Bipolar Junction Transistor
Session 11: Solid State Physics Bipolar Junction Transistor
1 1. I 2. 3. Outline 4. 5.
A B C D E F G H I J
2 1. I 2. 3. Introduction 4. 5.
In recent decades, the higher layout density and low-power advantage of CMOS technology has eroded the BJT ’s dominance in integrated- circuit products. (higher circuit density better system performance)
BJTs are still preferred in some integrated circuit applications because of their high speed and superior intrinsic gain. faster circuit speed larger power dissipation limits device density (~10 4 transistors/chip)
Si (npn, pnp) SiGe HBT GaAs (InSb) HBT
3 1. I 2. 3. BJT Types and Definitions 4. 5.
The BJT is a 3-terminal device, with two types: PNP and NPN pnp npn Asymmetric! Emitter Collector Emitter Collector
p+ n p- n+ p n- Base Base
4 1. I Review: Current Flow in a 2. 3. 4. Reverse-Biased pn Junction 5.
In a reverse-biased pn junction, there is negligible diffusion of majority carriers across the junction. The reverse saturation current is due to drift of minority carriers across the junction and depends on the rate of minority-carrier generation close to the junction (within ~one diffusion length of the depletion region). ⇒ We can increase this reverse current by increasing the rate of minority-carrier generation, e.g. by optical excitation of carriers ( e.g. photodiode) electrical injection of minority carriers into the vicinity of the junction…
5 1. I 2. 3. PNP BJT Operation (Qualitative) 4. 5. A forward-biased “emitter ” pn junction is used to inject minority carriers into the vicinity of a reverse-biased “collector ” pn junction. The collector current is controlled via the base-emitter junction
“F” “R” 0 0 hole p+ Emitter n Base Collector p- ≡ To achieve high current gain: • The injected minority carriers should not recombine within the quasi-neutral base region • The emitter junction current is comprised almost entirely of carriers injected into the base (rather than carriers injected into the emitter) 6 1. I Base Current Components 2. 3. 4. (Active Mode of Operation) 5.
The base current consists of majority carriers supplied for 1. Recombination of injected minority carriers in the base 2. Injection of carriers into the emitter 3. Reverse saturation current in collector junction (Reduces ) | | 4. Recombination in the base-emitter depletion region
“F” “R” 0 0 hole p+ Emitter n Base Collector p- ≡ 7 1. I 2. 3. BJT Circuit Configurations 4. 5.
CB: Common Base CE: Common Emitter CC: Common Collector
Output Characteristics for Common-Emitter Configuration
30 20 10 0
8 1. I 2. 3. BJT Modes of Operation 4. 5.
VBE p n p n p n 1 2 1 2 1 : F 1 : F EC E C 2 : R 2 : F
Active Saturation B B V E BC C Cut-Off Reversed
B Qp 1 : R 1 : R B Qn 2 : R 2 : F E
9 1. I 2. 3. BJT Electrostatics 4. 5. Emitter Base Collector pnp
p+ n p-
Under normal operating conditions, the BJT may be viewed electrostatically as two independent pn junctions 10 1. I 2. 3. BJT Electrostatics 4. 5. Emitter Base Collector pnp
p+ n p-
Under normal operating conditions, the BJT may be viewed electrostatically as two independent pn junctions 11 1. I 2. 3. BJT Electrostatics 4. 5. pnp Emitter Base Collector
p+ n p-
12 1. I 2. 3. BJT Electrostatics 4. 5. pnp Emitter Base Collector
p+ n p-
13 1. I 2. 3. BJT Performance Parameters (PNP) 4. 5.
“F” “R” Emitter Efficiency: 0 0 5 4 ≡ 3 relative to 1 hole 5 ↘ 1 2 2 p+ Emitter n Base Collector p- Base Transport Factor:
≡ relative to 1 ↘ 2 Common-Base d.c. Current Gain: ≡
14 1. I 2. 3. Collector Current (PNP) 4. 5.
