Modeling Photodiodes, LEDs and Laser Diodes

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Modeling Photodiodes, LEDs and Laser Diodes
Source: Brian Hirasuna, Cadence Design Systems, June 2000


LED and laser diode applications are increasing. They are used in diverse applications from complex optical communications systems to consumer electronics to material processing tools. Thus the motivation for this article -- to show some ways of creating PSpice models for LEDs and laser diodes, as well as photodetector counterparts for applications that use them.

LED LEDs are used as indicators (visible light) and to transmit optical power. Common wavelengths manufactured are 660nm (red), 850nm, and 940nm (infrared). An LED is characterized by its peak wavelength, output power, forward current and reverse capacitance and leakage characteristics. The LED can use a diode to model the diode impedance, and a controlled voltage source fit to the power output vs. forward current curve. The diode in the model can be created with the PSpice Model Editor. The controlled voltage source, modeling the power vs. current curve, can use a table or alternately a polynomial fit to the datasheet curve using Excel.

To demonstrate the technique, here is a model for the MLED96 part from Motorola. V(A,C) is the diode voltage. V(P) is the LED power output (as a voltage).

.SUBCKT MLED96 A C P
* A = Anode
* C = Cathode
* P = (light)Power as output voltage in Watts
* Diode forward, reverse leakage and breakdown, and capacitance
characteristics
DLED A VC MLED96
.MODEL MLED96 D
+ IS=10.000E-21
+ N=1.5219
+ RS=1.7433
+ CJO=50.000E-12
+ M=.3333
+ VJ=.75
+ ISR=100.00E-12
+ BV=5.3573
+ IBV=10
+ TT=5.0000E-9
* Leakage resistance
Rleak A VC 40Meg
* Input resistance
Rin P 0 1G
* Power output (as a voltage) - fit to total power output vs. forward
current curve.
ELED P 0 TABLE
{pwr(I(VLED),1.170105501)*EXP(-2.114426964-4.065819492*I(VLED))} (0,0) (1,1)
VLED VC C
.ends

Figure 1 shows the LED model waveform and test circuit.


Fig 1. Total output power vs. forward current for the MLED96 model. Output power is in Watts using voltage as an "analog".

Photodiode
Photodiodes are used to convert input optical power to electrical current. They typically come with peak wavelengths of 800nm and 940nm (infrared), and are also available for laser diodes. A photodiode is typically characterized by its peak wavelength, sensitivity, dark (leakage) current, output power, and its forward current and capacitance characteristics. The PSpice Model editor can be used to create the diode model using the forward current, reverse breakdown, dark current (reverse leakage), and capacitance curves on the datasheet. Then, use the spectral sensistivity (A/W) for the sensitivity multiplier in the Gpd device.

Here is a model for the Siemens BPY12 photodiode. V(A,C) is the diode voltage. V(P) is the input power (as a voltage).

* A = Anode
* C = Cathode
* P = (light)Power (in Watts) as voltage input
.SUBCKT BPY12 A C P
* Diode models dark (leakage) current, forward current, capacitance and
timing
DPD A C PhotoDet
.MODEL PhotoDet D IS=1.0000E-12 RS=10 CJO=138.22E-12 M=.50663
+ VJ=.3905 ISR=1.2315E-9 NR=4.9950 BV=20 IBV=100.00E-6 TT=1.4427E-9
.ends
* Controlled current source outputs input power multiplied by
sensitivity
GPD C A TABLE {V(P)* 0.60} (0,0) (1,1)

Figure 2 shows the photodiode model waveform and test circuit.


Fig 2. Spectral sensitivity vs. forward current curve for BPY12 photodiode model.

A phototransistor is a photodiode, formed by the base-collector junction, combined with an amplifier with a gain of 100 to 1000. Similar techniques can be used to create a phototransistor.

Laser Diode It is also possible to model a laser diode. Laser diodes can be used for applications such as bar code readers, and often contain a monitor diode in the same package. While the LED example used a polynomial controlled current source, this time we'll use a TABLE function (Eop) to model the power output vs. current curve. In this model of the Sony SLD1121VS part, the laser diode output power is V(op). Rather than being brought out of the model it is used in the expression for the monitor diode current.

The monitor diode is characterized by power vs. monitor current, capacitance and breakdown information on the datasheet. A TABLE function (Gopd) is used to model the power vs. monitor current curve. The pdetec diode models the capacitance and reverse breakdown specifications.

* LD = Laser diode cathode
* C = Common pin
* PD = Photodiode anode
.SUBCKT SLD1121VS LD C PD
* Laser diode power vs. current
Eop op 0 TABLE {I(Vid)} (0,0) (37m,0.3m) (40m,0.5m) (47m,5m) (100m,40m)
Rdummy op 0 1k
* Monitor diode power vs. current
Gopd C PD TABLE {V(op)} (0,0) (30m,1.5m)
* Laser diode forward current and breakdown
Dld C I dlaser
Vid I LD
.model dlaser D IS=5E-37 N=1 RS=2 BV=2 IBV=10u
*EG=2.8 XTI=3
* Detector capacitance and breakdown
Dpd PD C pdetec
.model pdetec D CJO=5p BV=15 IBV=10u
.ends

Figure 3 shows the laser diode model waveforms and test circuit.


Fig 3 Optical power and monitor diode current vs. laser diode current for the SLD1121VS.

References:

1. Motorola Optoelectronics Device Data, DL118/D, REV 4
2. 2. Sony SLD1121VS device data sheet, E93821-ST.

other link:
Regarding  laser diode model, I dare to submit to your attention the following information:
-opto.lib file of OrCAD9.0 nd the following versions contin the model of SONY SLD1121VS laser diode.
-diode5.lib of Intusoft's PSpice 7.0 contains the model of HITACHI's HL7801 laser diode.
-an improvement of this model can be find in EDN magazine.
-one accurate model can be found Journal of Lightwave Technology vol.15, no.4, april 1997, page 717.
You cn use these model text codes for improving your application. Please, take care on the differencies existing between the CADENCE and INTUSOFT SPICE versions.

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