Patent Publication Number: US-6982406-B2

Title: Simple CMOS light-to-current sensor

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates to a photo-detector used as a photometer to provide a measurement of the amount of light power incident on the detector. Particular applications for these photometers include power-saving control for street lights and domestic appliances, back-lighting of displays in cellular phones, notebook PCs, PDAs, video cameras, digital still cameras, and other equipment requiring luminosity adjustment. 
   BACKGROUND OF THE INVENTION 
   A photometer IC can be constructed by using a light-to-voltage sensor or a light-to-current sensor. A light-to-voltage sensor combines a photo-diode  10  and a trans-impedance amplifier  15  on a single monolithic IC, such as the TSL251R light-to-voltage optical sensor ( 1 ) described in its data sheet by Texas Advanced Optoelectronic Solutions Inc., and is illustrated in  FIG. 1 . The trans-impedance amplifier  15  senses the current Iph generated by the photo-diode  10  and outputs a voltage proportional linearly to the photo-generated current. A light-to-current sensor combines a photo-diode  20  and a current amplifier  25  on a single monolithic IC, such as the TPS851 light-to-current optical sensor ( 2 ) described in its data sheet by Toshiba Corp., and is illustrated in  FIG. 2 . Both sensors are widely used to measure the lighting brightness in the displays of cellular phones and portable devices. The trans-impedance amplifier  15  of the light-to-voltage sensor is quite complicated to implement as an integrated circuit, and the current amplifier  25  of the TPS851 light-to-current sensor is implemented in sophisticated bipolar integrated circuits and manufactured using expensive bipolar process technology. 
   Introductory technical reference for designing the trans-impedance amplifiers and the current amplifiers can be found in the book ( 3 ) titled “Analysis and Design of Analog Integrated Circuits” by Paul R. Gray and Robert G. Meyer. 
   As more functioning chips are packed into electronic portable devices, the demand for smaller and more cost-effective photo-sensor chips increases. 
   SUMMARY OF THE INVENTION 
   The photo-detector of this invention is a CMOS light-to-current sensor which is comprised of a photo-diode and two MOS transistors illustrated in  FIGS. 3 and 4 . As illustrated in  FIG. 5 , in a typical CMOS n-well process technology using a p-type substrate wafer, the photo-diode is constructed by an n+ diffusion layer thermally-diffused on top of the p-type substrate, and the two MOS transistors are p-channel transistors built in the n-well region. As illustrated in  FIG. 3 , the circuit configuration of the sensor is as follows: the photo-diode is connected in reverse biased condition having its p-type substrate node connected to the most negative potential of the sensor such as the ground and its n+ diffusion node connected to both the drain and the gate terminals of a p-type MOS transistor. The p-type MOS transistor functions as the load transistor for the photo-diode, the source terminal of this transistor is connected to the positive supply voltage of the sensor. A second p-type MOS transistor is connected as the current-mirror transistor for the first transistor, having its gate terminal connected to the gate terminal of the first transistor and its source terminal connected to the source terminal of the first transistor. The drain terminal of this second transistor is the output node of the sensor, which will output an amplified current linearly proportional to the photo-diode current to an external resistor. 
   The operation of this CMOS light-to-current sensor is described as follows: In the dark condition when no light is incident on the photo-diode, a small dark thermal-leakage current having the value of several nano-Amperes, (1 nano Ampere is equal to 1.0E-9 Amperes), will flow through the photo-diode and the load transistor. Under this condition, the gate-to-source voltage of the transistor is very close to the threshold voltage (Vtp) of the transistor. Because the second transistor is connected as the current-mirror transistor to the photo-diode load transistor, the current that flows through the second transistor to the external resistor will be linearly proportional to the dark leakage current of the photo-diode, and the voltage at the output node is very close to the ground potential. In the light luminance condition when the light photons illuminate on the photo-diode, the photo-generated electron and hole carriers beneath the photo-diode silicon area will diffuse to the space-charge region of the n+-p junction of the photo-diode and will be separated as the photo-generated current. The photo-generated current will flow through the load transistor and increase the voltage difference between the gate and the source terminals. Similarly, the current of the second current-mirror transistor will rise proportionally to the photo-diode current and will flow through the external resistor. 
   The linear proportional factor of the current of the second transistor to the photo-diode current depends on the number of the duplication of the first transistor used to form the second transistor. If an output current having a large multiplication factor to the photo-diode current is needed, it can be obtained by cascading multiple current-mirror circuits together. This will minimize the size of the chip. Sample circuit configurations for this requirement are illustrated in  FIG. 6  and  FIG. 7 . 
   The preliminary SPICE circuit simulation shows that the sensor of this invention can output an output current linearly, when the intensity of the light on the photo-diode varies from 1 lux to 1000 lux. The simulated transfer curve is illustrated in  FIG. 8 . 
   This invention demonstrates a very small, high performance, and cost-effective CMOS light-to-current sensor which is very suitable for applications in power-saving control of the display units of many portable electronic devices. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates the functional block diagram of a photometer constructed by a light-to-voltage sensor. 
       FIG. 2  illustrates the functional block diagram of a photometer constructed by a light-to-current sensor. 
