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SSPD_Chapter 7_Part 3_Basic Electrical Properties of MOSFET_continued7

Module by: Bijay_Kumar Sharma. E-mail the author

Summary: SSPD_Chapter 7_Part 3_Basic Electrical Properties of MOSFET_continued7 gives the differences in the key parameters of BJT and MOSFET.

SSPD_Chapter 7_Part 3_Basic Electrical Properties of MOSFET_continued7

7.3.14. Comparative aspects of Key Parameters of CMOS and BJT.

In the Table 7.3.14.1 we enumerate the major differences of CMOS and BJT in qualitative terms then we will deal with it in quantitative terms.

Table 7.3.14.1. Comparative study of CMOS and BJT.

Table 1
  Bipolar Junction Transistors CMOS
Carriers involved in transistor action Bipolar(both minority and majority cariers are involved) Unipolar (only majority carriers participate)
Input dynamic range Exponential device hence input dynamic range for linear applications is 5mV Quadratic Device hence input dynamic range for linear applications is 400mV
Noise Figure Much higher hence not suitable for Low Noise Amplifier At cryogenic temperatures its Noise Figure is nearly Unity hence MESFET is suitable for LNA
Packing Density Because of isolation diffusion BJT ICs are traditionally 1-G behind in level of integration. This has far ahead in packing density.
Speed Historically it is much faster because transit through base width limits the switching speed. It has reached more than 100GHz Here the transit through the channel length limits the speed. The maximum switching speed is 2GHz or more.
Transconductance 40mS at 1mA 1mS at 1mA
Drive capability(A/cm2) 10 times more than that of CMOSHence at the driver stage we use BiCMOS  

7.3.14.1. Quantitative analysis of the key parameters of BJT and CMOS.

In the Pentode region of operation:

Figure 1
Figure 1 (graphics1.png)

Figure 2
Figure 2 (graphics2.png)

Therefore:

Figure 3
Figure 3 (graphics3.png)

Where

Figure 4
Figure 4 (graphics4.png)

In circuit application books β is referred to as Kn , transcoductance parameter for NMNOS.

Substituting the values of the different parameters in Eq.7.3.12.4 we get:

Figure 5
Figure 5 (graphics5.png)
Figure 6
Figure 6 (graphics6.png)

Therefore β = (W/L)×69(µA/V2)

In Table 7.3.14.2 we get the special transconductance parameter for different aspect ratio.

Table 7.3.14.2. Special Transconductance Parameter for different geometries.

Table 2
W L W/L β(µA/V2)
20µm 1 µm 20 1380
60 µm 3 µm 20 1380
10 µm 0.25 µm 40 2760

From Section 7.3.5 we get:

Figure 7
Figure 7 (graphics7.png)
Figure 8
Figure 8 (graphics8.png)
7.3.23

Where

Figure 9
Figure 9 (graphics9.png)

Dividing Eq.7.3.23 by 7.3.12.3 we get:

Figure 10
Figure 10 (graphics10.png)

We will choose W/L = 10µm/0.25µm = 40.

Special transconductance parameter = β= 40×69µA/V2 = 2760 µA/V2

Therefore:

Figure 11
Figure 11 (graphics11.png)

Then VGS-VTn =0.851V.

Substituting this value in Eq.7.3.14.1

We get :

Figure 12
Figure 12 (graphics13.png)

Similarly if we analyze BJT we get :

Figure 13
Figure 13 (graphics14.png)

Figure 14
Figure 14 (graphics15.png)

IC = d.c. collector current and Vt = thermal voltage = kT/q = 25mV at 300K_Room Temperature.

Figure 15
Figure 15 (graphics16.png)

By comparing Eq 7.3.14.2 aand 7.3.14.4 we see that for same bias current 1mA, BJT delivers 17 times transconductace as compared to that of MOSFET.

7.3.14.2. Current Delivery Capability of BJT and MOSFET.

Rewriting Eq.7.3.12.3

Figure 16
Figure 16 (graphics17.png)

Where

Figure 17
Figure 17 (graphics18.png)

Therefore:

Figure 18
Figure 18 (graphics19.png)

Or

Figure 19
Figure 19 (graphics20.png)

Calculating for W/L = 10µm/0.25µm = 40. And D = 25nm we get:

Figure 20
Figure 20 (graphics21.png)

Analyzing for IDS = 1mA we get:

Figure 21
Figure 21 (graphics22.png)

Expressing it in A/cm2 we get:

Figure 22
Figure 22 (graphics23.png)

7.3.14.2.1. Current delivery capability of BJT.

From the Physics of BJT we have the following relationship:

Figure 23
Figure 23 (graphics24.png)

Therefore:

Figure 24
Figure 24 (graphics25.png)

Where WB = effective base width, npo = thermal equilibrium value of minority carrier in Base; λ = Exp(VBE/Vt).

From Device Physics we also know that the Collector Current is diffusion current given by the following expression:

Figure 25
Figure 25 (graphics26.png)

Since our reference current is IC = 1mA and we choose AE = 1(µm)2

Therefore

Figure 26
Figure 26 (graphics27.png)

Therefore

Figure 27
Figure 27 (graphics28.png)

Therefore

Figure 28
Figure 28 (graphics29.png)

Following are the various parameter values:q=1.6×10-19Coulombs, µn = bulk electron mobility=1450 cm2/(V-sec), Thermal Voltage= Vthermal = kT/q =25mV at 300Kelvin, npo = thermal equilibrium value of minority carriers in Base in a NPN transistor = 103/cm3 and WB = effective base width = 1µm=1×10-4cm.

Substituing the above values in Eq.7.3.14.12 we get:

Figure 29
Figure 29 (graphics30.png)

By inspection we see that it is a dimensionless quantity therefore:

Figure 30
Figure 30 (graphics31.png)

We substitute the value of λ=1.724×1015 in Eq.7.3.14.10:

Figure 31
Figure 31 (graphics32.png)

Eq.7.3.14.13 is dimensionally balanced.

Figure 32
Figure 32 (graphics33.png)

Comparing Eq 7.3.14.8 and Eq.7.3.14.14 we see that BJT has 10 times drive capability.

7.3.14.3.Quantitative differences in the key parameters

Table 3
  BIPOLAR TRANSISTOR MOSFET
Current drive capability for 1mA in units of A/cm2 4,31,000 40,000
Transconductance/Bias Current(1/V) 40 2.35
Transconductace(Siemens) Varies directly as bias current Varies as the square root of the bias current
Transconductance as a function of geometry Weak function Strongly dependent on aspect ratio and oxide thickness
Transconductance as a function of process technology Independent Dependent through oxide thickness

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