design of bjt transistors

8/13/2019 Design of BJT transistors

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COLLEGE OF ENGINEERING

DEPARTMENT OF ELECTRICAL ENGINEERING

MICROELECTRONIC DEVICES SECTION 1

Design and Fabrication of aBJT

NAME: ID#

ABDULLAH ALSAWAD 33811

AHMAD MHD SAID AL HAMWI 35566

NIZAR DADOUCH 27120


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Introduction

BJTs come in two types, or polarities, known as PNP and NPN based on the doping types of the

three main terminal regions. An NPN transistor comprises two semiconductor junctions that

share a thin p-doped anode region, and a PNP transistor comprises two semiconductor

junctions that share a thin n-doped cathode region.

In typical operation, the base emitter junction is forward biased, which means that the p-

doped side of the junction is at a more positive potential than the n-doped side, and the base

collector junction is reverse biased. In an NPN transistor, when positive bias is applied to the

base emitter junction, the equilibrium is disturbed between the thermally generated carriers

and the repelling electric field of the n-doped emitter depletion region. This allows thermally

excited electrons to inject from the emitter into the base region. These electrons diffuse

through the base from the region of high concentration near the emitter towards the region of

low concentration near the collector. The electrons in the base are called minority carriers

because the base is doped p-type, which makes holes the majority carrier in the base.


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Background and History

A bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies on the

contact of two types of semiconductor for its operation. BJTs can be used as amplifiers,

switches, or in oscillators. BJTs can be found either as individual discrete components, or in

large numbers as parts of integrated circuits.

Bipolar transistors are so named because their operation involves both electrons and holes.

These two kinds of charge carriers are characteristic of the two kinds of doped semiconductor

material. In contrast, unipolar transistors such as the field-effect transistors have only one kind

of charge carrier.

Charge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction

between two regions of different charge concentrations. The regions of a BJT are called emitter,

collector, and base. A discrete transistor has three leads for connection to these regions. By

design, most of the BJT collector current is due to the flow of charges injected from a high-

concentration emitter into the base where there are minority carriers that diffuse toward the

collector, and so BJTs are classified as minority-carrier devices.


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Method and Theory:

In order to manufacture a good Bipolar Junction Transistor (BJT) three important criteria must

be taken into consideration:

1. Emitter Injection efficiency: the ratio of the injected emitter current to the total emitter

current. In order to improve it, the device must be designed such as the base neutral

width is smaller than the electron diffusion length L e.

2. Base transport factor: The ratio of the electron current reaching the base-collector

junction to the current injected at the emitter-base junction. In order to make it

approach unity the neutral base width must be small without falling into the trap of

base width modulation and punch though.

3. Collector efficiency: it can be improved by reducing the lateral and vertical resistance of

the collector (n+ buried layer).

These criteria suggest that the base width must be small. Furthermore to avoid punch through,

the collector doping must be low compared to the base doping. And of course the emitter

doping must be high. Hence: Ne>>Nb>>NC .

Another important factor in the design of BJTs is the base intrinsic resistance (near the base

contact) the area under the base contact must be heavily doped (P+) in order to minimize it.

With these concepts in mind, a good BJT silicon transistor can be fabricated.


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Design Criteria:

Simulate the fabrication of a silicon npn transistor with the following parameters.Device length = 2.2 mEmitter width = 0.2 mBase width = 0.1 mCollector width = 0.7 mDoping concentrationEmitter = 4x1020 atoms cm-3Base = 1x1018 atoms cm-3Collector = 2x1016 atoms cm-3

Plot the donor and acceptor concentration profiles in the emitter, base, and collectorregions of the transistor.Using device simulator Atlas, plot the VBE-IB characteristics of the BJT and extract hie of

the transistorPlot the VCE-IC characteristics of the BJT and extract hfe of the transistor.Modify the device structure to increase hfe to be 5 times the present value.


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Doping Profile:

As discussed previously, Ne>>Nb>>NC . hence the doping profile must resemble figure 1.

Figure 1

This type of profile can be achieved using either diffusing or implantation. Since the design

criteria require a fairly small device, using implantation proved to be a difficult task. Hence

diffusion was used instead.

The first step is to diffuse boron into the substrate in a nitrogen environment to prevent

oxidation, and then a thin emitter layer is deposited by implanting a layer of poly silicon with

arsenic and slowly diffusing it onto the substrate. Further detail will follow in the next section of

the project report.


