1. Lecture 4 Carrier Transport Phenomena
DMT 234 Semiconductor Physic & Device
Lecture 4 Carrier Transport Phenomena
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2. DMT 234 Semiconductor Physic & Device
Preview
The net flow of the electron and holes in a semiconductor will generate current.
The process which these charged particles move is call transport.
Two basic transport : Drift & Diffusion.
The carrier transport phenomena are the foundation for finally determining the current-voltage characteristics of semiconductor devices.
Subtopic :
Carrier Drift
Carrier Diffusion
Hall Effect.
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3. DMT 234 Semiconductor Physic & Device
4.1 Carrier Drift
Transport : The process which charged particles ( holes or electrons) are move.
Understanding of the electrical properties ( I-V properties)
Basic current Equation :
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4.1.1 Drift Current Density
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4.1.1 Drift Current Density
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4.1.1 Drift Current Density
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4.1.1 Example carrier drift
To calculate the drift current density in a semiconductor for a given electric field. Consider a gallium arsenide sample at T=300K with doping concentration of Na = 0 and Nd = cm-3. Assume complete ionization and assume electron and hole mobilities given is un = cm2/v-s and up = 400 cm2/v-s. Calculate the drift current density if the applied electric field is E = 10 V/cm.
Since Nd > Na, the semiconductor is n type and the majority carrier electron concentration:
n = Nd – Na / 2 + √ ( Nd- Na/ 2)2 + ni2 = cm-3
The minority carrier hole concentration is :
p = n2i / n = (1.8 x 106)2 / 1016 = 3.24 x 10-4 cm-3
For this extrinsic n-type semiconductor, the drift current density is :
Jdrf = e ( unn + upp)E = eunNdE
Then , Jdrf = (1.6 x 10-19)(8500)(1016)(10) = 136 A/cm2
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4.1.2 Mobility Efects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.2 Mobility Effects
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4.1.3 Conductivity
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4.1.3 Conductivity
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4.1.3 Conductivity
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4.1.3 Conductivity
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4.1.3 Velocity Saturation
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4.2 Carrier Diffusion
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4.2 Carrier Diffusion
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4.2 Carrier Diffusion
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4.2 Carrier Diffusion
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28. DMT 234 Semiconductor Physic & Device 4.2 Example Carrier Diffusion
To calculate the diffusion current density given a density gradient. Assume that, in an n-type gallium arsenide semiconductor at T= 300 K, the electron concentration varies linearly from 1 x 1018 to 7 x cm-3 over a distance of 0.10 cm. Calculate the diffusion current density if the electron diffusion coefficient is Dn = 225 cm2/s.
The diffusion current density is given :
Jn|dif = eDn dn/dx
= (1.6 x 10-19)(225)(1 x 1018 – 7x1017/0.1)
= 108 A/cm2.
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4.2.1 Total current density
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4.2.1 Einstein relation
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4.2.1 Einstein relation
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4.2.1 Einstein relation
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4.3 Hall Effect
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4.3 Hall Effect
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4.3 Hall Effect
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4.3 Hall Effect
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4.3 Example Hall Effect
To determine the majority carrier concentration and mobility, given Hall effect parameters. Consider the geometry shown in Figure from previous slide. Let L = 10-1 cm ,W = 10-2 cm, and d = 10-3 cm. Also assume that Ix = 1.0 mA, Vx = 12.5 V, Bz = 500 gauss = 5 x 10-2 tesla and VH = mV.
A negative Hall voltage for this geometry implies that we have an n-type semiconductor using equation, we can calculate the electron concentration as :
n = -(10-3)(5 x 10-2)/ (1.6 x 10-19)(10-5)(-6.25 x 10-3) = 5 x 1021 m-3 = 5 x 1015 cm-3
The electron mobility is then determine from equation as :
Un = (10-3)(10-3) / (1.6 x 10-19)(5 x 1021)(12.5)(10-4)(10-5) = 0.1 m2/ V-s
Or Un = 1000 cm2/V-s.
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Q & A
Next week Topic :Nonequilibrium Excess Carriers in Semicondcutor.
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