Download presentation

Presentation is loading. Please wait.

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

Similar presentations

OK

“o” subscript denotes the equilibrium carrier concentration. Ideal diode equation.

“o” subscript denotes the equilibrium carrier concentration. Ideal diode equation.

© 2018 SlidePlayer.com Inc.

All rights reserved.

To make this website work, we log user data and share it with processors. To use this website, you must agree to our Privacy Policy, including cookie policy.

Ads by Google