Semiconductor Doping: N-Type and P-Type

• Adding impurity to a pure semiconductor is required to increase conductivity
• Intrinsic semiconductor has low conductivity at room temperature
• Adding a small amount (ppm) of suitable impurity increases conductivity, creating an extrinsic semiconductor

Doping

• Deliberately adding a desirable impurity
• Impurity atoms are dopants
• Doped semiconductor material does not distort original lattice
• Dopant and semiconductor atoms should be nearly the same size
• Silicon (Si) and Germanium (Ge) both:
• Are tetravalent
• Belongs to the fourth group in the periodic table
• Dopant element from nearby fifth or third group is chosen

Dopant Types

• Pentavalent (valency - 5) such as Arsenic (As), Antimony (Sb), and Phosphorus (P)
• Also called donor impurity
• Trivalent (valency - 3) such as Indium (In), Aluminium (Al), and Boron (B)
• Also called acceptor impurity
• Doping process provides two types of impure semiconductors

N-Type Semiconductor

• Doping Si or Ge with a pentavalent element
• Four of valence electrons bond with silicon neighbors
• Fifth electron remains weakly bound
• Ionization energy to free electron is small, free to move in the lattice at room temperature
• ~0.01 eV for germanium
• ~0.05 eV for silicon
• Pentavalent dopant donates an extra electron known as a donor impurity
• Number of electrons available depends on doping level and is independent of ambient temperature
• Free electrons and holes increase weakly with temperature
• Number of free electrons (ne) increases due to impurity
• Minority carrier holes (nh) is lesser: ne >> nh
• Majority charge carrier is electrons, the charge on electron is negative called "N"

P-Type Semiconductor

• Doping a crystal of Si or Ge with trivalent atoms such as Al, B, In
• Impurity atom arranges in place of Si or Ge, forms a covalent bond with three atoms
• Fourth bond has an electron deficiency and obtains a hole
• Trivalent foreign atom becomes negatively charged when it shares fourth electron with neighboring atom
• Dopant atom core is negative charge along with its associated hole, one acceptor atom provides one hole
• Majority charge carriers are holes, minority charge carriers are electrons
• Charge on hole is positive; called p-type semiconductor : ne << nh
• n-type or p-type semiconductors are electrically neutral
• Charge of the hole is the same as the charge at the core of the atom and the opposite charge

Energy Band Theory of Semiconductors

• Abundance of majority current carriers in extrinsic semiconductor
• Minority carriers are destroyed, reducing intrinsic concentration of minority carriers
• Semiconductor energy band structure affected by doping
• Additional energy states added due to donor impurities ED and acceptor impurities EA
• N-type of Si, donor energy level ED is slightly below the bottom EC of conduction band
• Electrons move into the conduction band with very little energy
• Most donor atoms get ionized at room temperature
• Many atoms of Si get ionized, and hence electrons go to conduction band from donor impurities
• P-type, acceptor energy level EA, is slightly above top EV of the valence band
• With very small energy electrons jump from valence band and ionize the acceptor negatively
• With very small supply of energy the hole from level EA sinks into the valence band EV
• Most acceptor atoms get ionized leaving holes in the valence band EV at room temperature
• Density of holes in valence band is mostly due to impurity in the extrinsic semiconductor

Energy Differences

• C (diamond) is 5.4 eV
• Si is 1.1 eV
• Ge is 0.7 eV
• Sn is a metal with an energy gap of 0 eV

Recombination

• Giving energy to the semiconductor causes creation of electron-hole pair
• Electrons and holes collide as per the law of thermodynamics
• Electron again occupies the hole
• Creation of electron hole pair and recombination process take place at the same time
• In thermal equilibrium, the rate of electron hole pair formation and their recombination is equal
• Recombination rate is proportional to ne nh; Recombination rate = Rne nh, where R is coefficient
• For intrinsic conductor ne=nh=ni and recombination rate is Rni
• In thermal equilibrium n2i=nenh

P-Type vs N-Type Semiconductors

• P-type uses trivalent impurity.
• Majority carriers in P-Type are holes and minority carriers are electrons.
• N-type uses pentavalent impurity.
• Majority carriers in N-Type are electrons and minority carriers are holes.
• Conduction by holes in P-Type mainly.
• Conduction by electrons in N-Type mainly.
• Hole number greater than electrons - P Type.
• Electron number greater then holes - N Type.

Intrinsic vs Extrinsic Semiconductors

• Intrinsic is a tetravalent pure crystal.

• Electrical conductivity is low.

• Conductivity is temperature dependent only.

• Free electrons = hole amount.

• Extrinsic uses impurity of the third and fifth periodic table group.

• Electrical conductivity is high.

• Conductivity dependent upon temp and impurity amount.

• N-type semicondutors have major electrons and P-type has major holes.

• Electron larger than holes in N-Type, vice versa for P-Type.

