1. Lecture 3

2. Intrinsic Semiconductor When a bond breaks, an electron and a hole are produced: n 0 = p 0 (electron & hole concentration) Also:n 0 p 0 = n i 2 Then: n 0 = p 0 = n i n i = intrinsic carrier concentration [cm -3 ] In Si at 300K (room temperature): n i = 1x 10 10 cm -3

3. Band Gap & Carrier Concentration Eg (eV)n i [cm -3 ] Ge0.672.4 x 10 13 Si1.11.5 x 10 10 GaAs1.435 x 10 7 Band gap and intrinsic carrier concentration for germanium, silicon and gallium arsenide at 300K

4. Extrinsic Semiconductor: Donor N d = donor concentration [cm -3 ] If N d << n i, doping irrelevant – Intrinsic semiconductor  n 0 = p 0 = n i If N d >> n i, doping controls carrier concentrations – Extrinsic semiconductor  n 0 = N d, p 0 = n i 2 /N d Note: n 0 >> p 0  n-type semiconductor

5. Extrinsic Semiconductor: Acceptor N a = acceptor concentration [cm -3 ] If N a << n i, doping irrelevant – Intrinsic semiconductor  n 0 = p 0 = n i If N a >> n i, doping controls carrier concentrations – Extrinsic semiconductor  p 0 = N a, n 0 = n i 2 /N a Note: p 0 >> n 0  p-type semiconductor

6. Extrinsic Semiconductor: Donor & Acceptor Carrier concentration can be engineered by addition of “dopants” (selected foreign atoms): – Pentavalent impurities (P, As, Sb)  n-type semiconductor: n 0 = N d, p 0 = n i 2 /N d – Trivalent impurities (B, Al, Ga)  p-type semiconductor: p 0 = N a, n 0 = n i 2 /N a

7. Properties of Crystals Two properties of crystals that are needed to calculate the current in a semiconductor: – First, we need to know how many fixed and mobile charges are present in the material. – Second, we need to understand the transport of the mobile carriers through the semiconductor.

8. Carrier Transport Two carrier transport mechanisms: – The drift of carriers in an electric field – The diffusion of carriers due to a carrier density gradient

10. Electron and holes will move under the influence of an applied electric field since the field exert a force on charge carriers (electrons and holes). These movements result a current of :  Carrier Drift drift current number of charge carriers per unit volume charge of the electron drift velocity of charge carrier area of the semiconductor

11.  Carrier Mobility, applied field mobility of charge carrier is a proportionality factor  So is a measure how easily charge carriers move under the influence of an applied field or determines how mobile the charge carriers are.

12.  n - type Si + - V n – type Si e-e- Electric field Electron movement Current flow Current carriers are mostly electrons.

13. + - V p – type Si hole Electric field Hole movement Current flow Current carriers are mostly holes.  p - type Si

14. The PN Junction Diode When a P-type semiconductor region and an N-type semiconductor region are in contact, a PN junction diode is formed. VDVD IDID +–

15. Diode Operating Regions In order to understand the operation of a diode, it is necessary to study its behavior in three operation regions: equilibrium, reverse bias, and forward bias. V D = 0V D > 0V D < 0

16. Carrier Diffusion across the Junction Because of the difference in hole and electron concentrations on each side of the junction, carriers diffuse across the junction: Notation: n n  electron concentration on N-type side (cm -3 ) p n  hole concentration on N-type side (cm -3 ) p p  hole concentration on P-type side (cm -3 ) n p  electron concentration on P-type side (cm -3 )

17. Depletion Region A region in a semiconductor device, usually at the juncture of P-type and N-type materials, in which there is neither an excess of electrons nor of holes.

18. Depletion Region As conduction electrons and holes diffuse across the junction, they leave behind ionized dopants. Thus, a region that is depleted of mobile carriers is formed. – The charge density in the depletion region is not zero. – The carriers which diffuse across the junction recombine with majority carriers, i.e. they are annihilated. width=W dep quasi- neutral region

19. Carrier Drift across the Junction Because charge density ≠ 0 in the depletion region, an electric field exists, hence there is drift current.

20. Video Links p-n-Juction-And-Diodes http://www.youtube.com/watch?v=W6QUEq0 nUH8 http://www.youtube.com/watch?v=W6QUEq0 nUH8 http://www.youtube.com/watch?v=jWh06oa G6LA