14-2-5Optoelectronics

Using a doped quantum well, transitions between subband can be used for light detection.
Ic Ib
The advantage of phototransistor:
high gain due to the transistor action(晶体管整流放大作用); low noise compared to an APD.
9. Metal-Semiconductor detectors
collector
N P N
base
发射极 E
— 集电区 集电结 发射区：作用：发射载流子 — 基区 特点：掺杂浓度高 发射结 — 发射区 基区：作用：传输载流子
各区主要作用及结构特点：
emitter
C
特点：薄、掺杂浓度低
集电区：作用：接收载流子 特点：面积大
B E

w bn
re gion de ple tion w BCJ Emitter Base
Cutoff wavelength for absorption
In band to band absorption, the photon energy must be larger than the bandgap to allow optical absorption.
●
Responsivity,R
●
The device is, however, not fast and is quite noisy.
●
TOPICS STUDIED
KEY OBSERVATIONS
phototransistor
● A detector
based upon BJT
technology.
Current due to carries generated in the base can be amplified to produce a high gain detector.
●
Photoconductive detector
A simple n-i-n(or p-i-p) detector in which the conductivity changes when light is incident.
●
The detector can have high gain if the recombination time is larger than the transit time.This allows an electron to go around the circuit several times before recombining with a hole.
The ratio of the photocurrent to the optical power impinging upon the semiconductor.
●
Quantum efficiency,Q
Tells us what fraction of photons result in e-h pairs collected at the contacts.
h
Contact
from minority carrier lifetime issues.
Se mitransp are nt SchottkyMe tal
n

Contact
n
Substrate Se mi Insulating
Light
Antire fletion c C oating
h
8. The phototransistor (光电晶体管)

Emitter is
heavy doped:
n+
Base layer
is narrow and lightly doped
p
+c b _e
n
Collector
The area of collector is large
NPN 型
集电极 C 基极 B
2 0
n
layer: lightly doped (Nd ~ 1015 cm3 ) , absorbing layer.
Advantage of M-S detectors : high speed (f 150GHz)
Ⅱ. M-S Junction
Before MSJ formation
E0
Ec E Fs
Wm
EFm
x
Ws
Ev
对于金属，绝对零度下，电子
ebn
h
Schottky barrier
EFm
Wm
x
Ws
E0
Ec E Fs
e(Vbi V)
Ev
E Fs

eVbi Wm Ws
ebn Wm
肖特基接触是指金属和半导体材料相接触的时候，在界面处半导体的能带弯曲，
by which a photon creates e-h pairs in semiconductors.
● The
process can involve an electron moving from a valence band to conduction band OR from valence band to an impurity level in the bandgap.
11. Advanced Detectors
Important issues in detectors : Tunability 可调性
Eg
Speed
Integration
响应速度
可集成化
t tr f
TOPICS STUDIED
KEY OBSERVATIONS
●Process
Optical absorption in semiconductors
TOPICS STUDIED
KEY OBSERVATIONS
Avalanche photodetector,APD
●
A reverse biased junction where electric fields are so high as to cause carrier multiplication. By proper device design, gain of up to 100 can be achieved.
Contact
Se mitransp are nt SchottkyMe tal
n
Contact
n
Substrate Semi Insulating
metal layer: thick enough to allow the Schottky barrier formation;
thin enough to allow light to pass through (d ~ 10 A )
BAND TO BAND ABSORPTION DETECTORS
QUANTUM WELL INTERSUBBAND DETECTORS
Narrow bandgap materials are “soft” and difficult to produce— Device yield is poor . It is difficult to fabricate tunable detectors.
ii) h Eg iii) h Eg
and and
V VB V ~ VB
similar to PIN similar to APD
Advantage of M-S detectors : high speed (f 150GHz)
Ⅲ. MSM detector
Interdigit ated Schottky Fingers
p
n
Collector

Collect
working state:
EB junction is forward biased; BC junction is reverse biased; Base is dangling.
Base
Emitter Collector
ICE 与 IBE 之比称为共发射 极电流放大倍数，一般有：
10. Quantum well intersubband detector
Detection of long wavelength radiation
Small bandgap material??
h
h
(a )
(b )
EC
EC EV

EV

Advantage of Q-W detector (such as GaAs/InP QwD): to detect long wavelength radiation ( ~ 5 20m)
●
Metal-semiconductor detector
●
A high performance detector that uses a Schottky barrier to sweep the e-h pairs to generate photocurrent.
Quantum well intra-subband detector
When light impinges upon the diode : i) Eg h qbn and V VB (breakdown voltage)
h
Electrons in the metal overcome the Schottky barrier to semiconductor