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Transcript of SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Bipolar Junction Transistor (1) 2013....
![Page 1: SOGANG UNIVERSITY SOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB. Bipolar Junction Transistor (1) 2013. 1. 24. SD Lab. SOGANG Univ. BYUNGSOO KIM.](https://reader036.fdocument.pub/reader036/viewer/2022062321/56649e725503460f94b712e3/html5/thumbnails/1.jpg)
SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Bipolar Junction Tran-sistor (1)
2013. 1. 24.SD Lab. SOGANG Univ.
BYUNGSOO KIM
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
Contents
1. Power Bipolar Junction Transistor Structure
2. Basic Operating Principles
3. Static Blocking Characteristics 3.1 Open-Emitter Breakdown Voltage 3.2 Open-Base Breakdown Voltage 3.3 Shorted Base–Emitter Operation
4. Current Gain 4.1 Emitter Injection Efficiency 4.2 Emitter Injection Efficiency with Recombination in the Deple-tion Region 4.3 Base Transport Factor 4.4 Base Widening at High Collector Current Density
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
1. Power Bipolar Junction Transistor Structure
• a positive bias to the collector terminal J1 becomes reverse biased supports the voltage N-drift region for the blocking voltage capability - the doping concentration of the N-drift region - the thickness of the N-drift region
• a negative bias to the emitter terminal J2 becomes forward biased J2 initiates the injection of electrons
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
• a base current:
• The common-emitter current gain(β ):
• the application of Kirchhoff’s current law
• The common-emitter power gain
2. Basic Operating Principles
Common-Emitter Configura-tion
input side
output side
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
• The common-base current gain(α):
• The common-emitter power gain:
• The relationship between the com-mon-base current and the common-emitter current:
• The relationship between the com-mon-Emitter current and the com-mon-base current:
2. Basic Operating Principles
Common-Base Configuration
input side
output side
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
3. Static Blocking Characteristics
• The bipolar power transistor structure is capable of sup-porting voltage in the first and third quadrants of opera-tion.
In the first quadrant - the collector=> positive bias - the incorporation of the N-drift region - blocking voltages of over 1,200 V
In the third quadrant - the collector=> negative bias - highly doped regions - usually less than 50 V
• A typical set of blocking characteristics depends on how the base terminal is connected Open-Base Shorted-Base the actual blocking voltage rating for the power bipolar
transistor is limited to the breakdown voltage of the open-base case.
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
3.1 Open-Emitter Breakdown Voltage
• If the emitter terminal is open circuited, the device operates like a diode be-tween the base and collector terminals.
the maximum blocking voltage is determined by the breakdown voltage of the J1.
open-emitter breakdown voltage (BVCBO) - the doping concentration of the N-drift region - thickness of the N-drift region an adverse impact on the resistance of the N-drift region - degrades the output characteristics - the increased thickness of the drift region => increases the turn-off time due to the larger stored charge
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
3.2 Open-Base Breakdown Voltage
• The currents flowing through the open-base transistor structure:
• the common base current gain:
• the multiplication coefficient (M):
• At low collector bias voltages, the multiplica-tion factor is equal to unity.
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
3.2 Open-Base Breakdown Voltage
• the open-base breakdown voltage can be rewritten as
• the common-emitter current gain:
n=6 for a P+/N diode • For a typical current gain (β) of between 50
and 100 in a power bipolar N–P–N transistor, The BVCEO is reduced to half of the BVCBO.
• The smaller doping concentration and larger width of the N-drift region
a larger on-state voltage drop slower switching speed
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
3.2 Open-Base Breakdown Voltage
• The doping concentration of the P-base region and its width (WP) are relatively large.
suppresses the extension of the depletion re-gion across J1 into the P-base region.
• The doping concentration of the P-base region and its width (WP) are small.
increase the current gain promotes the extension of the depletion re-
gion across J1 into the P-base region. lead to the complete depletion of the P-base
region the reach-through will be much smaller than
the BVCEO.
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
3.3 Shorted Base–Emitter Operation
• with the base and emitter terminals short cir-cuited
• a positive bias is applied to the collector the base–collector junction is reverse biased The leakage current flows to the base contact This current must flow via the RB of the P-base
region
• The voltage drop is well below the built-in po-tential of the base–emitter junction, there is no injection initiated from this junction.
• The device then supports voltage as in the case of operation with an open emitter.
• the blocking voltage is the same as BVCBO.
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
3.3 Shorted Base–Emitter Operation
• As the current increases, the voltage drop be-comes sufficient to promote the injection of minority carriers.
• Once the base–emitter junction begins to inject minority carriers, the device operates as a bipolar transistor with current gain that in-creases with increasing collector current.
=> produces a collapse in the voltage sup-ported by the transistor until the collector voltage be-comes equal to the BVCEO.
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4. Current Gain
• Current gain (α and β) determines the power gain.
• The current gain can be related its structural parameters.
• The common-base current gain:
The emitter injection efficiency:
The base transport factor:
The collector efficiency:
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.1 Emitter Injection Efficiency• a measure of the emitter current due to the injec-
tion of electrons into the P-base region
• The injected carrier concentrations are related to the corresponding minority carrier concentrations in equilibrium.