“R” “F” The collector current is comprised of: 0 0 Holes injected from emitter, which do not2 recombine in the base 5 4 3 Reverse saturation current of collector junction 1 3 hole 2 p+ Emitter n Base Collector p- where is the collector current which flows when 0
1 1
15 1. I 2. 3. Notation (PNP BJT) , Assumptions 4. 5. Emitter Base Collector
p+ n p-
0′′ 0 0′ Notations: ′′ ′
≫ Assumptions: 1) 1-D structure 2) and is constant 3) Like ≪ diode gen-rec in depletion region is negligible 4) Uniform doping + step junction 5) Low-level injection everywhere! 16 1. I 2. 3. “Game Plan” for I-V Derivation 4. 5. Emitter Base Collector
p+ n p-
currents 0′ ′′ 0′′ 0 ′ 0
′′ ′ 0 0
′′ 0′′ 0 0′′ 0′ ′ 0 0 0 ∆ ∆ ∆ ∆ ′′ ′′ minority-carrier diffusion equation (different set of B.C. for each region) minority-carrier diffusion currents at depletion region edges 17 1. I 2. 3. Minority Carriers 4. 5. Emitter Base Collector
p+ n p- Carriers: Steady-State solution is: 0′ ′′ 0′′ 0 ′ ∆ ∆
⁄ ⁄ ∆ ⁄ ⁄ ∆ ⁄ ⁄ ∆ ⁄ ∆ 0′′ 1 ⁄ ∆ 0 1 ⁄ ∆ 1 ⁄ ∆ 1 18 1. I 2. 3. Minority Carriers – Active 4. 5. Emitter Base Collector
p+ n p-
0′ ′′ 0′′ 0 ′
0
0 0 ′′ 0′′ 0 0′′ 0′ ′ Active: forward reverse 19 1. I 2. 3. Minority Carriers – Saturation 4. 5. Emitter Base Collector
p+ n p-
0′ ′′ 0′′ 0 ′
0
0 0
′′ 0′′ 0 0′′ 0′ ′ Saturation: forward forward 20 1. I 2. 3. Minority Carriers – Cut-off 4. 5. Emitter Base Collector
p+ n p-
0′ ′′ 0′′ 0 ′
0 0 0 ′′ 0′′ 0 0′′ 0′ ′ Cut-off: reverse reverse 21 1. I 2. 3. Ideal BJT! 4. 5. Emitter Base Collector “F” “R” p+ n 5 4 3 1 hole ′′ 0′′ 0 p+ n Base 2p-
( ) 0 ≪ 1 0
⁄ ⁄ ∆ ≅
∆ 0 ∆ ∆ ∆ 0 where ⁄ ∆ 1 ⁄ 0 ∆ 0 1
22 1. I 2. 3. Current Equations - IE 4. 5. Emitter Base Collector
∆ p+ n 0′′ ′′ ⁄ 1 ′′ 0′′ 0 0 → ∆ 0 0 ∆ 0 ∆ ⁄ ⁄ → 1 1 0′′
0 ′′ 0′′ 0 0 0
⁄ ⁄ 1 1 Dominant term 23 1. I 2. 3. Current Equations – IC , IB 4. 5. Base Collector No recombination in Base p- 0 0 ′ 0′ ⁄ 0 1 → → 0′ 0 0 ⁄ “R”0′ ′ 1 “F” ⁄ 5 1 4 3 hole 1 p+ n Base 2p-
⁄ ⁄ 1 1 usually > 24 1. I 2. 3. Considering Recombination in Base 4. 5.
⁄ ⁄ 0 ∆ ⁄ ∆ 1 → ⁄ ∆ 0 1 → 0 0
sinh sinh ∆ 1 1 0 sinh sinh
1 coth 1 1 sinh
1 1 coth 1 sinh
25 1. I 2. 3. Quasi-Ideal BJT! 4. 5.
