L16: Heterojunction Bipolar Transistor
L16: Heterojunction Bipolar Transistor
tanford University
EE 216 : Aneesh Nainani
1
Announcements • Final Exam – 3/21 (Th) from 12:15pm-3:15pm – at 200-203 (History Corner) – can bring 1 piece of paper – More focused on testing of concepts than number crunching
• iPad return – Before last class on 3/13 (Wed) – or Tech Desk at Meyer Library by 3/13, 5pm – Fine for late return ! tanford University
2
BJT recap N+
P
N- IC
• Small number of holes control the flow of large number of electrons • Large change in IC with a small change in IB tanford University
3
Quiz: Dopant profile design Emitter doping: •
As high as possible
Base doping: • •
As low as possible for high gain High to avoid current crowding
Collector doping: • Lower than base
Base width: • tanford University
As thin as possible without punch-thru 4
Quiz: Can I interchange terminal ? BJT E
C
MOSFET Source
Drain
BJT: NO
MOSFET: YES
Except when symmetric double heterojunction
Except in power MOSFET / DEnMOS
tanford University
5
High Frequency Applications
A. B. C. D. tanford University
6
A. Emitter Capacitance Charging Time
tanford University
7
B. Base Transit Time
tanford University
8
tanford University
EE 216 : Aneesh Nainani
9
C. Collector Depletion Layer Transit Time
τ = WBC ? vs tanford University
τ = WBC? 2vs 10
C. Collector Depletion Layer Transit Time
tanford University
EE 216 : Aneesh Nainani
11
C. Collector Capacitance Charging Time
tanford University
EE 216 : Aneesh Nainani
12
tanford University
EE 216 : Aneesh Nainani
13
Gain in BJT
• Cellphone @ 2.4 GHz • Need devices which can give gain upto 1 THz tanford University
14
How to improve BJT
Graded base
tanford University
Polysilicon Emitter
Heterojunction b/w B and E
EE 216 : Aneesh Nainani
15
Shockley BJTs..
Cartoon
tanford University
EE 216 : Aneesh Nainani
16
Modern BJT..
Polysilicon Emitter SiGe/Si Heterojunction tanford University
EE 216 : Aneesh Nainani
17
Polysilicon Emitter A high-performance BJT typically has a layer of As-doped N+ poly-silicon film in the emitter. The dashed line shows excess hole profile if the metal contact was made directly to the N+ Si emitter. The steep slope results because pn goes to 0 at metal/Si contact. This results in a large back injected base current reducing emitter injection efficiency γ. With N+ polySi emitter pn goes to 0 at metal/poly-Si contact. The slope of pn is reduced in Si-emitter to the “long base diode” value resulting in reduced back injected base current and higher γ. Slope in poly-Si is large because of small diffusion length due to grain boundaries. +
N -poly-Si emitter
pn
N+ poly-Si + emitter N Si emitter P-base
SiO 2 P-base N-collector
x tanford University
18
Polysilicon Emitter Q: why does poly suppress only hole current and not electron current ?
tanford University
EE 216 : Aneesh Nainani
19
How to improve BJT
Heterojunction b/w B and E
tanford University
EE 216 : Aneesh Nainani
20
Heterostructure in HBT
Key feature is that the E-B barrier for holes is much larger than that for electrons
tanford University
21
Aside: Heterostructure design • We previously saw this in multi junction solar cell • HBT is all about bandgap engineering !
tanford University
22
Bandgap & Lattice Matching
tanford University
23
Type I
tanford University
24
Type - I
tanford University
25
Type II
tanford University
EE 216 : Aneesh Nainani
26
Type III
= 4.03 eV = 4.9 eV
tanford University
EE 216 : Aneesh Nainani
27
Type III
tanford University
EE 216 : Aneesh Nainani
28
Abrupt HBT ݊ ܦ/ ∆ /
ܬൌ ݍ ݁ ா ݁ ܰ ܹ
݊ ܦ/ ∆ /
ܬൌ ݍ ݁ ா ݁ ܰ ܹ
ܰா ܦ ܹா ܰ ܦ ܹ ܰா ܦ ܹா ܰ ܦ ܹ
tanford University
EE 216 : Aneesh Nainani
29
Graded HBT
tanford University
EE 216 : Aneesh Nainani
30
How to improve BJT
Graded base
tanford University
EE 216 : Aneesh Nainani
31
Graded Base
tanford University
EE 216 : Aneesh Nainani
32
Graded Base HBT
tanford University
EE 216 : Aneesh Nainani
33
Quiz: Single / Double Heterostructure in HBT ?
