Photodetectors A photodetector converts electromagnetic radiation (photons) into an electrical signal (electrons) Many applications, including: 1. Detection of signals in optical communications systems (e.g. fibre-based systems) 2. General light detection (e.g. camera light meters) 3. Imaging (e.g. digital cameras) 4. Solar cells The photoconductor The simplest type of photodetector, consisting of a piece of semiconductor with two contacts. Incident light causes the conductivity of the semiconductor to change – this change is measured by an external circuit. light semiconductor contact I bias Conductivity σe= (μn n+ μ p ) p Where µn and µp are the electron and hole mobilities n and p are the electron and hole densities σ generally increases under illumination mainly because n and/or p increase (although µn or µp may also change) Types of photoconductivity p-doped n-doped INTRINSIC EXTRINSIC Intrinsic photoconductivity – band to band transitions involving creation of BOTH electrons and holes Extrinsic photoconductivity - band to impurity level transition (or other way) – only one type of carrier is created Extrinsic photoconductivity is generally used to detect low energy photons (long wavelength, far IR). Semiconductor must be cooled to prevent ionisation of the dopant atoms. For visible and near-infrared spectral region intrinsic photoconductivity is used. The energy limits of this process are determined by: Low energy (long wavelength) Incident photons must have sufficient energy to hν EG excite electrons across the bandgap hc hc νh ≥ EG ≥ ⇒ EG ∴ λ ≤ λ EG High energy (short wavelength) As the photon energy increases above the EG the absorption length in the semiconductor decreases rapidly (absorption coefficient increases) Increasing hν Decreasing λ Carriers created near to the surface may diffuse to the surface where they recombine rapidly before being able to produce a measurable photoeffect. The short wavelength limit results from the short absorption length and surface recombination. How a photoconductor operates Firstly, must consider the recombination dynamics of electrons and holes. Assume that a constant illumination of a semiconductor generates n0 excess carriers per unit volume. If the illumination is removed at time t=0 the excess carrier density, n, at a time t will be: t − T T = recombination time n= n0 e t dn ⎛ − ⎡ 1 ⎤⎞ − =n⎜ − eT −⎟ Carrier recombination rate = ⎜ 0 ⎢ ⎥⎟ dt ⎝ ⎣ T⎦⎠ t 1 − n =n eT = T 0 T hν, POPT Ohmic contact D W L Total incident optical power is POPT Assume the thickness of the semiconductor D >> absorption length of the incident light i.e. all incident light is absorbed. The number of incident photons/sec = POPT/hν Quantum efficiency is η (each incident photon creates η electrons and holes) Number of electrons / holes created per sec = ηPOPT/hν Photogenerated current IPH=eηPOPT/hν I hν =η⇒ PH . POPT e η will be a function of the photon energy and will have a value ≤ 1 The photogenerated current is not strictly a true current as the electrons and holes do not necessarily move. However, IPH has the same dimensions as current. ion for a continuously illuminated semiconductor is:The rate equat dn n 0=R ( − = equilibriu m ) ( 1 ) dt gen T P 1 R = η OPT . (generation rate / volume of device) gen hν WLD From (1), n = RgenT P 1 n= T η OPT . (2) hν WLD n is the steady state number of electrons/holes created by the light. We now need to relate n to the change in conductivity of the semiconductor. Only y.electrons are considered as holes generally have a much lower mobilit J = nevd where J is the current density and vd the electron drift velocity Hence IP = nevd.(WD) (3) IP is the additional current which flows between the electrodes as a result of illumination and is called the photocurrent. Using (2) to substitute for n in (3): P 1 IT= η OPT .. ev WD P hν WLD d ⎛ POPT ⎞ ⎛ Tv d ⎞ ⇒IP =⎜ e η ⎟.⎜ ⎟ ⎝ hν ⎠ ⎝ L ⎠ Now, IPH = eηPOPT/hν and L/vd = tr where tr is the carrier transit time between the contacts. T Hence II.P= PH tr I T and gain=P G = IPHt r For long carrier lifetimes (T) and short contact separation (small tr) G can be very large. Note that: 1. Even in the absence of light a background current (dark current) will flow between the contacts due to carriers produced by impurities or thermal excitation. The photo-signal must be detectable above this background – requires G to be large. 2. A value of G>1 does not imply that the number of photogenerated carriers is multiplied. A gain greater than one occurs because the presence of the photoexcited carriers affects the transport of all the carriers in the semiconductor. External Fall off as carriers created detector closer to the surface and hence efficiency T decreases E G Photon energy Operational parameters of a photoconductor 1. Gain 2. External efficiency – variation with hν 3. Response time – fast response times required in communication systems etc requires a short carrier lifetime T 4. Noise – should be as low as possible High gain and high speed are not mutually compatible as large G requires long T, high speed short T For high speed applications devices such as a PIN diode are generally used: A large reverse bias voltage creates a large electric field in the intrinsic region hν The intrinsic region is required to give sufficient thickness of semiconductor so that all the light is absorbed p-type intrinsic n-type Photons create electrons and holes mainly in the intrinsic region of the device. These carriers are very rapidly swept out of the device by the high electric field – the response time of the device is very fast. Each photon creates at most a single electron/hole which contributes to an externally measured current – the gain is hence equal to unity. Although the gain is generally much less than for a photoconductor the dark current will also be much less (for the PIN it is the small reverse bias leakage current) hence a smaller external signal will not have to compete with a large background. A gain >1 can be obtained by increasing the applied voltage to the point where the initial electron and hole are multiplied by avalanche effects. The device is then known as an ‘Avalanche Photodiode’ and is used to detect very low levels of light (single photons). Summary Photodetectors general applications Photoconductors operation intrinsic and extrinsic wavelength response definition of quantum efficiency Gain = IP/IPH = T/tr Parameters Gain Quantum efficiency Response time Noise P.I.N Photodiodes Next time – revision lecture, pn junctions (electrostatics and IV characteristics).