Avalanche photodiode & there bandwidth
- 1. PRESENTED BY:- BHUPESH SHARMA ROLL NO : 15S/EL017 AVALANCHE PHOTODIODE & THERE BANDWIDTH
- 2. INTRODUCTION The performance of Avalanche photo diode depends upon the efficiency. It convert light energy in to electrical signal.
- 3. PRINCIPLE Designed to operate in reverse bias condition. Photo electric effect:
- 4. Avalanche Photodiodes High gain due to avalanche multiplication effect Increased noise Silicon has high gain but low noise Si-InGaAs APD often used(diagram on right) n + p + pi Electricfield Depletion region
- 5. Avalanche Photodiodes (APDs) High resistivity p-doped layer increases electric field across absorbing region High-energy electron-hole pairs ionize other sites to multiply the current Leads to greater sensitivity
- 6. APD Detectors Signal Current s q i M P h APD Structure and field distribution (Albrecht 1986)
- 7. Detector Equivalent Circuits Iph Rd Id Cd PIN Iph Rd Id Cd APD In Iph=Photocurrent generated by detector Cd=Detector Capacitance Id=Dark Current In=Multiplied noise current in APD Rd=Bulk and contact resistance
- 8. Carrier transit time Transit time is a function of depletion width and carrier drift velocity td= w/vd
- 9. Detector Capacitance p-n junction xp xn For a uniformly doped junction Where: =permitivity q=electron charge Nd=Active dopant density Vo=Applied voltage V bi=Built in potential A=Junction area C A W w xp xn C A 2 2q Vo Vbi Nd 1/ 2 W 2(Vo Vbi) qNd 1/2 P N Capacitance must be minimized for high sensitivity (low noise) and for high speed operation Minimize by using the smallest light collecting area consistent with efficient collection of the incident light Minimize by putting low doped “I” region between the P and N doped regions to increase W, the depletion width W can be increased until field required to fully deplete causes excessive dark current, or carrier transit time begins to limit speed.
- 10. Bandwidth limit C=0K A/w where K is dielectric constant, A is area, w is depletion width, and 0 is the permittivity of free space (8.85 pF/m) B = 1/2RC
- 11. MERITS APD Large gain Greater level of sensitivity High speed
- 12. DEMERITS Much higher operating voltage required Much higher level of noise Output is not linear Requires high reverse bias for operation Not as widely used due to low reliability
- 13. APPLICATION Level of gain is of importance for high voltage requirement. Laser range finders Fast receiver modules for data communications High speed laser scanner (2D bar code reader) Speed gun