tanford University
34
Single / Double HBT
• Higher collector breakdown voltage • Reduced collector offset voltage tanford University
EE 216 : Aneesh Nainani
35
Single heterostructure HBT
Double heterostructure eliminates the built in voltage such that IC=0 at VCE=0 tanford University
36
Example of stack design
tanford University
EE 216 : Aneesh Nainani
37
HBT’s summary
• Have the potential to reach power gain till 1THz • Performance limited by parasitics, R, heating tanford University
EE 216 : Aneesh Nainani
38
Big Wins for HBTs 1. Current gain limited only by recombination in the E-B junction or the base 2. Current crowding and base resistance are greatly reduced because of high base doping 3. Improved high frequency performance from decreased base resistance and E-B capacitance 4. Eliminates base width modulation because depletion regions are in the more lightly doped emitter and collector regions 5. Completely eliminates punch-thru due to high base doping tanford University
39
Background
• • • •
First transistor BJT not MOSFET ! Point contact germanium BJT BJT: 2 Nobel prizes
tanford University
EE 216 : Aneesh Nainani
40
tanford University
EE 216 : Aneesh Nainani
1
Announcements • Final Exam – 3/21 (Th) from 12:15pm-3:15pm – at 200-203 (History Corner) – can bring 1 piece of paper – More focused on testing of concepts than number crunching
• iPad return – Before last class on 3/13 (Wed) – or Tech Desk at Meyer Library by 3/13, 5pm – Fine for late return ! tanford University
2
BJT recap N+
P
N- IC
• Small number of holes control the flow of large number of electrons • Large change in IC with a small change in IB tanford University
3
Quiz: Dopant profile design Emitter doping: •
As high as possible
Base doping: • •
As low as possible for high gain High to avoid current crowding
Collector doping: • Lower than base
Base width: • tanford University
As thin as possible without punch-thru 4
Quiz: Can I interchange terminal ? BJT E
C
MOSFET Source
Drain
BJT: NO
MOSFET: YES
Except when symmetric double heterojunction
Except in power MOSFET / DEnMOS
tanford University
5
High Frequency Applications
A. B. C. D. tanford University
6
A. Emitter Capacitance Charging Time
tanford University
7
B. Base Transit Time
tanford University
8
tanford University
EE 216 : Aneesh Nainani
9
C. Collector Depletion Layer Transit Time
τ = WBC ? vs tanford University
τ = WBC? 2vs 10
C. Collector Depletion Layer Transit Time
tanford University
EE 216 : Aneesh Nainani
11
C. Collector Capacitance Charging Time
tanford University
EE 216 : Aneesh Nainani
12
tanford University
EE 216 : Aneesh Nainani
13
Gain in BJT
• Cellphone @ 2.4 GHz • Need devices which can give gain upto 1 THz tanford University
14
How to improve BJT
Graded base
tanford University
Polysilicon Emitter
Heterojunction b/w B and E
EE 216 : Aneesh Nainani
15
Shockley BJTs..
Cartoon
tanford University
EE 216 : Aneesh Nainani
16
Modern BJT..
Polysilicon Emitter SiGe/Si Heterojunction tanford University
EE 216 : Aneesh Nainani
17
Polysilicon Emitter A high-performance BJT typically has a layer of As-doped N+ poly-silicon film in the emitter. The dashed line shows excess hole profile if the metal contact was made directly to the N+ Si emitter. The steep slope results because pn goes to 0 at metal/Si contact. This results in a large back injected base current reducing emitter injection efficiency γ. With N+ polySi emitter pn goes to 0 at metal/poly-Si contact. The slope of pn is reduced in Si-emitter to the “long base diode” value resulting in reduced back injected base current and higher γ. Slope in poly-Si is large because of small diffusion length due to grain boundaries. +
N -poly-Si emitter
pn
N+ poly-Si + emitter N Si emitter P-base
SiO 2 P-base N-collector
x tanford University
18
Polysilicon Emitter Q: why does poly suppress only hole current and not electron current ?
tanford University
EE 216 : Aneesh Nainani
19
How to improve BJT
Heterojunction b/w B and E
tanford University
EE 216 : Aneesh Nainani
20
Heterostructure in HBT
Key feature is that the E-B barrier for holes is much larger than that for electrons
tanford University
21
Aside: Heterostructure design • We previously saw this in multi junction solar cell • HBT is all about bandgap engineering !
tanford University
22
Bandgap & Lattice Matching
tanford University
23
Type I
tanford University
24
Type - I
tanford University
25
Type II
tanford University
EE 216 : Aneesh Nainani
26
Type III
= 4.03 eV = 4.9 eV
tanford University
EE 216 : Aneesh Nainani
27
Type III
tanford University
EE 216 : Aneesh Nainani
28
Abrupt HBT ݊ ܦ/ ∆ /
ܬൌ ݍ ݁ ா ݁ ܰ ܹ
݊ ܦ/ ∆ /
ܬൌ ݍ ݁ ா ݁ ܰ ܹ
ܰா ܦ ܹா ܰ ܦ ܹ ܰா ܦ ܹா ܰ ܦ ܹ
tanford University
EE 216 : Aneesh Nainani
29
Graded HBT
tanford University
EE 216 : Aneesh Nainani
30
How to improve BJT
Graded base
tanford University
EE 216 : Aneesh Nainani
31
Graded Base
tanford University
EE 216 : Aneesh Nainani
32
Graded Base HBT
tanford University
EE 216 : Aneesh Nainani
33
Quiz: Single / Double Heterostructure in HBT ?
tanford University
34
Single / Double HBT
• Higher collector breakdown voltage • Reduced collector offset voltage tanford University
EE 216 : Aneesh Nainani
35
Single heterostructure HBT
Double heterostructure eliminates the built in voltage such that IC=0 at VCE=0 tanford University
36
Example of stack design
tanford University
EE 216 : Aneesh Nainani
37
HBT’s summary
• Have the potential to reach power gain till 1THz • Performance limited by parasitics, R, heating tanford University
EE 216 : Aneesh Nainani
38
Big Wins for HBTs 1. Current gain limited only by recombination in the E-B junction or the base 2. Current crowding and base resistance are greatly reduced because of high base doping 3. Improved high frequency performance from decreased base resistance and E-B capacitance 4. Eliminates base width modulation because depletion regions are in the more lightly doped emitter and collector regions 5. Completely eliminates punch-thru due to high base doping tanford University
39
Background
• • • •
First transistor BJT not MOSFET ! Point contact germanium BJT BJT: 2 Nobel prizes
tanford University
EE 216 : Aneesh Nainani
40
