1. CHAPTER 5: BIPOLAR TRANSISTORS & RELATED DEVICES

2. INTRODUCTION
The bipolar junction transistor (BJT) is a semiconductor device constructed with three doped regions
The most common use of the BJT is in linear amplifier circuits (linear means that the output is proportional to input). It can also be used as a switch (in, for example, logic circuits). 
What’s meaning by BJT?
What’s the difference between PNP & NPN transistor?
Transistor Mode Operation and Characteristic I-V curves
The current gain
The cutoff frequency and switching time of a bipolar transistor
The advantages of heterojunction bipolar transistor
The power handling capability of thyristor and related bipolar devices

3. The Bipolar Junction Transistor
The term Bipolar is because two type of charges (electrons and holes) are involved in the flow of electricity
The term Junction is because there are two p-n junctions
There are two configurations for this device

4. NPN and PNP Transistors
NPN is more widely used
Majority carriers are electrons so it operates more quickly
PNP is used for special applications
The terminals of the transistor are labeled (Base, Emitter, and Collector)
The emitter is always drawn with the arrow.

5. Figure 5-1. Perspective view of a silicon p-n-p bipolar transistor.

6. Differences between NPN & PNP
Type of BJT
PNP-Type
NPN-Type 1
If the base is at a lower voltage than the emitter,
current flows from emitter to collector
If the base is at a higher voltage than the
emitter, current flows from collector to
emitter. 2
Small amount of current also flows from emitter to
base.
Small amount of current also flows from
base to emitter. 3
Emitter is heavily p-doped compared to collector. So,
emitter and collector are not interchangeable.
Emitter is heavily N-doped compared to
collector. So, emitter and collector are not
interchangeable. 4
The base width is small compared to the minority
carrier diffusion length. If the base is much larger,
then this will behave like back-to-back diodes.
The base width is small compared to the
minority carrier diffusion length. If the
base is much larger, then this will behave
like back-to-back diodes. 5
Voltage at base controls amount of current flow
through transistor (emitter to collector).
Voltage at base controls amount of current
flow through transistor (collector to
emitter). 6 7
Follow the arrow to see the direction of current flow
Follow the arrow to see the direction of
current flow

7. Operation of NPN Transistor
• In normal operation, the EB junction is forward biased and the BC junction is reverse biased
• The base region is very thin so the ratio L1:L2 is typically about 150:1
Figure (a) Idealized one-dimensional schematic of a p-n-p bipolar transistor and (b) its circuit symbol. (c) Idealized one-dimensional schematic of an n-p-n bipolar transistor and (d) its circuit symbol.

8. The E is more heavily doped than the C
B doping is less than the E doping but greater than the C
At thermal equilibrium – no net I flow  the Fermi level is a constant
Figure (a) A p-n-p transistor with all leads grounded (at thermal equilibrium). (b) Doping profile of a transistor with abrupt impurity distributions. (c) Electric-field profile. (d) Energy band diagram at thermal equilibrium.

10. BJT CONFIGRATIONS

11. Common-base configuration:
Note: depletion layer width of the E-B junction is narrower & C-B junction is wider compared with equilibrium case
E-B junction (forward biased) – holes injected from the p+ E into B, electron injected from the n B into E
C-B junction (reverse biased) – if B width is narrow, holes injected from the E can diffuse thru B to reach the B-C depletion edge and the “float up” into the C
E (emits/injects carriers)  C (collects carriers from nearby junction) : C hole I  E hole I
The transistor action: carriers injected from E junction  large I flow in C junction
Figure (a) The transistor shown in Fig. 3 under the active mode of operation.3 (b) Doping profiles and the depletion regions under biasing conditions. (c) Electric-field profile. (d) Energy band diagram.

12. Current gain
Assume no generation-recombination Is in the depletion regions
Holes injected from E: IEp (largest I component)
Most of injected holes will reach C junction - give rise ICp
IBB : electrons that must be supplied by the B to replace electrons recombined with the injected holes (IBB=IEp-ICp)
IEn: I arising from electrons injected from B to E – however not desirable
ICn: thermally generated electrons that are near the B-C junction edge and drift from C to B – direction of electron I is opposite the electron flow
Figure Various current components in a p-n-p transistor under active mode of operation. The electron flow is in the opposite direction to the electron current.

