48 
  
  
  
  
  
  
~ 
e 
] 
] 
8 
8 
8 
5 
relative transistor current 
relative current of the diode 
8 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
20r 100r 
10 L L 1 1 1 1 0 Aie 1 i Apa ii. A 
0 2 4 6 8 10 12 14 0 200 400 600 800 1000 1200 1400 1600 1800 2000 
Time in us Time in us 
Figure 6: Rising and falling edge of a Figure 7: Rising and falling edge of a 
photodiode step response phototransistor step response 
currents, generated by the photons of the optical signal, are added and amplified by the transistor. The optical 
sensitivity of this transistor (but not necessarily the SNR) is higher than the sensitivity of a single diode by a factor of 
the transistor current gain. Figure 4 shows the measured optical sensitivity of eight transistors, all with an area of 463 
um?, realised in a 2 u CMOS process. A further increased current gain has been obtained by Espejo by using a 
darlington phototransistor [6]. In figure 5 the sensitivity is shown for two transistors with same base geometry, but 
different emitter areas of 120 by 120 and 200 by 200 um‘. 
One problem of phototransistors is the low speed. Figure 6 shows the current through a reversed-biased photodiode. It 
tracks the optical power signal with a short delay, which depends on the intrinsic diode capacity, intrinsic diode resistor 
and the external current measurement resistor. Rise time and fall time obviously are in the same order of magnitude. In 
figure 7 the rising and falling edge of a phototransistor are shown. The rising time is comparable with that of the diode. 
The falling time is definitely longer, because the base capacity must be discharged via the base-emitter-diode, and the 
resistor of that diode increases with decreasing current. That means, the time constant of a phototransistor depends on 
the absolute optical power density levels before and after transition. 
MOS-receptor i 
— S : 
das | | 
S. | . 
  
P-channel 
  
  
  
voltage M 
o 
> 
  
  
x x xx 
  
10° 
  
10* 10* 10* 
incident optical power [mW/mm2] 
Figure 9: Sensitivity of a phototransistor with 
MOS diode as logarithmic characteristic 
  
Logarithmic i se | 
bipolar receptor 
. = . x x 
Figure 8: Logarithmic photoreceptors with s LU 
bipolar diode and MOS transistor 3 x 
  
  
  
4 l ; i ; 
10° 10° 10° 107 10° 
incident optical power [mW/qmm] 
Figure 10: Sensitivity of a phototransistor with 
bipolar diode as logarithmic characteristic 
IAPRS, Vol. 30, Part 5W1, ISPRS Intercommission Workshop "From Pixels to Sequences", Zurich, March 22-24 1995 
  
  
I