# Chapter 6: Photodetectors¶

## Example 6.1, Page Number: 224¶

In [1]:
import math

#variable declaration
h = 6.625*10**-34                          #planks constant(J*s)
C = 3*10**8                                #free space velocity(m/s)
Eg = 1.43*1.6*10**-19                      #(joules)

#calculation
LambdaC = h*C/Eg                           #wavelength(nm)

#result
print "Maximum wavelength for photodiode GaAs = ", round(LambdaC*10**9,0),"nm"

Maximum wavelength for photodiode GaAs =  869.0 nm


## Example 6.2, Page Number: 226¶

In [8]:
import math

#variable declaration
Ip_q = 5.4*10**6                     #electron-hole pair generated
Pin_hv = 6*10**6                     #number of incident photons

#calculation
etta = Ip_q / Pin_hv                 #Quantum efficiency

#result
print "Quantum efficiency at 1300nm =" ,etta*100,"%"

Quantum efficiency at 1300nm = 90.0 %


## Example 6.3, Page Number: 226¶

In [12]:
import math

#variable declaration
R = 0.65                                  #Responsivity of photodiode(A/W)
Pin = 10*10**-6                           #Optical power level(watts)

#calculation
Ip = R*Pin                                #Photocurrent(A)

#result
print "Photocurrent =",Ip*10**6,"uA"

Photocurrent = 6.5 uA


## Example 6.4, Page Number: 227¶

In [20]:
import math

#variable declaration
Lambda = 1300*10**-9                #Wavelength (m)
C = 3*10**8                         #freespace velocity(m/s)
q = 1.6*10**-19                     #Charge (coulombs)
etta = 0.9                          #Quantum efficiency
h = 6.625*10**-34                   #planks constant(J*s)
Eg = 0.73                           #energy gap(eV)

#calculation
R = (etta*q*Lambda)/(h*C)           #responsivity(A/W)
LambdaC = 1.24/ Eg                  #cutô€€€off wavelength(meters)

#result
print "Responsivity = ",round(R,2),"A/W"
print "Cutoff wavelength = ",round(LambdaC,1),"um"

Responsivity =  0.94 A/W
Cutoff wavelength =  1.7 um


## Example 6.5, Page Number: 230¶

In [26]:
import math

#variable declaration
etta = 0.65                         #Quantum efficiency
C = 3*10**8                         #freespace velocity(m/s)
Lambda = 900*10**-9                 #Wavelength (m)
q = 1.6*10**-19                     #Charge (coulombs)
h = 6.625*10**-34                   #planks constant(J*s)
Pin = 0.5*10**-06                   #optical power(W)
Im = 10*10**-06                     #multiplied photocurrent(uA)

#calculation
Ip = ((etta*q*Lambda)/(h*C))*Pin    #photocurrent(uA)
M = Im/Ip                           #multiplication

#result
print "Primary photocurrent = ",round(Ip*10**6,3),"uA"
print "Primary photocurrent is multiplied by " ,round(M+1,0)

Primary photocurrent =  0.235 uA
Primary photocurrent is multiplied by  43.0


## Example 6.6, Page Number: 234¶

In [13]:
import math

#variable declaration
Lambda = 1330.0*10**-9                  #Wavelength (m)
ID = 4.0*10**-9                         #photdiode current(nA)
etta = 0.90                             #Quantum efficiency
Pin = 300.0*10**-9                      #incident optical power(nW)
Be = 20.0*10**6                         #reciver bandwidth
q = 1.6*10**-19                         #Charge (coulombs)
h = 6.625*10**-34                       #planks constant(J*s)
T = 283.0                               #room temperature(kelvin)
KB = 1.38*10**-23                       #boltzmann's constant

#calculation
Ip = (etta*q*Pin*1.3*10**-6)/(h*C)        #primary current(uA)
Ishot = 2*q*Ip*Be                         #mean-squre shot noise current(A^2)
IDB = 2*q*ID*Be                           #mean-squre dark current(A^2)
IT = (4*KB*T)*Be/RL                       #mean-sqare thermal current(A^2)

#result
print "Primary current = ",round(Ip*10**6,3),"uA"
print "Mean-squre shot noise current = ",round(Ishot*10**18,2)*10**-18,"A^2  OR = ",round(math.sqrt(Ishot)*10**9,2),"nA"
print "Mean-squre dark current = ",round(IDB*10**20,2)*10**-20,"A^2  OR = ",round(math.sqrt(IDB)*10**9,2),"nA"
print "Mean-squre thermal current = ",round(math.sqrt(IT)*10**9,0),"nA"

Primary current =  0.283 uA
Mean-squre shot noise current =  1.81e-18 A^2  OR =  1.34 nA
Mean-squre dark current =  2.56e-20 A^2  OR =  0.16 nA
Mean-squre thermal current =  18.0 nA


## Example 6.7, Page Number: 239¶

In [9]:
import math

#variable declaration
CP = 3*10**-12                    #photodiode capacitance(pF)
CA = 4*10**-12                    #amplifier capacitance(pF)
CT = CP+CA                        #total capacitance
RT1 = 1000                        #load resistance 1(ohms)
RT2 = 50                          #load resistance 2(ohms)

#calculation
BC1 = 1/(2*math.pi*RT1*CT)        #circuit bandwidth 1 (Hz)
BC2 = 1/(2*math.pi*RT2*CT)        #circuit bandwidth 2 (Hz)

#result
print "Circuit bandwidth  for 1k ohms = ",round(BC1*10**-6,0),"MHz"
print "Circuit bandwidth  for 50 ohms = ",round(BC2*10**-6,0), "MHz"

Circuit bandwidth  for 1k ohms =  23.0 MHz
Circuit bandwidth  for 50 ohms =  455.0 MHz