15. Common-base current gain
Emitter efficiency
Base transport factor
ICBO : the leakage current between the C and B with the E-B junction open
Collector current

17. Carrier Distribution
To derive the I-V for an ideal transistor, assume:
The device has uniform doping in each region
The hole drift current in the base region as well as the C saturation current is negligible
There is low-level injection
There are no generation-recombination currents in the depletion regions
There are no series resistance
Holes from E to B to C : Once the minority-carrier distribution is determined (holes in the n-type B)  can obtain I from the minority-carrier gradient

18. pn (x): minority carrier (holes) in the base
pno: equilibrium minority carrier (holes) in the base
nE: electron conc. in E
nC: electron conc. in C
nEO: equilibrium electron conc. in E
nCO: equilibrium electron conc. in C
Figure Minority carrier distribution in various regions of a p-n-p transistor under the active mode of operation.

19. now look at what happens to the electrons injected into the base
now look at what happens to the electrons injected into the base. Because the base is made of p-type silicon, the electrons are minority carriers. The base is very thin so the electron concentration, np, will have a linear characteristic. The electron concentration will be highest at the emitter side of the base, and will be zero at the collector side. It is zero here because the CBJ is in reverse bias, causing all minority carriers to be attached to and swept across to the collector (by the same, majority carriers, holes, are repelled from the junction). We will term the electron concentration at the EBJ np(0).
The EBJ is in forward bias, so the concentration at the emitter side of the base np(0) will be proportional to evBE/vT:

20. Minority carrier in the base region:
: equilibrium minority-carrier concentration in the base
: Exponential factor of the increased minority carrier concentration at the edge of the E-B depletion region (x=0)

21. Ideal Transistor Currents for Active Mode Operation:
The emitter current
The collector current
The base current
Note:
Is in the 3 terminals are mainly determined by the minority carrier distribution in the base region
Common-base current gain 0 can be obtained

22. Modes of operation of p-n-p transistor
Active mode:
E-B junction is forward biased, B-C junction is reverse-biased
Saturation mode:
both junctions are forward biased
corresponds to small biasing V & large output I – transistor is in a conducting state & acts as a closed (or on) switch
Cutoff mode:
both junctions are reverse biased
corresponds to the open (or off) switch
Inverted mode:
inverted active mode
E-B junction is reverse biased, C-B junction is forward biased
Figure Junction polarities and minority carrier distributions of a p-n-p transistor under four modes of operation.

23. I-V of Common-Base
Ic is equal to IE (i.e 01) & independent of of VBC
Ic remains constant even down to 0V for VBC (holes are still extracted by C)
Hole distributions (See fig. 5-9)
Figure (a) Common-base configuration of a p-n-p transistor. (b) Its output current-voltage characteristics.

24. I-V of Common-Base
Hole at x=W changes only slightly from VBC>0 to VBC=0 (IC remains the same) – fig. (a)
to reduce IC to 0 – apply a small forward bias (about 1V to B-C junction) – fig. (b)
The forward bias will increase the hole density at x=W to make it equal to that of the emitter at x=0 (horizontal line)
The hole gradient at x=W & IC will reduce to 0
Figure Minority carrier distributions in the base region of a p-n-p transistor. (a) Active mode for VBC = 0 and VBC > 0. (b) Saturation mode with both junctions forward biased.

25. I-V of Common-emitter Common-emitter current gain:
C-E leakage current:
Figure (a) Common-emitter config. of a p-n-p transistor. (b) Its output I-V characteristics.
Common-base current gain

26. Heterojunction bipolar transistor
The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is using mostly RF systems.
Heterojunction transistors have different semiconductors for the elements of the transistor.
Usually the emitter is composed of a larger bandgap material than the base. This helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias and increases emitter injection efficiency.
The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode.
Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by epitaxy techniques like MOCVD and MBE.