P-N Junctions

• Primary unit for diode and transistor is the P-N Junction
• Function of Junctions must be understood to grasp semiconductor compositions
• Two electrodes in P-N Junction, also called a P-N Junction Diode

Depletion Layer Creation

• When acceptor impurity(Al) added on one side of a Silicon wafer, it will be P-Type
• Donor Immurity(As) will be N-Type
• Metallic junction forms between the two region

Diffusion and Drift

• Number density is number of holes or electrons per unit volume
• Upon creation of P-N Junction differences in hole densities will start diffusion from P to N
• Likewise, electrons flow from N to P
• Motion of charge carriers gives direction to the current across the junction direction
• Electron diffuses from N to P and leaves behind ionized donor on N-Side
• Ionized charges cannot move
• Region of space on each side has space charged ions or depletion layer

Depletion Region

• N-Region does not have electrons (charge carrier) while P-Region does not contain holes
• Empty region where majority charge carriers used to be.
• N-Material loses electron and P-Material acquires electrons
• The motion prevents further electron movement and is called 'barrier potential'
• One tenth of a micrometer thick (0.5 micro meters)
• Electric field is directed from positive to negative charge
• Electrons move to N-Side and Holes move to P-Side due to drift
• Initially diffusion is larger and drift smaller
• As diffusion continues charge regions extent due to high electric field strength (drift).
• Until the diffusion current equals drift then the P-N Junction is formed

Equilibrium and Bias

• Under equilibrium there is no current
• Diffusion of carriers in P-N decreases number of electrons while increasing in P
• A potential difference is developed, polarity opposes flow of carriers, equilibrium is formed
• Barrier potential created, obtained in depletion region

Semiconductor Diode

• Metallic contact of junction, has two electrical terminal
• Called 'semiconductor diode"
• Symbol of P-N Junction diode in circuit
• Arrow indicates conventionaly direction of current, equilibrium can be altered by voltage

Biasing Junctions

• P-N Junction can be biased in two ways:
• Forward Bias
• Reverse Bias

Forward Bias

• Positive Terminal connected to P-Side and Negative to N-Side of Junction
• Connection known as forward bias, battery applied mostly accross the depletion region
• Resistance very high in depletion region.
• Applied voltage goes to built in potential, layer width decreases due to high height
• Height under forward bias: V0-V
• If no external battery effective depletion barrier is shown as first in voltage output If voltage applied effective changes to second, and hight applied shows as three in figure
• Height reduced and more carriers required for current
• Movement called 'minority carrier inject

Minority Concentration

• Increases the closer you are to the junction due to gradient
• Electrons on P-Side move to the opposite and holes on N-Side do as well.
• Motion is what makes the current
• Magnitude of current typically in mA (miliamperes)
• Forward bias Junction has low resistance
• External barrier is ~ 1.5V

Reverse Bias

• Voltage (V) can be applied that N-Side is positive and P-Side is negative
• Depletion Width, W and height shown figure
• As a resulting barrier height increases and depletion region widens.
• When connected the potential barrier is now Voltage + V0
• Suppresses any diffusion and current is disputed
• However, minority carriers pass through the junction due to high voltage

Static Characteristics

• Reverse current does not dependend only on applied voltage
• Is limited by magnitude of the applied voltage and concentration of morinority carriers
• Can be sufficient voltage to go accross junction
• Reverse bias varies from ~10V to 15V and shown in microamperes
• Ratio of of voltage to current is also called dynamic resistance
• Resistence of diode forward is 10-100 ohms. Reverse bias is 10 to the sixth power (Megaohms)

Forward vs Reverse Bias

• Connect positive to P and ngative to N, vs negative connected to side.
• Former called forward bias and has current due to majority charge (miliamperes)
• Depletion layer width is decreased as voltage increased, reverse bias happens the opposite
• Forward bias' resitance is 10-100 ohms.
• Depletion larger and is 1 million ohms.

P-Type and N-Type Doping

• Physically cant join both - slab smoothness much is much larger than inter-atomic crystal spacing of 2-3 Amstrong.
• Junction will behave as discontinuity.

Doping Problems

• Crystal has Si and doped by ppm (parts per million of pentavalent As arsenic.
• We can caluclate electrons and holes by finding thermally generated electrons equal 10 to the sixteeth power
• ppm One part per million, divide amount of atoms due for dopping electron equal, N D number of atoms
• Number density small with number of doping
• As a result N E is roughly same as ND or 5 to tenth power of 22.

Diode Characteristics

• Using the Voltage to current graph, take any diode and find resistence at both
• When diode is straigh can caluclate resistence by slope to point
• If not from D at -10 use points A or C and ohms law with resistence.

Learn about doping semiconductors with pentavalent and trivalent impurities to create N-type and P-type semiconductors. Understand how adding impurities increases conductivity. Explore donor and acceptor impurities like Arsenic, Boron and Aluminium.