1. ספרות עזר : פרופ ' אדיר בר - לב, מוליכים למחצה והתקנים אלקטרוניים, עמ ' 165-168. P.A. Tipler, Modern Physics, pp 332-342 Mc Kelvey, Solidstate and Semiconductor Physics, pp 461-469 p-n junction, Wikipedia: http://en.wikipedia.org/wiki/PN_junctionhttp://en.wikipedia.org/wiki/PN_junction

2. The PN junction is the name of the junction that is formed when joining a P-type semiconductor to an N-type semiconductor. Remember: P-type semiconductors are doped with acceptors which when ionized induced positive mobile charge in the form of extra holes in the material valance band. In contrast N-type semiconductors are doped with donors which when ionized induced negative mobile charge in the form of extra electrons in the material conduction band. Note that In both cases the net space charge (fixed + mobile) will be zero. Conduction band Valence band EAEFEAEF Conduction band Valence band EFEDEFED ++ ++ + ++ ++ + - - -- --- - -- Mobile Holes Fixed Ions ++ ++ + ++ ++ + - - -- --- - -- Mobile Electrons P-type semiconductor N-type semiconductor

3. At equilibrium the net current (diffusion + drift) of holes and electrons across the junction is zero. The resulting built in voltage in the junction in equilibrium will depends on the majority and the intrinsic carrier concentrations of the materials: Note that this internal potential difference cannot be measured with a voltmeter! The voltmeter measures the difference in the Fermi Energy between two points which is zero under equilibrium conditions. Energy Band diagram of the PN junction under zero bias voltage, V=0.

4. At first, due to the concentration gradient electrons start to defuse from the n-type material to the p-type material leaving fixed positively charged ions unmasked. Holes do the opposite and diffuse from the p-type material to the n-type material leaving negatively charged fixed ions unmasked. An area depleted from mobile charge is formed in the junction. The fixed unmasked ions in this area induce an electric field which opposes the diffusion process. An equilibrium is reached.

5. Energy Band diagram of the PN, V>0. When the diode forward-bias-voltage is increased, the barrier for electron and hole diffusion current decreases linearly. Since the carrier concentration decreases exponentially with energy in both bands, diffusion current increases exponentially as the barrier is reduced.

6. Energy Band diagram of the PN, V<0. As the reverse-bias-voltage is increased, the barrier for electron and hole diffusion grows and the diffusion current decreases rapidly to zero, since the fall-off in current is exponential. When the reverse-bias-voltage is increased, the net electric field increases, but drift current does not change. In this case, drift current is limited not by how fast carriers are swept across the depletion layer, but rather how often. In equilibrium the drift current is limited by the number of minority carriers which are thermally generated within a diffusion length of the junction.

7. |I drift | does not change with applied voltage (V), while |I diff | varies exponentially with applied voltage,|I diff | = I 0 exp (eV/kT) where I 0 is a constant. The net current I = I diff – I drift At equilibrium, V = 0 and the net current I = 0 we get: At any applied voltage, V: since I drift = I 0 at any voltage.

8. + + The PN junction can serve as a photo-voltaic cell and generate electricity from light. Photons with energy greater then the material band gap will be absorbed and its energy will excite an electron from the valance band to the conduction band there by creating an electron hole pair. (excess energy will be converted to heat) The built in electric field of the PN junction physically separates the created electron-hole pairs, which will otherwise recombine. P N EFEFEFEF ECECECEC EVEVEVEV - - Holes accumulate on the P side while electrons accumulate on the N side. The resulting potential difference, referred to as an open circuit, can be picked up by an electrometer. The voltage measured is equal to the difference in the quasi Fermi levels of the minority carriers, i.e. electrons in the p-type portion and holes in the n-type portion. When electrodes are provided at both sides, a current can flow between them. Note that the photo generated current is oposit in direction to the diode diffusion current

9. Neglecting shunt and serial resistance (R S =0,R SH = ∞ ) the current through the photo-voltaic cell can be written as: The open circuit voltage (I=0) is therefore: The short circuit current (V=0) will be: Photo-voltaic cell equivalency circuit: I L – photo generated current I D – diode current R S – serial resistance R SH – shunt resistance

10. Take the I-V characteristic of the PN junction, shift it downward by an amount corresponding to the light-injected current (I=I L -I D ), and flip it, and the result is the following I-V curve that is often presented for photo-voltaic cells. The point at which a curve intersects the vertical axis is the short circuit condition: V=0, I=I SC The point at which a curve intersects the horizontal axis is the open circuit condition: V=V OC, I=0 behavior of a photo-voltaic cell at particular intensities of solar radiation