Mechanically-stable BJT with reduced base-collector capacitance
View Patent ↗A bipolar junction transistor (BJT) requires the fabrication of a BJT structure and of a support post which is adjacent to, but physically and electrically isolated from, the BJT structure. The BJT structure includes a semi-insulating substrate, a subcollector, a collector, a base, and an emitter. Metal contacts are formed on the subcollector and emitter to provide collector and emitter terminals. Contact to the structure's base is accomplished with a metal contact which extends from the top of the support post to the edge of the base nearest the support post. The contact bridges the physical and electrical separation between the support post and the base and provides a base terminal for the device. The base contact need extend over the edge of the base by no more than the transfer length associated with the fabrication process. This results in the smaller base contact area over the collector than would otherwise be necessary, and a consequent reduction in base-collector capacitance. The invention is particularly useful when forming heterojunction bipolar transistors (HBTs), built on a compound semiconductor substrate such as indium phosphide (InP).
1. A bipolar junction transistor (BJT), comprising:
a BJT structure, comprising:
a semi-insulating substrate,
a subcollector formed on said substrate,
a collector formed on said subcollector,
a first metal contact on said subcollector which provides a collector contact for said BJT,
a base formed on said collector,
an emitter formed on said base,
a second metal contact on said emitter which provides an emitter contact for said BJT;
a first support post formed on said substrate which is physically and electrically isolated from said BJT structure; and
a third metal contact which extends from the top of said support post to the edge of said base nearest said support post, thereby bridging the physical and electrical separation between said first support post and said base and providing a base contact for said BJT.
2. The BJT of claim 1 , wherein said semi-insulating substrate is a compound semiconductor.
3. The BJT of claim 1 , wherein said semi-insulating substrate comprises indium phosphide (InP).
4. The BJT of claim 1 , wherein said base contact extends over the edge of said base by about X μm, where X is the transfer length associated with the process used to fabricate the BJT.
5. The BJT of claim 1 , wherein said support post is formed simultaneously with said BJT structure, said support post comprising:
a first layer on said substrate, said first layer comprising the material which comprises said subcollector;
a second layer on said first layer, said second layer comprising the material which comprises said collector; and
a third layer on said second layer, said third layer comprising the material which comprises said base.
6. The BJT of claim 1 , wherein said base contact is approximately comb-shaped having a top portion and periodically-spaced teeth extending perpendicularly from said top portion, said top portion of said comb-shaped base contact contacting said base and said teeth extending from said top portion to said first support post.
7. The BJT of claim 1 , further comprising:
a second support post formed on said substrate which is physically and electrically isolated from said BJT structure and is located on the opposite side of said BJT structure from said first support post; and
a fourth metal contact which extends from the top of said second support post to the edge of said base nearest said support post, thereby bridging the physical and electrical separation between said second support post and said base and providing a second base contact for said BJT.
8. The BJT of claim 1 , wherein said subcollector comprises indium phosphide (InP) or indium gallium arsenide (InGaAs).
9. The BJT of claim 1 , wherein said collector comprises indium phosphide (InP), indium gallium arsenide (InGaAs), indium aluminum arsenide (InAlAs), or indium aluminum arsenide phosphide (InAlAsP).
10. The BJT of claim 1 , wherein said base comprises indium gallium arsenide (InGaAs).
11. The BJT of claim 1 , wherein said emitter comprises indium phosphide (InP) or indium aluminum arsenide (InAlAs).
12. The BJT of claim 1 , wherein said semi-insulating substrate is a compound semiconductor and said BJT structure is arranged to form a heterojunction bipolar transistor (HBT).
13. A heterojunction bipolar transistor (HBT), comprising:
an HBT structure, comprising:
a semi-insulating substrate comprising a compound semiconductor,
a subcollector formed on said substrate,
a collector formed on said subcollector,
a first metal contact on said subcollector which provides a collector contact for said HBT,
a base formed on said collector,
an emitter formed on said base,
a second metal contact on said emitter which provides an emitter contact for said HBT;
a first support post formed on said semi-insulating substrate which is physically and electrically isolated from said HBT structure, said first support post comprising:
a first layer on said substrate, said first layer comprising the material which comprises said subcollector;
a second layer on said first layer, said second layer comprising the material which comprises said collector; and
a third layer on said second layer, said third layer comprising the material which comprises said base; and
a third metal contact which extends from the top of said support post to the edge of said base nearest said support post, thereby bridging the physical and electrical separation between said support post and said base and providing a base contact for said HBT, said base contact extending over the edge of said base by about X μm, where X is the transfer length associated with the process used to fabricate the HBT.
14. The HBT of claim 13 , wherein said semi-insulating substrate comprises indium phosphide (InP).
15. The HBT of claim 13 , wherein said base contact is approximately comb-shaped having a top portion and periodically-spaced teeth extending perpendicularly from said top portion, said top portion of said comb-shaped base contact contacting said base and said teeth extending from said top portion to said first support post.
16. The HBT of claim 13 , further comprising:
a second support post formed on said substrate which is physically and electrically isolated from said HBT structure and is located on the opposite side of said HBT structure from said first support post; and
a fourth metal contact which extends from the top of said second support post to the edge of said base nearest said support post, thereby bridging the physical and electrical separation between said second support post and said base and providing a second base contact for said HBT.
17. The HBT of claim 13 , wherein said subcollector comprises indium phosphide (InP) or indium gallium arsenide (InGaAs).
18. The HBT of claim 13 , wherein said collector comprises indium phosphide (InP), indium gallium arsenide (InGaAs), indium aluminum arsenide (InAlAs), or indium aluminum arsenide phosphide (InAlAsP).
19. The HBT of claim 13 , wherein said base comprises indium gallium arsenide (InGaAs).
20. The HBT of claim 13 , wherein said emitter comprises indium phosphide (InP) or indium aluminum arsenide (InAlAs).
21. A heterojunction bipolar transistor (HBT), comprising:
an HBT structure, comprising:
a semi-insulating substrate comprising indium phosphide (InP),
a subcollector formed on said substrate,
a collector formed on said subcollector,
a first metal contact on said subcollector which provides a collector contact for said HBT,
a base formed on said collector,
an emitter formed on said base,
a second metal contact on said emitter which provides an emitter contact for said HBT;
first and second support posts formed on said semi-insulating substrate on opposite sides of said HBT structure, each of said support posts physically and electrically isolated from said HBT structure, each of said support posts comprising:
a first layer on said substrate, said first layer comprising the material which comprises said subcollector;
a second layer on said first layer, said second layer comprising the material which comprises said collector; and
a third layer on said second layer, said third layer comprising the material which comprises said base;
a third metal contact which extends from the top of said first support post to the edge of said base nearest said first support post, thereby bridging the physical and electrical separation between said first support post and said base and providing a first base contact for said HBT; and
a fourth metal contact which extends from the top of said second support post to the edge of said base nearest said second support post, thereby bridging the physical and electrical separation between said second support post and said base and providing a second base contact for said HBT;
wherein each of said base contacts is approximately comb-shaped having a top portion and periodically-spaced teeth extending perpendicularly from said top portion, the top portion of said first base contact contacting a first edge of said base and the top portion of said second base contact contacting an edge of said base opposite said first edge and said teeth extending from the top portions of said first and second base contacts to said first and second support posts, respectively, with each of said base contacts extending over the edge of said base by about X μm, where X is the transfer length associated with the process used to fabricate the HBT.