• The holes within the emitter region obey the conti-nuity equation:
• If the emitter thickness is much greater than the diffusion length for holes in the emitter
• The hole current density at the base–emitter junc-tion
• the mobility for holes in the emitter region
• the hole current component of the total emitter cur-rent:
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.1 Emitter Injection Efficiency
• the continuity equation for electrons in the P-base region:
• the electron concentration decreases linearly from an nB(0) at the base–emitter junction to zero at the base–collector junction:
• the above electron carrier distribution:
• the mobility for electrons in the base region
• the total emitter current:
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.1 Emitter Injection Efficiency
• The emitter injection efficiency can be obtained by using the electron and hole current compo-nents:
• the minority carrier concentrations in equilibrium:
• the intrinsic carrier concentrations in the base and emitter regions are not equal due to the dif-ference in doping concentrations, which impacts the band-gap narrowing for the regions.
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.1 Emitter Injection Efficiency
• The common-emitter current gain (βE):
• for the current densities
• for the minority carrier concentrations in equilib-rium,
• it is desirable to obtain a high-current gain to con-trol a large load (collector) current with a small input drive (base) current.
• a high gain a large doping concentration for the emitter re-
gion a low doping concentration for the base region
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.1 Emitter Injection Efficiency
• The heavy doping effects in practice
a reduction of the diffusion length for holes in the emitter
a large increase in the intrinsic carrier concentra-tion in the emitter due to band-gap narrowing
• An optimum doping concentration
the emitter is about 1 × a base doping concentration is 1 × the common-base current gain is 0.96 The common-emitter current gain is 25
• a lightly doped base region with a narrow base width
compromise the blocking voltage capability due to the reach-through phenomenon
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
• At low-current densities, it is necessary to ac-count for the recombination current at the base–emitter junction.
• The recombination current in the depletion re-gion:
• The emitter injection efficiency at low-current levels
• As the space-charge generation lifetime is in-creased, a high gain is retained to lower collector current levels.
• The rate of falloff in the current gain with de-creasing collector current density is a strong function of the space-charge generation lifetime.
4.2 Emitter Injection Efficiency with Recombination in the Depletion Region
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.3 Base Transport Factor
• a measure of the ability for the minority carriers injected from the base–emitter junction to reach the base–collector junction
• The diffusion length for electrons (LnB)in the P-base region is much larger than its width (WB).
=> the base transport factor is equal to unity.
• However, in a power bipolar transistor, the base width can be relatively large to prevent reach-through breakdown at high collector bias voltages.
• The diffusion equation for electrons in the P-base region
• At the base–emitter junction (y = 0), the electron concentration
• At the base–collector junction, the electron concentration is zero due to the reverse bias:
• The electron concentration profile:
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• the electron current at the base–emitter junc-tion (JnE) and the base–collector junction (JnC):
• the base transport factor:
4.3 Base Transport Factor
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.3 Base Transport Factor
• the base transport factor
• The base transport factor is determined by the width of the P-base region
• When the diffusion length is much larger than the base width,
the base transport factor becomes equal to unity.
• As the base width is increased to suppress reach-through breakdown,
The base transport factor becomes less than unity.
• The common-emitter current gain
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4.4 Base Widening at High Collector Current Density
• the Kirk effect
When the collector current density is large, an-other phenomenon that reduces the current gain is an increase in the effective base width
occurs when the bipolar transistor is biased in its forward active regime of operation with a large collector bias voltage.
The collector bias is supported across the base–collector junction with a triangular profile at low collector current densities
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.4 Base Widening at High Collector Current Density
• Poisson’s equation with the doping concentra-tion of the N-drift region determining the charge in the depletion region
• The concentration of the electrons in the deple-tion region
• The Poisson’s equation that governs the electric field distribution at high collector current densi-ties
• The electric field profile:
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4.4 Base Widening at High Collector Current Density
• the profile is linear in shape and that its slope becomes smaller as the collector current den-sity is increased.
• the reduction of the slope for the electric field in the drift region promotes its punch-through to the N+ substrate
• becoming equal to zero occurs at a collector current density:
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• The electric field profile
• at an even larger collector current density, the electric field becomes equal to zero at the base–collector junction
• The maximum electric field occurs at the inter-face between the N-drift region and the N+ substrate at a distance y = WN:
• The collector voltage supported by the electric field profile
• The collector current density
4.4 Base Widening at High Collector Current Density
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.4 Base Widening at High Collector Current Density
• the Kirk current density (JK) The collector current density at which the elec-
tric field becomes equal to zero at the base–col-lector junction
a current-induced base region develops within the collector drift region when the collector cur-rent density exceeds its magnitude.
a neutral region develops in the drift region ad-jacent to the base–collector junction.
The electrons injected into the P-base region must now diffuse not only through the WB but also through an extra distance called the cur-rent-induced base width (WCIB)
This enlargement of the base width reduces the base transport factor and the current gain for the bipolar transistor.
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.4 Base Widening at High Collector Current Density
• The electric field profile
• the width of the space-charge region
• The width of the current-induced base region
• the effective base width
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SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.
4.4 Base Widening at High Collector Current Density
• The physical basis for the increase in the width of the current-induced base with increasing col-lector current density is related to the change in the electric field profile
JC1: a lower collector current density JC2: a higher collector current density
• The same collector bias voltage is then sup-ported across a smaller depletion width, with a larger maximum electric field, producing an en-largement of the current-induced base width.
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4.4 Base Widening at High Collector Current Density
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4.4 Base Widening at High Collector Current Density