Text of AEI105.1201 Types of Semiconductors Semiconductors can be classified as: 1.Intrinsic Semiconductor....
AEI105.1201 Types of Semiconductors Semiconductors can be classified as: 1.Intrinsic Semiconductor. 2.Extrinsic Semiconductor. Extrinsic Semiconductors are further classified as: a. n-type Semiconductors. b. p-type Semiconductors.
AEI105.1202 Intrinsic Semiconductor Semiconductor in pure form is known as Intrinsic Semiconductor. Ex. Pure Germanium, Pure Silicon. At room temp. no of electrons equal to no. of holes. Si FREE ELECTRON HOLE Fig 1.
AEI105.1203 Intrinsic semiconductor energy band diagram Fermi level lies in the middle Conduction Band Valence Band Energy in ev FERMI LEVEL Fig 2.
AEI105.1204 Extrinsic Semiconductor When we add an impurity to pure semiconductor to increase the charge carriers then it becomes an Extrinsic Semiconductor. In extrinsic semiconductor without breaking the covalent bonds we can increase the charge carriers.
AEI105.1205 Comparison of semiconductors Intrinsic Semiconductor 1.It is in pure form. 2. Holes and electrons are equal. Extrinsic Semiconductor 1.It is formed by adding trivalent or pentavalent impurity to a pure semiconductor. 2.No. of holes are more in p- type and no. of electrons are more in n-type.
AEI105.1206 (Cont.,) 3. Fermi level lies in between valence and conduction Bands. 4. Ratio of majority and minority carriers is unity. 3. Fermi level lies near valence band in p-type and near conduction band in n-type. 4. Ratio of majority and minority carriers are equal.
AEI105.1207 Comparison between n-type and p-type semiconductors N-type Pentavalent impurities are added. Majority carriers are electrons. Minority carriers are holes. Fermi level is near the conduction band. P-type Trivalent impurities are added. Majority carriers are holes. Minority carriers are electrons. Fermi level is near the valence band.
AEI105.121 to 1228 N-type Semiconductor When we add a pentavalent impurity to pure semiconductor we get n-type semiconductor. As Pure si N-type Si Fig 1.
AEI105.121 to 1229 N-type Semiconductor Arsenic atom has 5 valence electrons. Fifth electron is superfluous, becomes free electron and enters into conduction band. Therefore pentavalent impurity donates one electron and becomes positive donor ion. Pentavalent impurity known as donor.
AEI105.121 to 12210 P-type Semiconductor When we add a Trivalent impurity to pure semiconductor we get p-type semiconductor. Ga Pure si P-type Si Fig 2.
AEI105.121 to 12211 P-type Semiconductor Gallium atom has 3 valence electrons. It makes covalent bonds with adjacent three electrons of silicon atom. There is a deficiency of one covalent bond and creates a hole. Therefore trivalent impurity accepts one electron and becomes negative acceptor ion. Trivalent impurity known as acceptor.
AEI105.121 to 12212 Carriers in P-type Semiconductor In addition to this, some of the covalent bonds break due temperature and electron hole pairs generates. Holes are majority carriers and electrons are minority carriers.
AEI105.121 to 12213 P and N type Semiconductors + + + +++ + + ++ + N - - - - - -- - - - - P Acceptor ionDonor ion Minority electron Minority hole Majority holesMajority electrons Fig 3.
AEI105.121 to 12214 Comparison of semiconductors Intrinsic Semiconductor 1.It is in pure form. 2.Holes and electrons are equal. 3.Fermi level lies in between valence and conduction Bands. Extrinsic Semiconductor 1.It formed by adding trivalent or pentavalent impurity to a pure semiconductor. 2.No. of holes are more in p- type and no. of electrons are more in n-type. 3.Fermi level lies near valence band in p-type and near conduction band in n-type.
AEI105.121 to 12215 Conduction in Semiconductors Conduction is carried out by means of 1. Drift Process. 2. Diffusion Process.
