Driving Led Fast

Driving LED in a Nanosecond Regime by a Fast Operational Amplifier
Joachim Rose, Stella Bradbury, Isabel Bond, Paul Ogden, Andrew Price, Richard Oliver and Yerbol Khassen
Department of Physics and Astronomy, Faculty of Mathematics and Physical Sciences, University of Leeds, LS2 9JT, 31 July 2009

Email: fy06yk@leeds.ac.uk Online at: http://www.leeds.ac.uk/~fy06yk/research/led.pdf

AbstractIt is widely believed that the generation of high speed optical signals is not the job for an LED. However this work is done to show that there are techniques which can be used to produce nanosecond square pulses from a diode. Rise and fall times of a typical 10ns long signal were 1-2 ns and the intensity of the emission could be controlled by the supply voltage. The wavelength of the radiationwas 472 nm, which is blue in colour, but any longer or even shorter wavelengths can similarly be used. The consistency of the experiment and its theoretical model was analysed by computer simulations using OrCAD and PSPICE.

Keywords: Operational Amplifier, SPICE modelling, LED.

Introduction
An LED consists of a semiconductor pn junction where electrons recombine with holes when forwardbiased and power dissipated in the form of light. The frequency of light depends on the band gap of the semiconductor element, which is usually based on InGaN (indium gallium nitride). The blue LED was chosen to be used due to the design of a particular application although longer wavelengths are easier to handle because of the smaller band gap. Current flows easily from the pside (anode) to the n-side(cathode) but not in the opposite direction. The reason is that the electrons are in energetically higher state of the conduction band so they try to obtain the ground state by reaching the valence band. The blue LEDs are less efficient than the red or green LEDs and it causes the leakage in the form of thermal energy which may result in hot temperatures across the junction of about 150 . ˚CFigure 1 illustrates the processes in the diode with potentials applied.

Figure 1 Schematic diagram of a circuit with a diode where electrons combine with holes and produce output emission. Below is the process of recombination in the junction; the larger the band gap the shorter the wavelength of the photon emitted [5].

1

When forward biased, the ideal diode is simply a short circuit andwhen reverse biased, an open circuit. According to Veledar (2007), the equivalent circuits of a diode further classified into DC (up to ~10 kHz) and AC models (>10 kHz), because higher frequencies result in new parameters such as parasitic capacitance and resistance. The model of the diode for this experiment is called large signal and it has a schematic form drawn in Figure 2, but there are alsomodels called small bias and large bias. Massobrio and Antognetti (1993) explained that the aspect of charge storage must be considered in large signal model, because ‘if there was no charge storage, the devices would be infinitely fast as there would be no charge inertia and the currents could be changed in zero time’. The diode capacitance CD is the sum of two capacitances: the junctioncapacitance Cj and the diffusion capacitance Cd given by

 τ D dQD  CD = = dVD  τ D 

 V dI D + C j (0)1 − D  ϕ dVD 0  C j (0) dI D + dVD (1 − FC )1+ m

   

−m

for for

VD < FC × ϕ 0 VD ≥ FC × ϕ 0
(1)

 mVD  1 − FC (1 + m ) +   ϕ0   

with m being equal to 0.33 for a linearly graded junction or 0.5 for an abrupt junction and FC being a factor between 0 and 1.Detailed meanings of the terms can be found in Appendix. The junction capacitance is due to the dopant concentrations in the depletion region which is dominant term for reverse and small forward biases. For moderate forward and beyond, the diffusion capacitance dominates due to the minority-carrier charges injected into the neutral regions and effects of Cj are therefore neglected. It is assumed that...