27. The Heterojunction Bipolar Transistor
Emitter – wide band gap (AlGaAs)
Base – lower band gap (GaAs)
Large band gap difference (between E-B)  common-emitter current gain can be extremely large
Homojunction: no band gap difference – doping concentration in the E & B must be very high
EV increases the valence-band barrier height  reduce injection of holes from B to E
can use heavily doped base, maintain a high E efficiency & current gain
Figure (a) Schematic cross section of an n-p-n heterojunction bipolar transistor (HBT) structure. (b) Energy band diagram of a HBT operated under active mode.

28. InP-based material systems Advantages: very low surface recombination
Advanced HBTs
InP-based material systems
Advantages:
very low surface recombination
Higher electron mobility in InGaAs than in GaAs – superior high-freq performance (in fig., cutoff freq: 254GHz)
InP collector region has higher velocity at high fields than GaAs collector
InP collector breakdown voltage is higher than GaAs
Figure Current gain as a function of operating frequency for an InP-based HBT.
Cutoff freq., fT= 254GHz

29. Si/SiGe material system
high-speed capability – because the base is heavily doped (band gap difference)
Small trap density at Si surface minimizes the surface recombination current – high current gain at low Ic
Lower cutoff freq – because lower mobility in Si compared to GaAs- & InP- based HBTs
Problem: E efficiency & Ic suffer (caused by EV)
To improve: graded-layer & graded-base heterojunction
Figure (a) Device structure of an n-p-n Si/SiGe/Si HBT (b) Collector and base current versus VEB for a HBT and bipolar junction transistor (BJT).

30. Wg: thickness of graded layer
Graded profile in base region: reduction of the band gap from E to C
Ebi: built-in electric field:
reduce minority-carrier transit time
increase the common-emitter current gain
increase the cutoff freq.
Thicker C layer:
improve breakdown voltage (B-C junction)
increase transit time
Carrier move thru C at a saturation velocity because of very large electric fields are maintained in C
Figure Energy band diagrams for a heterojunction bipolar transistor with and without graded layer in the junction, and with and without a graded-base layer.

31. THYRISTOR & RELATED DEVICES
Thyristor: designed for handling high V & large I
Used for switching applications that require the device to change from an off or blocking state to an on conducting state
Thyristors have much wider range of I- & V- handling capabilities
Available with I ratings from a few miliamperes to over 5000A, V ratings extending above 10,000V

32. Called p-n-p-n diode
Add gate electrode at the inner p-layer (p2)  3-terminal device called semiconductor-controlled rectifier (SCR) or thyristor
Figure (a) Four-layer p-n-p-n diode. (b) Typical doping profile of a thyristor. (c) Energy band diagram of a thyristor in thermal equilibrium.

33. Figure 5-23. Current-voltage characteristics of a p-n-p-n diode.
Basic I-V characteristics of p-n-p-n diode – exhibits 5 distinct regions:
0-1: The device is in forward-blocking or off-state and has very high impedance. Forward breakover (switching) occurs where dV/dI=0; & at point 1 defined as forward breakover voltage VBF & switching current IS
1-2: The device is in a negative-resistance region – the I increases as the V decreases sharply
2-3: The device is in forward-conducting or on-state & has low impedance. At point 2, where dV/dI=0, the holding current Ih & holding voltage Vh
0-4: The device is in the reverse-blocking state
4-5: The device is in the reverse-breakdown region
Figure Current-voltage characteristics of a p-n-p-n diode.

34. Exercise 1
For an ideal p-n-p transistor , the current components are given by IEp=3mA, IEn=0.01mA, ICp=2.99mA, and ICn=0.001mA. Determine:
The emitter efficiency 
The base transport factor T
The common-bas current gain 0
ICBO

35. Solution
Emitter efficiency is:
(b) The base transport factor :
(c) The common-base current gain
(d)

36. Exercise 2
Referring to Exer. 1, find the common-emitter current gain 0. Express ICEO in terms of 0 and ICBO and find the value of ICEO

37. Solution
The common-emitter current gain in Exer. 1 is Hence we can obtain 0 by