cap formed on the emitter;
a pair of collector contacts formed on the subcollector;
a pair of base contacts formed on the base; and
an emitter contact formed on the emitter cap.
10. The heterojunction bipolar transistor of claim 9 wherein the emitter cap comprises:
a first cap formed on the emitter;
a second cap formed on the first cap; and
a third cap formed on the second cap.
11. The heterojunction bipolar transistor of claim 7 wherein the subcollector comprises Gallium-Arsenide.
12. The heterojunction bipolar transistor of claim 11 wherein the first collector level is formed from Gallium-Arsenide over the subcollector.
13. The heterojunction bipolar transistor of claim 12 wherein:
the first layer of the second collector level is formed from Gallium-Arsenide compositionally graded into Aluminum-Gallium-Arsenide;
the second layer of the second collector level is formed from Aluminum-Gallium-Arsenide; and
the third layer of the second collector level is formed from Aluminum-Gallium-Arsenide.
14. The heterojunction bipolar transistor of claim 13 wherein the base is formed from Gallium-Arsenide.
15. The heterojunction bipolar transistor of claim 14 wherein the emitter is formed from Indium-Gallium-Arsenide.
16. The heterojunction bipolar transistor of claim 15 wherein the subcollector is approximately 8000 angstroms thick and has a doping concentration approximately equal to 4e18 cm −3 .
17. The heterojunction bipolar transistor of claim 16 wherein the first collector level is approximately 9650 angstroms thick and has a doping concentration approximately equal to 4e15 cm −3 .
18. The heterojunction bipolar transistor of claim 17 wherein:
the first layer is approximately 300 angstroms thick and has a doping concentration approximately equal to 4e15 cm −3 ;
the second layer is approximately between 1500 angstroms and 3000 angstroms thick and has a doping concentration approximately equal to 4e15 cm −3 ;
a doping spike is formed of Aluminum-Gallium-Arsenide between the second layer and the third layer wherein the doping spike is approximately 50 angstroms thick and has a doping concentration approximately equal to 2e18 cm −3 ; and
the third layer is approximately less than 600 angstroms thick and has a doping concentration approximately equal to 4e15 cm −3 .
19. The heterojunction bipolar transistor of claim 18 wherein the base is approximately 800 angstroms thick and has a doping concentration approximately equal to 4e19 cm −3 .
20. The heterojunction bipolar transistor of claim 19 wherein the emitter is approximately 500 angstroms thick and has a doping concentration approximately equal to 3e17 cm −3 .
21. A heterojunction bipolar transistor comprising:
a base formed over a collector wherein a base-collector junction is formed between the base and the collector;
an emitter formed over the base wherein a base-emitter junction is formed between the base and the emitter, the emitter having a bandgap near the base-emitter junction approximately equal to 1.32 electronvolts (eV); and
the collector having a bandgap near the base-collector junction that is approximately equal to 1.42 eV so that a turn-on voltage of the base-collector junction is higher than a turn-on voltage of the base-emitter junction.
22. The heterojunction bipolar transistor of claim 21 wherein the bandgap of the collector decreases as a distance from the base-collector junction increases.
23. A method for manufacturing a heterojunction bipolar transistor comprising:
providing a collector on a subcollector;
providing a base on the collector such that a base-collector junction is formed between the base and the collector, wherein the base-collector junction is configured to become forward-biased at a first turn-on voltage;
providing an emitter on the base such that a base-emitter junction is formed between the base and the emitter, wherein the base-emitter junction is configured to become forward-biased at a second turn-on voltage, which is lower than the first turn-on voltage;
providing an emitter cap on the emitter;
providing a pair of collector contacts on the subcollector;
providing a pair of base contacts on the base; and
providing an emitter contact on the emitter cap.
24. The method of claim 23 wherein providing the collector comprises:
providing a first collector level on the subcollector; and
providing a second collector level on the first collector level wherein a doping concentration of the second collector level is graded such that a bandgap corresponding to a portion of the second collector level near the base-collector junction is wider than a bandgap corresponding to a portion of the base near the base-emitter junction.
25. The method of claim 24 wherein providing the second collector level comprises:
providing a first layer on the first collector level;
providing a second layer on the first layer; and
providing a third layer on the second layer.