0 ∆ ∆ ∆ 0 ∆ 2 → → 0 0 1 1 2 Valid for all regions of operation! → Hence: 0 4 1 1
1 1 2 2
5 ~4 10 ~7.5 10 4 26 1. I 2. 3. Quasi-Ideal BJT! 4. 5.
1 1 2 2
5 ~4 10 ~7.5 10 4
1 1 2 2 ~7 10 ~7.5 10
Hence: ≅ ≅
≅
27 1. I 2. 3. CB: Common Base 4. 5. Input Characteristic Output Characteristic - Active + p+ n p- very little 3 - + dependence to 2 output voltage Saturation 1 Cut-Off 0 0.4 Ideal: 1 1 → 1 Non ideal: 1 ⇈ 1 ≅ → cosh 1 Leakage current of BC diode Determined by as p+ n p- ≫ 28 1. I 2. 3. 4. 5.
29 1. I 2. 3. CE: Common Emitter 4. 5.
Input Characteristic Output Characteristic - p- - Saturation Active 0 n very little 30 B 0 + p+ + dependence to 20 output voltage 10 Cut-Off 0 0.2 ≪ _ → ∙ ∙ 0 → ⇊ 0 ⇈
30 1. I 2. 3. ICEO and ICBO 4. 5.
p+ n p- ≫ p-
p- n n p+
p+
31 1. I 2. 3. Ebers-Moll Equations 4. 5. Normal Inverted
∆ ∆ 0 ∆ 0 ∆
0 0 Normal 0 Inverted ∆ 0 ∆ ; ∆ coth ∆ 0 ∆ ; ∆ csch
≡ ⁄ ∆ ∆ 1 ∆ ∆ ⁄ ≡ 1 1 ; ∆ 0 1 ; ∆ 0 32 1. I 2. 3. Ebers-Moll Equations 4. 5.
1 1 1 1 E C 1 1 B
33 1. I 2. 3. Charge Control Analysis 4. 5. Normal Inverted Generally :
1 1 1 1 ≡ ≡ ≡ ⁄ ≡ ⁄
1 1 Small- ≡ signal 1 1 model 34 1. I 2. 3. Transient Response 4. 5. C p- n B p+
saturation Cut-off active
0 0 0 saturation active 0 0 35 1. I 2. 3. Base Transient Time 4. 5. active
∆ 0 ∆ 0, ∆ 2 0 ∆ 0 2 /2
2 36 1. I 2. 3. Turn ON 4. 5.
Turn-on:
⁄ → I.C.: ⁄ 0 0 → 1
⁄ 1 0 →
1 → ln 1 37 1. I 2. 3. Turn OFF 4. 5.
Turn-off:
⁄ → I.C.: → 0 ∞
⁄ → 1
→ ⁄ 1
1 → ln 38 1. I 2. 3. Non-Ideal BJT 4. 5.
Deviations from Ideality:
1. Base width modulation (Early effect) 2. Punch-through 3. Avalanche Breakdown 4. Geometrical effects 5. Generation-Recombination in depletion regions 6. High-level injection
39 1. I 2. 3. Base Width Modulation 4. 5. “F” “R” Emitter Base Collector
p+ n p-
Saturation
30 ⁄ ≅ 20 10 ↗ ⟶ ↘ ⟶ ↗ Early voltage 0 Show that Active: HW: 1 40 1. I 2. 3. Punch Through 4. 5. “F” “R” Emitter Base Collector
p+ n p-
41 1. I 2. 3. Avalanche Breakdown 4. 5.
1 / ~4 42 1. I 2. 3. Geometry Considerations 4. 5. p-n-p Individual device n-p-n Integrated circuit BJT
Current Crowding
43 1. I 2. 3. G-R in Depl. Region , High-level injection 4. 5. p-n-p Individual device
High-level injection, Kirk Effect ln , ∝
high injection ∝
∝ In depletion region
44 1. I 2. 3. Small-Signal Model 4. 5. Low Freq: B C + / - . E High Freq:
B C +
- E 1 1 . 45 /