PHYSICAL ELECTRONICS ECX 5239 PRESENTATION 01 PRESENTATION 01 Name : A.T.U.N Senevirathna. Reg, No : Center : Kandy

Introduction What is a semiconductor ? Introduction about Current density, Resistivity, Conductivity, Drift velocity, Mobility. How to solve my problems according to above equations.

What Is a Semiconductor? Many materials, such as most metals, allow electrical current to flow through themMany materials, such as most metals, allow electrical current to flow through them These are known as conductorsThese are known as conductors Materials that do not allow electrical current to flow through them are called insulatorsMaterials that do not allow electrical current to flow through them are called insulators A material whose properties are such that it is not quite a conductor, not quite an insulator That material called as semiconductor. That material called as semiconductor.

Semiconductors Some common semiconductors Some common semiconductors  Si - Silicon (most common)  Ge – Germanium Silicon is the best and most widely used semiconductor. The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit. The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.

Doping n To make the semiconductor conduct electricity, other atoms called impurities must be added. n “Impurities” are different elements. n This process is called doping. n Some impurities are As, P, B

Doping with Boron n Boron has 3 electrons are in its outer shell. n We remove a silicon atom from the crystal lattice. n Then we replace it with a boron atom. n Notice we have a hole in a bond – this hole is thus free for conduction n This type of silicon is called p-type p-type will be shown like this.

Semiconductors can be Conductors n An impurity, or element like arsenic, has 5 valence electrons. arsenic n we remove a silicon atom from the crystal lattice and replace it with a arsenic atom. n We now have an electron that is not bonded – it is thus free for conduction. n This type of silicon is called n-type. n-type will be shown like this.

Carrier Drift n When apply an electric field to semiconductor, charged particles move according to electric field. This process is called drift. n Charged particles move with an average velocity. This velocity proportional to the electric field. n The proportionality constant is the carrier mobility. Hole velocity Electron velocity Notation:  p  hole mobility (cm 2 /V·s)  n  electron mobility (cm 2 /V·s) Hole velocity Electron velocity

n Drift current is proportional to the carrier velocity and carrier concentration: Drift Current (current density) J = =

Drift Current Equations

Electrical Resistance 1 2 Using 1,2 we can get eq. 3 3

Problems Question No 08 : A current density of 10 A/ m 2 flows through an n-type germanium which has resistivity 0.05 ohm-m. Calculate the time taken for electrons in the material to travel 50 μ m. A current density of 10 A/ m 2 flows through an n-type germanium which has resistivity 0.05 ohm-m. Calculate the time taken for electrons in the material to travel 50 μ m. According to question we can get Current density = 10 A/ m 2 Current density = 10 A/ m 2 Resistivity = 0.05 ohm-m Resistivity = 0.05 ohm-m Distance = 50 μ m Distance = 50 μ m Charge of electron = 1.6 x c Charge of electron = 1.6 x c Electron mobility = 0.39 m 2 / vs because n –type germanium. Electron mobility = 0.39 m 2 / vs because n –type germanium. Where is Drift velocity 1

2 Density of electrons 3 Conductivity Conductivity Using 2,3 we can get eq. 4 4 Using 1,4 we can get eq Using 5,6 can get,

Question No 09 : Intrinsic silicon has a resistivity of 2000 ohm-m at R.T. and the density of conduction electrons is 1.4 x calculate resistivity's of samples containing acceptor concentrations of and m -3 Assume that μ h remains as for intrinsic silicon and that μ h = 0.25 μ e. According to question we can get Density of electrons Resistivity Conductivity of Intrinsic silicon Charge of electron Using above data we can get, Electron mobility 4 3

Hole mobility of intrinsic B Using 4,B we can get 5 Now we can get eq. for conductivity of sample 1 We use another eq. for number of electrons and number of holes Where N a is acceptor concentration and NdNd Is donor atom concentration/impurity concentration Let And assume this is p-type semiconductor Hence 2 Because room temperature 2,3,4,5,6 apply to 1 we can get

Hence resistivity of sample (ρ) = 1/ = ohm.m Similarly, when We can get (ρ) = ohm.m Question No 10 : A rod of p-type germanium 6mm long, 1mm wide and 0.5mm thick has and electrical resistance of 120 ohm. What is the impurity concentration ? Assume μ e = 0.39, μ h = 0.19 m 2 /v.s and n i = 2.5× m -3 what proportion of the conductivity is due to electrons in the conduction band ? According to question we can get, μ e = 0.39, μ h = 0.19 m 2 /v.s, n i = 2.5× m -3 Length of rod (L) = 6mm, Resistance of rod (R) =120Ω, Area of rod (A) = 0.5 m 2 Then we can get conductivity of rod 1 Let As p-type germanium 2 3

Using 1,2,3 we can get 4 Solving eq. 4 Using previous data we can get or Solving eq. 2 we can get Hence let Because p>n as p-type Finally we can get impurity concentration

Reference  Course material of physical electronics. END  Electronic materials and Devices. (By S.O.Kasap)  Internet resources.  Electronic materials and Devices. John Allis0n