Semiconductors: Diodes

EEE 1: Lecture 15

Semiconductors

• Conductors – any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminals

• Insulators – material that offers a very low level of conductivity under pressure from an applied voltage source

• Semiconductors – material whose conductivity level is in between that of an insulator and a conductor

Resistivity

• 𝑅 = 𝜌 ∗𝑙

𝐴 𝑜ℎ𝑚𝑠

– 𝜌 measured in units of 𝛺-cm

• Copper (conductor) – 𝜌 = 10-6

• Mica (insulator) – 𝜌 = 1012

• Germanium (semiconductor) – 𝜌 = 50

• Silicon (semiconductor) – 𝜌 = 50x103

Semiconductor materials

• Intrinsic materials – impurities reduced to a very low level

• Ge and Si show a reduction in resistance with increase in temperature (negative temperature coefficient)

– Opposite for conductors

Energy gap

• Energy required to be absorbed by an electron in order to break away from the atomic structure to enter the conduction band

• The energy is measured in electron volts (eV)

– W = QV = 1.6E-19 C * 1V

– 1eV = 1.6E-19 J

Energy gap

• Silicon = 1.1eV

• Germanium = 0.67eV

• Gallium Arsenide = 1.41eV

Extrinsic Materials

• Semiconductor materials subjected to doping processes to alter their electrical characteristics

• N-type, P-type

• Still electrically neutral

N-type extrinsic materials

• Excess of electrons

• Example includes the doping with a antimony impurity (pentavalent)

• Energy gap is significantly less than the intrinsic versions

• Majority carrier – electron

• Minority carrier – holes

P-type extrinsic material

• Common doping materials are boron, gallium, and indium (3 valence electrons)

• Excess of holes

• Majority carrier – holes

• Minority carrier – electrons

Ideal Diode

• Symbol

– P – Anode

– N – Cathode

Ideal Diode

• Depletion region/layer – region where the holes and electrons immediately combine

Diode Voltage

• Application of voltage across diodes

– VD = 0 V – no bias

– VD > 0 V – forward bias

– VD < 0 V – reverse bias

No Bias

• Holes in n-type will be in the depletion region and will pass into p-type material

• Electrons in the n-type must overcome the attractive forces of the positive ions in the n-type material and the shield of negative ions in the p-type

• A large number of electrons means there is a small number of electrons with sufficient kinetic energy to cross into p-type material

No Bias

• Applying the same assumptions from the p-type side, will result in a net electron/hole movement of zero

– A charge barrier is formed at the depletion region, restricting movement of majority carriers but help movement of minority carriers

– The small number of minority carriers will trigger a cancellation of vectors and zero current flow

Reverse Bias

• The application of a positive voltage at the n-type side will cause the majority carriers to be drawn to the terminal, leaving the minority carriers to build up an even larger depletion region, reducing majority carrier flow through it to zero – Same condition for the p-type side

• There will be the same minority carrier flow through the depletion region, causing a small-scale current flow from n-type to p-type material – Saturation current (Is)

- VD +

Forward Bias

• Applying a negative voltage at the n-type terminal will cause an attraction of the minority carriers and will repel the majority carriers causing pressure to recombine with the majority carriers of the p-type material

• Minority carrier flow remains unchanged, but majority carrier flow is significantly larger as the charge barrier has

been reduced

+ VD -

Diode Equation

• Diode current is defined by the following equation

𝐼𝐷 = 𝐼𝑆 𝑒𝑘𝑉𝐷𝑇𝐾 − 1

Where

- IS = reverse saturation current

- K = 11600/𝜂 (𝜂=1 for Ge, 𝜂=2 for Si)

- TK = temperature in Kelvin

Diode Characteristics

Zener Region

• If the reverse biasing voltage is large enough, the diode will reach a point of operation wherein the diode current will increase rapidly in the opposite direction

– Zener potential (VZ)

• The velocity of minority carrier movement increases which triggers a large enough value of kinetic energy to release additional carriers

– Avalanche breakdown region – Avalanche current

Diode Characteristics

Diode Equivalent Circuits

• Piecewise-Linear Equivalent

• Simplified Equivalent

• Ideal Equivalent

Diodes

• Piecewise-Equivalent Model

Biasing Diode Voltage (VD)

Diode Current (ID)

Effective Resistance

Reverse < VT 0 mA Infinite

Forward >= VT (VD – VT)/rav A rav 𝛺

VT - Threshold Voltage • VT = 0.7 V for Si • VT = 0.2 for Ge

Diodes

• Simplified Diode Model

Biasing Diode Voltage (VD)

Diode Current (ID)

Effective Resistance

Reverse < VT 0 mA Infinite

Forward >= VT >= 0 A 0 𝛺

VT - Threshold Voltage • VT = 0.7 V for Si • VT = 0.2 for Ge

Diodes

Biasing Diode Voltage (VD)

Diode Current (ID)

Effective Resistance

Reverse < 0 V 0 mA Infinite

Forward > 0 V >= 0A 0 𝛺

As a switch, • Forward Bias == ON • Reverse Bias == OFF

• Ideal Diode Model

Example

• For the series diode configuration, determine VD, VR, and ID. Is the diode forward or reverse biased?

600m

V1 R1

1.2k

+ VR -

+ VD - ID

Example

• Determine Vo, ID1, ID2, and IA. Both D1 and D2 are Silicon diodes

D2D1R1

100

10

V1

+ VD -

ID2 ID1

IA