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Budapest University of Technology and EconomicsDepartment of Electron Devices

Solution of the 1st mid-term test

20 October 2009

20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 2

Budapest University of Technology and Economics

Department of Electron Devices

Short questions► What does MFS stay for? What does it mean? In what order

of magnitude is the typical value of the MFS of advanced process today? MFS = minimal feature size, Today much below 0.1µm, in case of

the most advanced processes it is at around 45nm.

► In case of a p-type silicon substrate (ni = 1010/cm3|T=300K) the acceptor concentration is Na = 1017/cm3. What will be the concentration of electrons in this material at around room temperature(T=300K)? The majority carriers are the holes, pp Na. The minority carrier

concentration according to the mass effect law will be:

np = ni2/pp ni

2/Na = 1020/1017 = 103/cm3

20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 3

Budapest University of Technology and Economics

Department of Electron Devices

Short questions► On which side of a pn junction is the depletion layer wider:

on the more heavily doped side or on the less doped side? On the less doped side.

► What is the expression of the drift current for electrons!

p

n

d

a

S

S

N

N

EnqJ nn

20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 4

Budapest University of Technology and Economics

Department of Electron Devices

► 5. Under forward biased conditions at large currents which capacitance of the pn junction dominates? What is it directly proportional with? the diffusion capacitance dominates, it is directly proportional with the

forward current

► Draw the cross-sectional view of a discrete BJT chip!

Short questions

IconstCD

emitter

base

collector

20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 5

Budapest University of Technology and Economics

Department of Electron Devices

Problem solving► The forward voltage of Si diode at 1mA forward current and

25oC temperature is 800mV. What would be the value of its forward voltage if the temperature is elevated to 45 oC? What would be the differential resistance of that diode at that temperature?

I

Ur Td

CmVdT

dU oF /2 UF(45oC) = 800mV – (45 – 25)2mV = 760mV

261

26298,1 KmAdr

74.27298

31826318,1 KmAdr

20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 6

Budapest University of Technology and Economics

Department of Electron Devices

Essay question ► Describe the steps of photolithography which are needed to

open a contact window through a SiO2 layer on a silicon wafer! Provide drawings and explanations for these steps!)

1. wafer cleaning

2. coating by photo resists

3. UV exposure

4. development

5. oxide etching

6. window openphoto resist

photo resist

photo resist

maskoxide window