       FIG. 3  illustrates the circuit diagram of a photometer constructed by a CMOS light-to-current sensor built on a p-type substrate wafer. 
       FIG. 4  illustrates the circuit diagram of a photometer constructed by a CMOS light-to-current sensor built on an n-type substrate wafer. 
       FIG. 5  illustrates a cross sectional view of the COMS light-to-current sensor presented in  FIG. 3 . 
       FIG. 6  illustrates the circuit diagram of a photometer constructed by a CMOS light-to-current sensor with cascading current-mirror circuits built on a p-type substrate wafer. 
       FIG. 7  illustrates the circuit diagram of a photometer constructed by a CMOS light-to-current sensor with cascading current-mirror circuits built on an n-type substrate wafer. 
       FIG. 8  illustrates a transfer curve of the light-to-current sensor presented in  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The photo-detector of this invention is a CMOS light-to-current sensor which is comprised of a photo-diode and two MOS transistors. 
   Referring to  FIG. 3  of the present invention, a CMOS light-to-current sensor built on a p-type substrate wafer is illustrated, the photo-diode  100  is an n+-p junction photo-diode and the two MOS transistors  110  (G 1 ) and  120  (G 2 ) are p-channel transistors built in the n-well region. Also referring to  FIG. 4  of the present invention, a CMOS light-to-current sensor built on an n-type substrate wafer is illustrated, the photo-diode  200  is a p+-n junction photo-diode and the two MOS transistors  210  (G 1 ) and  220  (G 2 ) are n-channel transistors built in the p-well region. 
   As illustrated in  FIG. 5 , in a typical CMOS n-well process technology using a p-type substrate wafer  300 , the photo-diode  100  is constructed by an n+ diffusion layer  310  thermally-diffused on top of the p-type substrate  300 , and the two MOS transistors  110  (G 1 ) and  120  (G 2 ) are p-channel transistors built in the n-well region  320 . As illustrated in  FIG. 3 , the circuit configuration of the sensor is as follows: the photo-diode  100  is connected in reverse biased condition having its p-type substrate  300  connected to the most negative potential of the sensor such as the ground  330  and its n+ diffusion node  310  connected to both the drain D 1  and the gate terminals G 1  of a p-type MOS transistor  110 . The p-type MOS transistor  110  functions as the load transistor for the photo-diode  100 , the source terminal S 1  of this transistor is connected to the positive supply voltage Vcc of the sensor. A second p-type MOS transistor  120  is connected as the current-mirror transistor for the first transistor  110  having its gate terminal G 2  connected to the gate terminal G 1  of the first transistor  110  and its source terminal S 2  connected to the source terminal S 1  of the first transistor  110 . The drain terminal D 2  of this second transistor  120  is the output node  350  of the sensor, which will output an amplified current linearly proportional to the photo-diode current to an external resistor R  125 . 
   The operation of this CMOS light-to-current sensor is described as follows: In the dark condition when no light is incident on the photo-diode  100 , a small dark thermal-leakage current having the value of several nano-Amperes, (1 nano-Ampere is equal to 1.0E-9 Ampere), will flow through the photo-diode  100  and the load transistor  110 . Under this condition, the gate-to-source voltage of the load transistor  110  is very close to the threshold voltage (Vtp) of the transistor. Because the second transistor  120  is connected as the current-mirror transistor to the photo-diode load transistor  110 , the current that flows through the second transistor  120  to the external resistor will be linearly proportional to the dark leakage current of the photo-diode  100 , and the voltage of the output node  350  is very close to the ground potential  330 . In the light luminance condition when the light photons illuminate on the photo-diode  100 , the photo-generated electron and hole carriers beneath the photo-diode silicon area will diffuse to the space-charge region of the n+-p junction of the photo-diode  100  and will be separated as the photo-generated current. The photo-generated current will flow through the load transistor  110  and increase the voltage difference between the gate and source terminals G 1  and S 1 . Similarly, the current of the second current-mirror transistor  120  will rise proportionally to the photo-diode current and will flow through the external resistor. 
   The linear proportional factor of the current of the second transistor  120  to the photo-diode current depends on the numbers of the duplication of the first transistor  110  used to form the second transistor  120 . If an output current with a large multiplication factor to the photo-diode current is needed, it can be obtained by cascading multiple current-mirror circuits together. This will minimize the size of the chip. Sample circuit configurations for this requirement are illustrated in  FIG. 6  and  FIG. 7  with cascaded current mirrors comprising transistors  130  to  160  and  230  to  240  respectively. 
   The preliminary SPICE circuit simulation shows that the sensor of this invention can output an output current linearly, when the intensity of the light on the photo-diode varies from 1 lux to 1000 lux. The simulated transfer curve representing the functional relationship between Vout versus the variation of luminance is illustrated in  FIG. 8 . 
   This demonstrates a very small, high performance, and cost-effective CMOS light-to-current sensor which is very suitable for applications in the power-saving control of the display units of many portable electronic devices. 
   The above disclosure is not intended as limiting. Those skilled in the art will readily observe that numerous modifications and alternations of the device may be made while retaining the substance of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.