Introduction to Optical Detectors (Nofziger)

Transcription

1 1 Intrductin t Optical Detectrs (Nfziger) Optical detectrs are at the heart f mst mdern-day ptical systems. They have largely replaced the human eye r tgraic film as the detectin medium. They may be categrized int tw main types pint detectrs and array detectrs. A pint detectr (individual detectr) is used t measure the ttal amunt f light in an ptical beam mtin sensrs, bar cde scanners, etc. An array detectr is used in ptical systems where an image needs t be detected digital cameras (visible light), thermal cameras (infrared energy), CCD cameras used n telescpes, etc. In actuality, array detectrs are made up f individual pint detectrs. When yu buy a 5-megapixel camera, yu are actually buying 5 millin individual pint detectrs (regularly spaced very clsely in an array!). Detectrs may further be categrized int thermal and tn detectrs. Thermal detectrs perate by absrbing all f the light energy incident n the detectr, and prduce a crrespnding temperature increase in sme material. In turn, the temperature increase is sensed as a change in resistance, vltage, current, capacitance, etc. Thermal detectrs prduce an utput that is directly prprtinal t the pwer Φ (jules/sec) incident n the detectr. Phtn detectrs perate by absrbing sme f the light energy incident n the detectr, and prduce a crrespnding electrical current. Phtn detectrs prduce an utput that is directly prprtinal t the tn flux Φ (number f tns/secnd) incident n the detectr. In this lab we will measure the basic prperties f the mst cmmnly-used kind f tn detectr the silicn tvltaic detectr. Basic Physics f Phtvltaic Detectrs (Infrmatin in this sectin is brrwed frm the OPTI 400/500 class ntes, Radimetry, Surces, and Detectrs, Dr. James Palmer) The mst ppular tn detectr is the tvltaic detectr, which invlves an internal ptential barrier. The mst cmmn barrier is frmed with a p-n junctin in semicnductr materials. If tw adjacent regins in the same semicnductr crystal are dped such that ne is n-type (dnr) and the ther is p-type (acceptr), then a ptential barrier is frmed at the junctin. In the n-type material (dnr, dpants are As, Sb, P), the electrns are the majrity carriers and the hles are the minrity carriers. In the p- type material (acceptr, dpants are Al, B, In, Ga), the hles are the majrity carriers and the electrns are the minrity carriers. Majrity carriers are far mre mbile than are minrity carriers, and they are the primary cntributrs t current flw.

2 2 During the prcess f junctin frmatin, the fllwing events ccur: 1. Free electrns in the n-regin are attracted t the psitive charge in the p-regin, and they drift n ver. 2. Free hles in the p-regin are attracted t the negative charge in the n-regin, and they drift n ver. 3. This carrier drift leaves the n-regin with a net psitive charge and the p-regin with a net negative charge. The whle crystal stays neutral; n net carriers are gained r lst. 4. A ptential barrier is thus frmed at the junctin between the p-regin and the n- regin. Fabricatin A p-n junctin tdide may be fabricated by ne f several techniques. The mst cmmn is t start with an n-type f material and diffuse a p-type regin thrugh an xide windw as shwn belw in Fig ACTIVE AREA A-R COAT P+ DIFFUSION DEPLETION REGION SiO 2 N-TYPE BULK SILICON METAL CONTACT Fig. 1.1 Diffused p-n junctin tdide.

3 3 Barrier Height, Energy Gap, and Cut-ff Wavelength The barrier height depends upn the dnr and acceptr levels and cncentratins. This structure is shwn in Figure 1.2. The regin between the n-type and the p-type is called the depletin regin, and there is an electric field acrss it. CONDUCTION BAND N-TYPE E g E f DONOR LEVEL ACCEPTOR LEVEL φ P-TYPE VALENCE BAND DEPLETION REGION Fig. 1.2 Energy levels in a p-n junctin. The difference in energy between the bttm f the cnductin band and the tp f the valence band is called the energy gap, E g. Incident tns with energy > E g create electn-hle pairs, primarily within the depletin regin. The lngest wavelength at which this prcess can ccur is the cutff wavelength λ c : λ c = ( λ in μm, Eg in ev ) (1-1) E g Nte that fr silicn, E g = 1.12 ev, and the cutff wavelength is 1100 nm.

