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1 Summary of electron and hole concentration in semiconductors Intrinsic semiconductor: E G n kt i = pi = N e 2 0 Donor-doped semiconductor: n N D where N D is the concentration of donor impurity Acceptor-doped semiconductor: n p = n i 2 p N A where N A is the concentration of donor impurity n p = n i 2

2 Semiconductor p-n junction basics

3 p-n junction formation p-type material Semiconductor material doped with acceptors. Material has high hole concentration Concentration of free electrons in p-type material is very low. n-type material Semiconductor material doped with donors. Material has high concentration of free electrons. Concentration of holes in n-type material is very low.

4 p-n junction formation p-type material Contains NEGATIVELY charged acceptors (immovable) and POSITIVELY charged holes (free). Total charge = 0 n-type material Contains POSITIVELY charged donors (immovable) and NEGATIVELY charged free electrons. Total charge = 0

5 p-n junction formation What happens if n- and p-type materials are in close contact? p-type material Contains NEGATIVELY charged acceptors (immovable) and POSITIVELY charged holes (free). Total charge = 0 n-type material Contains POSITIVELY charged donors (immovable) and NEGATIVELY charged free electrons. Total charge = 0

6 p- n junction formation What happens if n- and p-type materials are in close contact? Being free particles, electrons start diffusing from n-type material into p-material Being free particles, holes, too, start diffusing from p-type material into n-material Have they been NEUTRAL particles, eventually all the free electrons and holes had uniformly distributed over the entire compound crystal. However, every electrons transfers a negative charge (-q) onto the p- side and also leaves an uncompensated (+q) charge of the donor on the n-side. Every hole creates one positive charge (q) on the n-side and (-q) on the p-side

7 p- n junction formation What happens if n- and p-type materials are in close contact? p-type n-type Electrons and holes remain staying close to the p-n junction because negative and positive charges attract each other. Negative charge stops electrons from further diffusion Positive charge stops holes from further diffusion The diffusion forms a dipole charge layer at the p-n junction interface. There is a built-in VOLTAGE at the p-n junction interface that prevents penetration of electrons into the p-side and holes into the n-side.

8 p- n junction current voltage characteristics What happens when the voltage is applied to a p-n junction? p-type n-type The polarity shown, attracts holes to the left and electrons to the right. According to the current continuity law, the current can only flow if all the charged particles move forming a closed loop However, there are very few holes in n-type material and there are very few electrons in the p-type material. There are very few carriers available to support the current through the junction plane For the voltage polarity shown, the current is nearly zero

9 p- n junction current voltage characteristics What happens if voltage of opposite polarity is applied to a p-n junction? p-type n-type The polarity shown, attracts electrons to the left and holes to the right. There are plenty of electrons in the n-type material and plenty of holes in the p-type material. There are a lot of carriers available to cross the junction. When the applied voltage is lower than the built-in voltage, the current is still nearly zero When the voltage exceeds the built-in voltage, the current can flow through the p-n junction

10 Diode current voltage (I-V) characteristics Semiconductor diode consists of a p-n junction with two contacts attached to the p- and n- sides V p n 0 qv I = I S exp 1 kt I S is usually a very small current, I S A When the voltage V is negative ( reverse polarity) the exponential term -1; The diode current is I S ( very small). When the voltage V is positive ( forward polarity) the exponential term increases rapidly with V and the current is high.

11 The I-V characteristic of the diode qv I = I S 1 exp kt I S

12 The experimental I-V characteristic of a Si diode

13 p- n diode circuit notation p n When plus is applied to the p-side, the current is high. This voltage xp qv kt 1 polarity is called FORWARD. I S When plus is applied to the n-side, the current is nearly zero. This voltage polarity is called REVERSE.

14 p- n diode applications: current rectifiers xp qv kt I S 1 Voltage Current Time Time

15 p- n diode applications: Light emitters + - P-n junction can emit the light when forward biased p-type n-type Electrons drift into p-material and find plenty of holes there. They RECOMBINE by filling up the empty positions. Holes drift into n-material and find plenty of electrons there. They also RECOMBINE by filling up the empty positions. The energy released in the process of annihilation produces PHOTONS the particles of light

16 p- n diode applications: Photodetectors + - P-n junction can detect light when reverse biased p-type n-type When the light illuminates the p-n junction, the photons energy RELEASES free electrons and holes. They are referred to as PHOTO-ELECTRONS and PHOTO-HOLES The applied voltage separates the photo-carriers attracting electrons toward plus and holes toward minus As long as the light is ON, there is a current flowing through the p-n junction