AEI105.121 to 12216 Drift process CB VB Electrons move from external circuit and in conduction band of a semiconductor. Holes move in valence band of a semiconductor. A B V Fig 4.
AEI105.121 to 12217 Diffusion process X=a Moving of electrons from higher concentration gradient to lower concentration gradient is known as diffusion process. Fig 5.
AEI105.12318 P and N type Semiconductors + + + +++ + + ++ + N - - - - - -- - - - - P Acceptor ionDonor ion Minority electron Minority hole Majority holesMajority electrons Fig 1.
AEI105.12320 Formation of pn diode A P-N junction is formed, if donor impurities are introduced into one side,and acceptor impurities Into other side of a single crystal of semiconductor Initially there are P type carriers to the left side of the junction and N type carriers to the right side as shown in figure 1
On formation of pn junction electrons from n- layer and holes from p-layer diffuse towards the junction and recombination takes place at the junction. And leaves an immobile positive donor ions at n- side and negative acceptor ions at p-side. AEI105.12321
AEI105.12322 Formation of pn diode A potential barrier develops at the junction whose voltage is 0.3V for germanium and 0.7V for silicon. Then further diffusion stops and results a depletion region at the junction.
Depletion region Since the region of the junction is depleted of mobile charges it is called the depletion region or the space charge region or the transition region. The thickness of this region is of the order of 0.5 micrometers AEI105.12323
AEI105.12324 Circuit symbol of pn diode A K Arrow head indicates the direction of conventional current flow. Fig 3.
AEI105.12426 Working of P-N Junction under FB P N Potential barrier V Fig. 2 Working of P-N junction
AEI105.12427 Forward Bias An ext. Battery applied with +ve on p-side, ve on n- side. The holes on p-side repelled from the +ve bias, the electrons on n- side repelled from the ve bias. The majority charge carriers driven towardsthe junction. This results in reduction of depletion layer width and barrier potential. As the applied bias steadily increased from zero onwards the majority charge carriers attempts to cross junction.
AEI105.12428 Holes from p-side flow across to the ve terminal on the n-side, and electrons from n-side flow across to the +ve terminal on the p-side. As the ext. bias exceeds the Junction barrier potential (0.3 V for Germanium, 0.7 V for Silicon ) the current starts to increase at an exponential rate. Now, a little increase in forward bias will cause steep rise in majority current. The device simply behaves as a low resistance path.
AEI105.12429 Features: Behaves as a low resistor. The current is mainly due to the flow of majority carriers across the junction. Potential barrier, and the depletion layer is reduced
AEI105.12430 Current components Fig. 3 Current components
AEI105.12532 P-N Junction working under reverse bias P N Potential barrier V Fig.2 P-N Junction Diode working under RB
AEI105.12533 P-N Junction Diode- Reverse Bias External bias voltage applied with +ve on n-side, ve on p- side. This RB bias aids the internal field. The majority carriers i.e. holes on p-side, the electrons on n- side attracted by the negative and positive terminal of the supply respectively. This widens the depletion layer width and strengthens the barrier potential.
AEI105.12534 Few hole-electron pairs are created due to thermal agitation (minority carriers). As a result small current flows across the junction called as reverse saturation current I 0 (uA for Germanium, nA for Silicon). Behaves as a high impedance element.
AEI105.12535 Further rise in reverse bias causes the collapse of junction barrier called breakdown of the diode. This causes sudden increase in flow of carriers across the junction and causes abrupt increase in current.
AEI105.12636 P-N JUNCTION Fig 1.
AEI105.12637 JUNCTION PROPERTIES 1.The junction contains immobile ions i.e. this region is depleted of mobile charges. 2.This region is called the depletion region, the space charge region, or transition region. 3.It is in the order of 1 micron width. 1.The cut-in voltage is 0.3v for Ge, 0.6v for Si.
AEI105.12638 (Contd..) 5. The reverse saturation current doubles for every 10 degree Celsius rise in temperature. 6.