4 4 Biasing the Junctin If we apply external bias acrss the junctin, we can change the energy level structure. If we apply FORWARD BIAS, where the psitive terminal f the bias surce (shwn here as a battery) is applied t the p-type regin, the barrier height is reduced by the amunt f applied vltage. The psitive terminal f the bias surce attracts carriers frm the ther side f the junctin (n-type) and vice versa. The cnsequence is a high current flw due t cnductin by majrity carriers. Since the barrier height is lwered, the depletin regin becmes narrwer. N-TYPE CONDUCTION BAND E f DONOR LEVEL Vf E g ACCEPTOR LEVEL φ-vf VALENCE BAND DEPLETION REGION P-TYPE + Fig. 1.3 Applicatin f frward bias t a p-n junctin. Vf If we apply REVERSE BIAS, where the psitive terminal f the bias surce (shwn here as a battery) is applied t the n-type regin, the barrier height is increased by the amunt f applied vltage. The psitive terminal f the bias surce repels carriers frm the ther side f the junctin (n-type) and vice versa. The cnsequence is a lw current flw due t cnductin by minrity carriers. Since the barrier height is increased, the depletin regin becmes wider. The advantage f reverse-biasing a tdide is t greatly increase its speed f respnse t high-frequency time-varying (AC) signals.

5 5 CONDUCTION BAND E g N-TYPE Vr ACCEPTOR LEVEL P-TYPE E f DONOR LEVEL φ+vr VALENCE BAND DEPLETION REGION + Vr Fig. 1.4 Applicatin f reverse bias t a p-n junctin. Current-Vltage Characteristics f a p-n Junctin The equatin f the I-V (current-vltage) characteristics f a p-n junctin is derived frm a cntinuity equatin: qv kt I = I e β 1 where q = electrnic charge = x C k = Bltzmann cnstant = x J/K T = abslute temperature in Kelvins q/(kt) = 38.7 at 300K β = cnstant t make the equatin fit the data; varies with applied vltage, usually 1 but as high as 3; smetimes called the ideality factr I = reverse saturatin current With ptical radiatin incident, a current is generated, which adds t the dark current as: (1-2) I qv kt I β = e 1 I (1-3)

6 6 I is called the tcurrent, and is given by: I λ = η q Φ = η q Φ (1-4) hc where: η is the quantum efficiency f the detectr (independent f wavelength) λ is the wavelength f the light n the detectr h is Planck s cnstant (6.626x10-34 J sec) c is the speed f light Φ is the tn flux (tns/sec) n the detectr Φ is the pwer (in watts) f the light n the detectr The tcurrent generated by the incident radiatin is directly prprtinal t the tn flux Φ, reduced by the quantum efficiency. This says that a tvltaic detectr is a quantum, r tn detectr. Every tn absrbed within the depletin regin is cnverted int an electrn (electrn-hle pair) that cntributes t the tcurrent. The same tn flux Φ at tw different wavelengths will prduce the same tcurrent. At a given wavelength, the current is als directly prprtinal t the incident pwer Φ, als reduced by the quantum efficiency. Hwever, at tw different wavelengths, the same pwer incident n the detectr will NOT prduce the same tcurrent. This is simply a cnsequence f the fact that a tdide is a tn detectr. The cnversin f tn flux t ptical pwer scales with wavelength: Φ = Φ hc ( energy per tn = Φ ( hν ) = Φ (1-5) λ ) The same ptical pwer Φ at tw different wavelengths will NOT prduce the same tcurrent. Rather, the tcurrents will be in direct prprtin t the tw wavelengths.

