4.1.3 Structure of Actual Transistors

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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 II SSSS is 10~100 times larger than II SSSS Figure 4.7: Cross-section of an npn BJT 1

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 v CB down to approximately -0.4V Only after this point will diode begin to really conduct Fig 4.8 2

3 4.1.4 Operation in Saturation Mode Fig 4.5(c) Fig 4.9 collector current (eq4.14) (eq6.14) : in saturation region 3 I S C = vbe / VT vbc / VT ic = ISe ISCe base current (eq4.15) (eq6.15) : i in saturation region (eq4.16) (eq6.16) forced β : BC β B S vbe / VT = + C forced ib saturation As v is increased, the value of β is forced lower and lower. = i I e β β this terms plays bigger role as vbc exceeds 0. 4V I SC e v BC / V T

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 i C /i B less than β.? Collector-to-emitter voltage (vv CCCC ) of a saturated transistor VV CCCCssssss = VV BBEE VV BBCC VV CCCCCCCCCC 0.1 tttt 0.3 VV : CBJ has a larger area than the EBJ VV CCCCCCCCCC = 0.3 VV at the edge of saturation VV CCCCCCCCCC = 0.2 VV in deep saturation 4

5 4.1.5 The pnp Transistor Figure 6.10: Current flow in a pnp transistor biased to operate in the active mode. 5

6 4.1.5 The pnp Transistor Figure 4.11: Two large-signal models for the pnp transistor operating in the active mode 6

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. 7

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. 8

9 4.2.1 Circuit Symbols and Conventions 9

10 Collector-Base Reverse Current (I CB0 ) 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 nanoampere range Many times higher than its theoretically-predicted value Contains a substantial leakage component Dependent on v CB Depend strongly on temperature (doubling every 10 C rise) 10

11 4.2.2 Graphical Representation of Transistor Characteristics i C I vbe / VT = Se -2mV for each rise of 1 C Figure 4.16/17: (left) The i C -v BE characteristic for an npn transistor. (right) Effect of temperature on the i C -v BE characteristic. Voltage polarities and current flow in transistors biased in the active mode. 11

12 4.2.3 Dependence of i C on Collector Voltage The Early Effect When operated in active region, practical BJT s show some dependence of collector current on collector voltage As such, i C -v CB characteristic is not straight Common emitter characteristics Early voltage (10-100V) 12

13 4.2.4 Common-Emitter Characteristics The Common-Emitter Current Gain A second way to quantify β is changing base current by i B and measuing incremental i C vv The Saturation Voltage V CEsat and Saturation Resistance (RR CCCCCCCCCC CCCC ) ii CC iibb =II BB,ii CC =II CCCCCCCC RR CCCCCCCCCC : 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). 13

14 Figure 4.21: A simplified equivalent-circuit model of the saturated transistor. 14

15 4.3 BJT Circuits at DC Apply only dc voltages VV BBBB : 0.7V for conducting transistor VV CCEE : 0.2V for saturated transistor Neglect the Early effect In which mode is the transistor operating? Is the CBJ forward-biased by more than 0.4V? Is the ratio i C /i B less than β.? Important!!! Example 4.4~4.12 Figure 4.21: A simplified equivalent-circuit model of the saturated transistor. 15

16 4.3 BJT Circuits at DC 16

Structure of Actual Transistors

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