1. http://www.eet.bme.hu Budapest University of Technology and Economics Department of Electron Devices Solution of the 1 st mid-term test 20 October 2009

2. Budapest University of Technology and Economics Department of Electron Devices 20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 2 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 (n i = 10 10 /cm 3 |T=300K) the acceptor concentration is N a = 10 17 /cm 3. What will be the concentration of electrons in this material at around room temperature(T=300K)?  The majority carriers are the holes, p p  N a. The minority carrier concentration according to the mass effect law will be: n p = n i 2 /p p  n i 2 /N a = 10 20 /10 17 = 10 3 /cm 3

3. Budapest University of Technology and Economics Department of Electron Devices 20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 3 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!

4. Budapest University of Technology and Economics Department of Electron Devices 20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 4 ► 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 emitter base collector

5. Budapest University of Technology and Economics Department of Electron Devices 20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 5 Problem solving ► The forward voltage of Si diode at 1mA forward current and 25 o C temperature is 800mV. What would be the value of its forward voltage if the temperature is elevated to 45 o C? What would be the differential resistance of that diode at that temperature? U F (45 o C) = 800mV – (45 – 25)  2mV = 760mV

6. Budapest University of Technology and Economics Department of Electron Devices 20-10-2009 Microelectronics BSc course, 1st mid-term test: solutions © BME-EET 2009 6 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 open photo resist mask oxide window