7 7 A typical set f I-V curves fr varius incident levels f Φ is shwn in Figure E-05 8E-06 6E-06 4E-06 CURRENT 2E E-06-4E-06-6E-06 DARK LI'L LITE MED LITE -8E-06 LOTSA LITE -1E VOLTAGE Fig. 1.5 I-V curves fr a tdide with several light levels. Mdes f Operatin There are several ways in which ne can perate a tvltaic detectr: Shrt-circuit current. If we set V = 0 in the I-V equatin, the result is that I = -I which is linear with radiant pwer. This linearity is easily demnstrated up t 7 decades in radimetric-quality silicn tdides. This is the mst imprtant and useful way t perate a tdide fr radimetric and tmetric applicatins. Open-circuit vltage. Set I = 0 in the I-V equatin and slve fr V: V c βkt I + I = ln q I (1-6) If I >> I, the result is that V c is lgarithmic with radiant pwer. This is typically nt as useful as the shrt-circuit mde f linear peratin. In this lab, we will learn hw t perate a tdide in the shrt-circuit current mde, and demnstrate its linear respnse t light.

8 8 Respnsivity One f the mst useful merits f peratin is the detectr s respnsivity, R. This is defined as the utput tcurrent divided by the input ptical pwer (amps/watt): I λ R = ηq (1-7) Φ hc This can be pltted vs. wavelength. The ideal tvltaic detectr (η = 1) has a spectral respnsivity prprtinal t wavelength ut t the cutff wavelength, λ c, which is determined by the energy gap (see equatin 1-1) RESPONSIVITY (A/W) WAVELENGTH (um) Fig. 1.6 Respnsivity vs. wavelength ideal (upper curve) and actual (lwer curve). Phtvltaic Detectr Interfacing The Transimpedance Amplifier The fllwing circuit is used t perate a tvltaic detectr (als called a tdide). The circuit is knwn as a Transimpedance Amplifier, r TIA fr shrt. It is the interface between the tdide and the electrnics used t measure the utput signal. Fundamentally, it cnverts the tcurrent, I t a vltage, V. The tdide is cnnected between the inverting pin f the p-amp (pin 2) and grund, and the nn-inverting pin (pin 3) is tied directly t grund. (Nte that grund refers t the grund terminal f the pwer supply that supplies pwer t this entire circuit.) The p-amp wrks t maintain a vltage difference f 0 vlts between pins 2 and 3, which means that ΔV=0 acrss the tdide. This perates the tdide in the shrt-circuit mde, which means that the tcurrent is linear with ptical pwer.

9 9 The relatinship between I and the utput vltage is linearly related t the feedback resistr R f, and fllws Ohm s Law: V = I R (1-8) f The feedback resistr determines the gain f the current-t-vltage cnversin prcess. The negative sign indicates that the utput vltage is negative fr a psitive tcurrent, r psitive fr a negative tcurrent. Physically, this is because the tdide is cnnected t the inverting pin f the p-amp. (Nte that the sign f the tcurrent may be changed by simply reversing the detectr leads within this circuit.) Substituting frm equatin (1-4) gives the fllwing system equatin fr the TIA: V η q = λ Φ hc R f (1-9) η q Nte that is a cnstant. The remaining terms are variable and directly determine hc the vltage measured at the utput f the TIA (pin 6). Please nte the fllwing: The utput vltage is directly prprtinal t the pwer f the incident light. The utput vltage is directly prprtinal t the value f the feedback resistr, which is user-selectable. On mst cmmercial TIA s, there is a range switch that allws yu t select different values f R f, typically 10 3, 10 4, 10 5, 10 6, r 10 7 hms. The switch will be labeled as either Ohms r Vlt/Amp. - The maximum gain available is determined by the pwer supply vltage t the pamp, ±V cc. Yu must keep V < ±V cc r the circuit will saturate (V will n lnger increase with increasing tcurrent). In saturatin, yur readings will n lnger be valid! - Saturatin f the tcurrent itself ccurs at a current density f 50 μa/mm 2 fr silicn tdides. NOTE: the area t cnsider is the area f the light beam incident n the detectr (nt necessarily the area f the detectr itself). In ther wrds, the tcurrent will saturate lcally within the depletin regin. The utput vltage is directly prprtinal t the wavelength f the incident light. If 1 watt f light frm a He-Ne laser (633nm) falls n the detectr, cmpared t 1 watt f light frm a He-Cd laser (442nm), the utput f the TIA will be (633/442) times higher fr the red light cmpared t the vilet light.