Microelectronic Circuits, Kyung Hee Univ. Spring, 2016...

of 16/16
Microelectronic Circuits, Kyung Hee Univ. Spring, 2016 1 4.1.3 Structure of Actual Transistors Figure 4.7 shows a more realistic BJT cross-section Collector virtually surrounds entire emitter region This makes it difficult for electrons injected into base to escape collection Unity α, large β Device is not symmetrical As such, emitter and collector cannot be interchanged Device is uni-directional is 10~100 times larger than Figure 4.7: Cross-section of an npn BJT
  • date post

    24-May-2018
  • Category

    Documents

  • view

    236
  • download

    8

Embed Size (px)

Transcript of Microelectronic Circuits, Kyung Hee Univ. Spring, 2016...

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    1

    4.1.3 Structure of Actual Transistors Figure 4.7 shows a more realistic BJT cross-section Collector virtually surrounds entire emitter region

    This makes it difficult for electrons injected into base to escape collection Unity , large

    Device is not symmetrical As such, emitter and collector cannot be interchanged Device is uni-directional is 10~100 times larger than

    Figure 4.7: Cross-section of an npn BJT

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    2

    4.1.4 Operation in Saturation Mode

    For BJT to operate in active mode, CBJ must be reverse biased

    However, for small values of forward-bias, a pn-junction does not operate effectively

    As such, active mode operation of npn-transistor may be maintained for vCB down to approximately -0.4V

    Only after this point will diode begin to really conduct

    Fig 4.8

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    3

    4.1.4 Operation in Saturation Mode

    this termsplays biggerrole as

    exceeds 0.

    /

    4V

    /collector current (eq6.14) : in saturation region

    base current (eq6.15)

    in saturation

    BC TB

    BC

    E

    C

    T

    SI

    v Vv VC S SC

    v

    i I I

    =

    =

    e e

    As is increased, the value of is forced lower and lowe

    /

    r.

    /: region

    (eq6.16) forced :

    B

    BC TBE T

    C

    v Vv VSB SC

    Cforced

    B saturation

    v

    Ii I

    ii

    = +

    =

    e e

    (eq4.14)

    (eq4.15)

    (eq4.16)

    Fig 4.5(c)Fig 4.9

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    4

    4.1.4 Operation in Saturation Mode

    Two questions must be asked to determine whether BJT is in saturation mode, or not:

    Is the CBJ forward-biased by more than 0.4V? Is the ratio iC/iB less than .?

    Collector-to-emitter voltage () of a saturated transistor = 0.1 0.3 : CBJ has a larger area than the EBJ = 0.3 at the edge of saturation = 0.2 in deep saturation

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    5

    4.1.5 The pnp Transistor

    Figure 6.10: Current flow in a pnp transistor biased to operate in the active mode.

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    6

    4.1.5 The pnp Transistor

    Figure 4.11: Two large-signal models for the pnp transistor operating in the active mode

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    7

    4.2 Current-Voltage Characteristics

    Figure 4.12: Circuit symbols for BJTs.

    Figure 4.13: Voltage polarities and current flow in transistors

    biased in the active mode.

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    8

    4.2.1 Circuit Symbols and Conventions

    Figure 4.14 Graphical representation of the conditions for operating the BJT in the active mode and in the saturation mode.

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    9

    4.2.1 Circuit Symbols and Conventions

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    10

    Collector-Base Reverse Current (ICB0) Previously, small reverse current was ignored

    Carried by thermally-generated minority carriers

    However, it does deserve to be addressed The collector-base junction current (ICBO) is normally in the nano-

    ampere range Many times higher than its theoretically-predicted value Contains a substantial leakage component Dependent on vCB Depend strongly on temperature (doubling every 10 C rise)

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    11

    4.2.2 Graphical Representation of Transistor Characteristics

    Figure 4.16/17: (left) The iC-vBE characteristic for an npn transistor. (right) Effect of temperature on the iC-vBE characteristic. Voltage polarities and current flow in

    transistors biased in the active mode.

    /

    BE Tv VC Si I= e -2mV for each rise of 1 C

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    12

    4.2.3 Dependence of iC on Collector Voltage The Early Effect

    When operated in active region, practical BJTs show some dependence of collector current on collector voltage

    As such, iC-vCB characteristic is not straight

    Common emittercharacteristics

    Early voltage(10-100V)

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    13

    4.2.4 Common-Emitter Characteristics The Common-Emitter Current Gain

    A second way to quantify is changing base current by iB and measuingincremental iC

    The Saturation Voltage VCEsat and Saturation Resistance ( =,=

    )

    : few ohms to a few tens of ohms

    Figure 4.20: Common-emitter characteristics. (a) Basic CE circuit; note that in (b) the horizontal scale is expanded around the origin to show the saturation region in some detail. A much greater expansion of the saturation region is shown in (c).

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    14

    Figure 4.21: A simplified equivalent-circuit model of the saturated transistor.

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    15

    4.3 BJT Circuits at DC Apply only dc voltages : 0.7V for conducting transistor : 0.2V for saturated transistor Neglect the Early effect

    Figure 4.21: A simplified equivalent-circuit model of the saturated transistor.

    In which mode is the transistor operating? Is the CBJ forward-biased by more than 0.4V? Is the ratio iC/iB less than .?

    Important!!! Example 4.4~4.12

  • Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

    16

    4.3 BJT Circuits at DC

    4.1.3 Structure of Actual Transistors4.1.4 Operation in Saturation Mode4.1.4 Operation in Saturation Mode4.1.4 Operation in Saturation Mode4.1.5 The pnp Transistor4.1.5 The pnp Transistor4.2 Current-Voltage Characteristics4.2.1 Circuit Symbols and Conventions4.2.1 Circuit Symbols and ConventionsCollector-Base Reverse Current (ICB0)4.2.2 Graphical Representation of Transistor Characteristics4.2.3 Dependence of iC on Collector Voltage The Early Effect4.2.4 Common-Emitter Characteristics 144.3 BJT Circuits at DC4.3 BJT Circuits at DC