Studying of light-emitting diode

2) Goal: Studying of electric energy transformation into light one by the LED and measuring its electric and light characteristics.

3) Scheme of laboratory research facility:

B - supply battery mA – miliamperemeter; V – voltmeter; R3 – 1 – 9999 Ω resistors bank; LED – light emitting diode; PD – photodiode; Rsh – shunt resistance μA - microamperemeter

4) Table of measuring instruments:

 

№	Name	Type	Serial №	Grid limit	Grid unit	Accuracy class
1.	Milliamperemeter	М906		25 mА	1 mА	1 %
2.	Microamperemeter	М906		500 μА	20 μА	1 %
1000 μА	40 μА
3.	Voltmeter	DT803B		2000 mV	1mV	0,5 %
4.	Resistor bank R3	Р333	-	9999 Ω	1 Ω	0,2 %

Accesories: the LED, the photodiode.

5) Equations for calculation:

1. Electrical power consumed by LED:

;

where I _{1 (LED)} – current through LED, U – forward bias voltage.

2. Light emission power:

;

where I _{2 (PD)} – current through photodiode, S – photodiode sensitivity.

3. LED energy efficiency:

;

where P_LIGHT - light emission power, P_E - electrical power consumed by LED.

4. LED quantum efficiency:

.

where e = 1.6·10^-19 - charge of electron, λ - wavelength of light, h =6.63·10^-34J·s - Planck constant, c = 3·10⁸ m/s - speed of light, P_LIGHT - light emission power, I _{1 (LED)} – current through LED.

6) Table of measurements

S = … μA/mW - the photodiode sensitivity (imprinted on the box),

λ = … nm,

R_sh = 1 kΩ (grid limit 500 μА); R_sh = 500 Ω (Grid limit 1000 μА).

№	I 1 (LED) , mА	I 2 (PD) , μА	U, V	PLIGHT, mW	PE, mW	η 1 (EN) , %	η 2 (Q) , %
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.

7) Quantities calculation:

U_CUT = … V;

R_RES = … Ω;

8) Graph:

8.1 LED CVC	8.2 Emission power and efficiencies

9) Final results:

U_CUT = … V; R_RES = … Ω; η _{1 (EN)} (50mA) = …%; η _{2 (Q)} (50mA) = …%.

10) Conclusions:

11) Work done by:Work checked by:

 

 

LABORATORY WORK № 6 – 10

Topic: STUDYING OF A PHOTOELECTRIC EFFECT IN A P-N JUNCTION

Goal of the work

Studying the photoelectric effect in a p-n junction and measuring of dark and light current-voltage characteristics.

Main concepts

3.1 P-N junction ( see work 6-9 )

Photodiode properties

Figure 18 – Band diagram of non-illuminated (а) and illuminated (b) p-n junction.

Photovoltaic effect consists in appearing of a photo-emf in a valve (i.e. rectifying) contact while it’s illuminated. The lowest frequency f₀ (the highest wavelength λ ₀ ) at which the photoelectric effect still possible called the photoelectric effect red border. It can be determined for given semiconductor from the condition:

hf₀ = E_g,

that means that energy of photons hf₀ must be not less than the forbidden energy gap width.

On the Fig. 19 (curve 1) represented current-voltage characteristic of non-illuminated p-n junction. It can be described by such equation:

(46)

Figure 19 – CVC of the non-illuminated (curve 1) and illuminated (curve 2) р-n-junction.

here I_S – saturation current of non-illuminated p-n junction, k – Boltzman constant, e – elementary charge, T – temperature in Kelvins, U – external applied voltage.

Figure 20 – Principle of operation of the photocell.

Figure 21 – photocells’ CVC at different light intensities.

Now let’s p-n junction be illuminated by light with energy of photons not less then forbidden gap width of the semiconductor. The internal photoelectric effect will lead to generation of electron-hole pairs, concentration of which will decrease with increasing of distance from illuminated surface. Electrons and holes will move to the junction, where they will be separated. Majority carriers of the region will be held by internal built-in electric field. Accelerated minority carriers will freely path through p-n junction, and create photocurrent I_PHOTO, which flows in the reverse direction (from n- to p - region).

If the circuit is open, then on the borders of p-n junction appears volume charge that opposes to further migration of charge carriers. This volume charge produces photo-emf U₀ with polarity reverse to polarity of built-in potential. Potential barrier of depletion region decreases (Fig. 18b). That leads to appearance of leakage current I_L flowing in the reverse direction. The photo-emf will be increased until the photocurrent will be compensated by increasing current of majority carriers.

If we connect to p-n junction load resistance R_LOAD (Fig. 20), than, in obtained circuit will flow a current I which can be represented as:

I = I_PHOTO­ - I_L (47)

Leakage current I_L can be calculated by formula (46) for non-illuminated p-n junction when an external forward bias voltage U_LOAD = I×R_LOAD­ is applied:

.

Let’s consider the critical modes of operation of such photocell.

1.Short-circuit mode appears if R_LOAD­ = 0. Then U_LOAD = 0, I_L = 0 and short circuit current I_SC is equal to photocurrent I_PHOTO which is proportional to light intensity Ф:

I_SC = I_PHOTO; I_PHOTO ~ Ф (48)

2.Open-circuit mode will take place if R_LOAD­ = ¥. The open-circuit voltage

V_OC = U_PHOTO, total current I = 0 and I_PHOTO = I_L­. Equation (46) gives

(49)

from where we get

(50)

where U_PHOTO – is a photo EMF generated in the photocell.

Figure 22 – Scheme of work 6-10 research facility

Thus, valve photocells allow to transform radiant energy directly into electric one. That is why they also called the photogalvanic elements and used as sources of EMF in the solar cell batteries.

The current-voltage characteristic of illuminated p-n junction shown on the Fig.19 (curve2). Segment OA (U = 0) refers to short-circuit current (R_LOAD = 0), segment OB (I = 0) – to open-circuit voltage (R_LOAD =∞). Changing external load from 0 to ∞ we’ll get segment AB, which is a current-voltage characteristic of p-n junction in a photogalvanic mode at constant light intensity Ф. Segment BC characterize work of the photocell at forward bias voltage, segment AD – at reverse bias (photodiode mode).

The CVCs will be displaced and change their shape under the change of light flux intensity. The photocell CVCs in a photogalvanic mode for different light intensities are shown on the figure 21. Straight line, drawn from the coordinate origin under the angle α (), intersects the CVC at point, which X coordinate is equal to voltage drop on the load R_LOAD, and Y coordinate – current in the external circuit (for example U ₁ = I ₁· R_{LOAD 1}). Highlighted area (Fig.21) is proportional to power P, generated on the load R_{LOAD 1}:

(51)

Optimal load resistance chosen in such way that this power will be maximal. Efficiency of the photogalvanic cell η determined as:

 

(52)

where Ф – light intensity in Watts, P – useful power, emitted on the load.

4 Data processing (Same as in lab № 8.5)