17 Semiconductor p-n junction basics: band diagram technique

18 Semiconductor Energy Bands and Fermi Energy concept free electrons free holes Conductance band Fermi Energy Valence band Fermi Energy E F is an average energy of all the free carriers in a sample. In equilibrium, the Fermi Energy MUST be uniform over the semiconductor sample (compare to the temperature distribution over any sample in equilibrium)

19 Fermi energy level and electron concentration The probability to find free electron with the energy E depends on how much this energy differs from E F At the energy levels just 0.1 ev above the E F level the electron concentration is very low 1.2 f(e) (20 C) f(e) E-E F, ev

20 Fermi Dirac and Maxwell Boltzmann electron distributions in semiconductors Fermi Dirac is a general function describing the probability to find a free electron: In most cases, this probability is << 1, because [(E E F )/kt] >> 1 E EF E f( E) = exp = exp kt kt where E is the energy counted from the Fermi level This energy distribution is called Maxwell Boltzmann function

21 Fermi level position in doped semiconductors E F E F E F Intrinsic semiconductor: n = p = n i; E F ~ (E C + E V )/2 Donor doped semiconductor (n-type): n >> p E F ~E C Acceptor doped semiconductor (p-type): p>> n E F ~ E V

22 Formation of the p-n junction: the energy band diagram language 1. Two separate bits of semiconductor, one is an n-type, the other is a p-type E F E F 2. Bits joined together: not in equilibrium yet! E F (n) E F (p) E F E F

23 Formation of the p-n junction: the energy band diagram language 3. Junction comes into the equilibrium by balancing the Fermi level E F E F

24 Formation of the p-n junction: the energy band diagram language 3. Junction coming to the equilibrium by balancing the Fermi level p n E F E F The balance is achieved by electrons diffusing into a p-side (bringing an extra negative charge in there) and by the holes diffusing into an n-side (bringing an extra negative charge in there)

25 Built-in voltage of the p-n junction The voltage drop between n- and p- parts, V bi creates a difference in the electron and hole energies, φ pn = q V bi E c E F n(x) ϕ(x) n n n ~ e ϕ kt E v ; n( x) = n n e ϕ ( x) kt p n X E C (p-side) = E C (n-side) + ϕ pn

26 Electron (n), hole (p) and net charge (ρ) distribution in the p-n junction Log (Concentration) p p0 N A ; n p0 << N A ; p p0 n p0 = n i2 ; ρ = 0; N A p N D n n n0 N D ; p n0 << N D ; p n0 n n0 = n i2 ; ρ = 0; x p << N A ; n p << n << N A ; p n= n i2 ; ρ -q N A ; n << N D ; p n << p << N D ; p n= n i2 ; ρ +q N D ;

27 Built-in voltage of the p-n junction n( x) = n n e ϕ ( x) kt As the point x moves from the n-side to the p-side, the potential energy (with respect to the conduction band edge on the n-side) changes from 0 to ϕ pn. Far on the n-side, n = n n0 ; Far on the p-side, n = n p0 = n i2 /p p0 ; For x >> depletion region width: n( ) = n ϕ pn ( x) kt n0 e = np0 ϕ pn = kt ln (n n0 /n p0 ) V bi = ϕ pn /q = (kt/q) ln (n n0 /n p0 )

28 Alternative expressions for the built-in voltage. Using n p = n i 2 n n0 p n0 = p p0 n p0 = n i2. Or, n n0 / n p0 = p p0 / p n0 ; Therefore, V bi = (kt/q) ln (n n0 /n p0 ) = (kt/q) ln (p p0 /p n0 ) Note that n n0 = N D, p p0 = N A. n p0 = n i2 /p p0 = n i2 /N A. From these: V bi k T ln N B D = 2 q ni N A

29 Example Find the built-in voltage for a Si p-n junction with N A = cm -3 and N D = cm -3 Assume n i = cm -3 ; V bi k T ln N B D = 2 q ni Answer: V bi = V N A

30 Built-in voltage summary: = 2 ln i A D B bi n N N q T k V = 0 0 p n B bi n n q T k V ln = 0 0 n p B bi p p q T k V ln B B k T k T ev V q. ;. At room temperature (T 300 K):

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