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Course Name: Semiconductor Devices Course Code: 10B11EC411

Tutorial Sheet -11

1. For a p-n-p BJT with NE > NB >NC, show the dominant current components, with proper arrows, for directions in the normal active mode. If IEp=10 mA, IEn =100A, ICp= 9.8 mA, and ICn = 1A, calculate the base transport

factor, emitter injection efficiency common-base current gain, common-emitter current gain, and ICBO. If the

minority stored base charge is 4.9X10-11C, calculate the base transit time and lifetime.

2. Si p-n-p transistor has the following properties at room temperature:

n=p=0.1S, Dn=Dp=10 cm2/s, NE=10

19cm-3=emitter concentration,

NB=1016cm-3 =base concentration, NC=10

16cm- =collector concentration,

WE = emitter width= 3m, A = cross-sectional area= 10-5 cm2.

W =metallurgical base width = 1.5m = distance between base-emitter junction and base-collector junction,

Calculate the neutral base width Wb for VCB = 0 and VEB = 0.2 V. Repeat for 0.6V.

3. For the BJT in Prob. 2, calculate the base transport factor and the emitter injection efficiency for VEB = 0.2 and 0.6V.

4. For the BJT in Prob. 2, calculate , , IE, IB, and IC for the two values of VEB.

5. Assume that a p-n-p transistor is doped such that the emitter doping is 10 times that in the base, the minority

carrier mobility in the emitter is one-half that in the base and the base width is one-tenth the minority carrier

diffusion length. The carrier lifetimes are equal. Calculate and for this transistor.

6.

(a) In a BJT, we increase the base doping by a factor of 10 and halve the base width. Calculate approximately by what factor the collector current changes in the normal active mode, assuming that everything else stays the

same.

(b) In a certain BJT, the emitter doping is 100 times greater than the base doping, the emitter width is 0.1 times the base width, and we can assume both base and emitter widths to be much shorter than the carrier diffusion

lengths Ln and Lp. What is the emitter injection efficiency? What is the base transport factor?

(c) Suppose Ln = Lp. Now assuming that both emitter and base ate much wider than the diffusion lengths, what is the emitter injection efficiency and base transport factor?

Explain the various components of current flow and current directions in an n+pn BJT for the normal active

mode of operation. Draw the energy band diagram for equilibrium and this bias condition.