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Fabrication process:

Figure 2


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The first step is to define a wafer with back ground doping such as Arsenic. This wafer is then

mirrored vertically and an N+ region is created for the collector. This layer reduces the collector

resistance and improves the collector efficiency:

Figure 3


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The second step is to implant boron into the wafer in order to create a p-region. The doping of

this region must be greater than the background doping and less than the emitter doping by at

least one magnitude:

Figure 4


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After creating the P-region, poly silicon was deposited onto the wafer to act as the emitter. This

reduces contact resistance and improves the efficiency of the emitter. Furthermore this

technique is very useful when a shallow emitter is required (figure 4). The poly silicon is

implanted with arsenic. Etched and then diffused. (figure 5)

Figure 5


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The final step is to anneal the device from defects reflect it across the Y axis and place and

ohmic contacts for testing.

Figure 6


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Testing the device

A good way to check the gain of a BJT and inspect whether the device is behaving in an ideal

manner is to utilize a gummel-poon plot. The plot shows the recombination effects at low bias

and the kirk/auger effects at high transport. This allows the designer to quickly realize a

working device (figure 7).

Figure 7


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Figure 10

The original gain is 10V/V at VCE=3V (close up available in the appendix)

While after modifications the gain is 69V/V

An improvement of 10x was not achieved due to the non-ideal effects (kirk/auger) and

the base width modulation. A heterojunction bjt may be more suitable due to the

difference in the bandgaps of the the materials.


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Appendix

CODE:

go athena

# Establish initial grid and substrate material

line x location=0.0 spacing=0.08line x location=0.5 spacing=0.05line x location=0.7 spacing=0.05line x location=1.2 spacing=0.08line x location=2.2 spacing=0.18#line y location=0.0 spacing=0.01line y location=0.1 spacing=0.02line y location=0.5 spacing=0.05line y location=.6 spacing=0.15#

init orientation=100 silicon c.arsenic=2e15 two.dstructure outf=tmpr1.str two.dinit inf=tmpr1.str flip.yimplant arsenic energy=20 dose=1e16diffuse time=3 temp=1000struct outf=tempinit inf=temp flip.yimplant boron energy=1.5 dose=.60e14diffuse time=2.0 temp=1000 t.final=920 nitrogen# Mask and implant the emitterdeposit poly thick=.12 divis=5implant arsenic energy=42 dose=10.5e15etch poly right p1.x=1.4deposite oxide thick=.12 divis=5etch oxide righ p1.x=1.6

diffuse time=2.5 temp=950 t.final =960 c.boron=2e21diffuse time=180 temp=600 nitrogen c.boron=2e21

etch oxide# Deposit and pattern the contact metal

deposit aluminum thick=0.1 div=2

etch aluminum start x=1.4 y=10.etch cont x=1.4 y=-10.etch cont x=1.6 y=-10.etch done x=1.6 y=10.

structure reflect left

# Define the electrodeselectrode name=emitter x=0.0electrode name=base x=2.0electrode name=collector backsideelectrode name=base1 x=-2.0# Define impurity characteristics in each material

structure outfile=BJT.str

go atlas


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contact name=base1 common=base short# Material parameter and model specification

material material=Si taun0=1e-7 taup0=1e-7model bgn consrh auger fldmob conmob

# Initial solutionsolve init

save outf=bjtex04_0.str

tonyplot bjtex04_0.str -set bjtex04_0.set

# Gummel plot

method newton autonr trapsolve vcollector=0.025solve vcollector=0.1solve vcollector=0.25 vstep=0.25 vfinal=2 name=collector

solve vbase=0.025solve vbase=0.1solve vbase=0.2

log outf=InputChara.logsolve vbase=0.3 vstep=0.05 vfinal=1 name=base

tonyplot InputChara.log

#IC/VCE with constant IB

#ramp Vb

log offsolve init

solve vbase=0.025solve vbase=0.05

solve vbase=0.1 vstep=0.1 vfinal=0.7 name=base

# switch to current boundary conditions

contact name=base current

# ramp IB and save solutionssolve ibase=1.e-6save outf=bjt_1.str mastersolve ibase=2.e-6

save outf=bjt_2.str mastersolve ibase=3.e-6save outf=bjt_3.str master

# load in each initial guess file and ramp VCEload inf=bjt_1.str masterlog outf=bjtout_1.logsolve vcollector=0.0 vstep=0.25 vfinal=5.0 name=collector


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load inf=bjt_2.str masterlog outf=bjtout_2.logsolve vcollector=0.0 vstep=0.25 vfinal=5.0 name=collector

load inf=bjt_3.str masterlog outf=bjtout_3.logsolve vcollector=0.0 vstep=0.25 vfinal=5.0 name=collector

# plot resultstonyplot -overlay bjtout_1.log bjtout_2.log bjtout_3.logquit

Beta extraction:

BETA=6.97 E-5/1 E-6= 69.7

design of bjt transistors

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