1. Semiconducting junctions
Lecture 12
Semiconducting junctions

2. The PN-Junction
One of the simplest bipolar devices, important for the understanding of more complex devices (bipolar = both electrons and holes contribute to device characteristics).

3. Semiconductor devices: Inhomogeneous semiconductors
All solid-state electronic and opto-electronic devices are based on doped semiconductors.
In many devices the doping and hence the carrier concentrations are non-homogeneous.
In the following section we will consider the p-n junction which is an important part of many semiconductor devices and which illustrated a number of key effects

4. Nonlinear I-V characteristics
Diode
Nonlinear I-V characteristics
Forward bias n p V I A + - + - A A + -
Reverse bias

5. The p-n semiconductor junction: p-type / n-type semiconductor interface
We will consider the p-n interface to be abrupt. This is a good approximation.
n-type ND donor atoms per m3
p-type NA acceptor atoms per m3
Consider temperatures ~300K Almost all donor and acceptor atoms are ionised.
impurity atoms m - 3 N A D x a
= 0 p
type n
(x) = N
(x>0) = 0 (x<0)
(x<0) = 0 (x>0)
p-n interface at x=0.

6. Electron and hole transfer
n-type semiconductor
p-type semiconductor E C V m
Electrons
Holes
Electron and hole transfer
Consider bringing into contact p-type and n-type semiconductors.
n-type semiconductor: Chemical potential, m (Fermi level) below bottom of conduction band
p-type semiconductor: Chemical potential, m above top of valence band.
Electrons diffuse from n-type into p-type filling empty valence states.

7. Band Bending
Electrons diffuse from n-type into p-type filling empty valence band states.
The p-type becomes negatively charged with respect to the n-type material.
Electron energy levels in the p-type rise with respect to the n-type material.
A large electric field is produced close to the interface.
Dynamic equilibrium results with the chemical potential (Fermi level) constant throughout the device.
Note: Absence of electrons and hole close to interface -- depletion region

8. Junction
At equilibrium the Fermi level gradient equals zero!

9. p-n junction I
IV characteristics :

10. The principle working of a pn-junction
P-doped
N-doped
Positively charged holes + negatively charged immobile acceptors
Negatively charged electrons + positively charged immobile donors
P-doped
N-doped
holes - +
electrons
No electrons or holes, only charged donors/acceptors (DEPLETION LAYER)

11. The principle working of a pn-junction+ -
electrons
holes
No Voltage
P-doped
N-doped I
IV characteristics :
Current + -
electrons
holes
Forward bias
current
Voltage
“No” current
(Leakage current)
Large current - +
electrons
holes
Reverse bias
“no” current
Circuit symbol:

12. Ec No bias Ev Forward Ec bias Reverse Ec bias Drift (thermally exc.)
Diffusion (E-field) e- e- Ec
No bias Ev
Forward
bias Ec j V
Reverse
bias Ec V V

13. Io = Ind + Ipd = (C1 Np + C2Nn) exp (-eVbi/(kT))
I-V Characteristics
Hole current:
diffusion Ipd = C1Npexp (-eVbi/(kT))
drift Ipu = CNpn = Ipd = C1Npexp (-eVbi/(kT))
at forward bias IpF = C1 Np exp (-e(Vbi- V) /(kT))
Ip = IpF - Ipu = C1Np exp (-e(Vbi- V) /(kT)) – C1Np exp (-eVbi/(kT)) =
C1Npexp [-eVbi/(kT)][exp(eV/(kT)-1] =Ipd [exp(eV/(kT))-1]
Electron current:
In = Ind [exp(eV/(kT)) with Ind = C2Nn exp (-eVbi/(kT))
I = Io [exp(eV/(kT)-1]
Io = Ind + Ipd = (C1 Np + C2Nn) exp (-eVbi/(kT))

14. Rectifier
Ac transfers into dc a) b) I t

15. photodiode Based on the photovoltaic effect -solar cell
-photodetectors

16. Avalanche diode Powielanie lawinowe (Vprzebicia>6Eg/e) p n -
elektrony
uzyskują energię +
aby kreować pary elektron-dziura
przez zderzenie nieelastyczne

17. Zener diode
Wykład VI

18. photodiode Short-circuit (U = 0) Isc = q Nph(Eg)
Light is absorbed if ; EHP are created; electric field separates carriers
Short-circuit (U = 0)
ID (A)
VD (V) - E C V EF hf
Isc
Isc = q Nph(Eg)

19. photodiode Open circuit Voc Id = Io [exp(eVoc /kT)-1]
ID (A)
VD (V) EC EV
qVbi
qVOC
Voc
Id = Io [exp(eVoc /kT)-1]
This current balances photogenerated current, Isc
Isc – Id = 0

20. Solar cell Transfers solar energy into electric energy
P = I x U=I2 x R= U2/R

21. LED Ge Si
GaAs

22. Semiconductor laser