sogang university sogang university. semiconductor device lab. bipolar junction transistor (1) 2013....

SOGANG UNIVERSITYSOGANG UNIVERSITY. SEMICONDUCTOR DEVICE LAB.

Bipolar Junction Tran-sistor (1)

2013. 1. 24.SD Lab. SOGANG Univ.

BYUNGSOO KIM


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


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


• a base current:

• The common-emitter current gain(β ):

• the application of Kirchhoff’s current law

• The common-emitter power gain


Common-Emitter Configura-tion

input side

output side


• 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:


Common-Base Configuration

input side

output side


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.


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


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.



• 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



• 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.


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.


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.


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:


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:


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:



• 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.



• 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



• 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


• 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


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:


• the electron current at the base–emitter junc-tion (JnE) and the base–collector junction (JnC):

• the 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


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



• 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:



• 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:


• 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




• 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.



• The electric field profile

• the width of the space-charge region

• The width of the current-induced base region

• the effective base width



• 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.

sogang university sogang university. semiconductor device lab. bipolar junction transistor (1) 2013....

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