R.S

UNIT-IIIP-N JUNCTION-1

BY:- YASHU SWAMIASST. PROF., DEPTT. OF EE

DEI, DAYALBAGH

OPEN CIRCUIT PN JUNCTION

OPEN CIRCUIT PN JUNCTION

OPEN CIRCUIT PN JUNCTION

Since potential energy = potential x charge, the curve in (d) is proportional to potential energy of hole and curve (e) is proportional to the negative of (d) curve i.e. electron.

REVERSE BIAS PN JUNCTION

FORWARD BIAS PN JUNCTION

CURRENT CHARACTERISTICS PN JUNCTION

CURRENT CHARACTERISTICS PN JUNCTION

The minority diffusion current (Hole) at junction (x=0) is given by :

LAW OF JUNCTION

The forward bias V lowers the barrier height and allows more carrier to cross the junction.Pn(0) depends exponentially on VExpression is given by :

Pn(0) is the hole conc. At the edge of n region(x=0) and Pno is the hole conc. Away from the Junction edge.

TOTAL CURRENT:

Since current is same throughout a series circuitI is independent of x.It is represented by horizontal line.

TOTAL DIODE CURRENT

TOTAL DIODE CURRENT

TOTAL DIODE CURRENT

Fig: Minority (solid line) and majority (dashed line)Current vs. distance in pn junction.

Volt – Ampere Characteristics

Volt – Ampere Characteristics

The unity in parentheses may be neglected.Except near the origin, the current increases Exponentially. When diode is reverse biased and V>>VT , The reverse current is constant Independent of applied voltage.Io is called as reverse saturation current.

Shockley diode equation

Where I is the diode current, IS is the reverse bias saturation current (or scale current), VD is the voltage across the diode, VT is the thermal voltage, n is the ideality factor, also known as the quality factor or sometimes emission coefficient. The ideality factor n varies from 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to beapproximately equal to 1 (thus the notation n is omitted). VT Thermal Voltage = 25.85 mV at 300 K.

Breakdown Region• In reverse bias voltage Vz, large reverse

current flows. Diode is said to be in Break down region.

• The Voltage at which the diode enters the breakdown region is called as reverse voltage, breakdown voltage (Vz).

BREAKDOWN VOLTAGE• There is a limit to how much reverse voltage a

diode could withstand before it is damaged. • If you continue to increase the reverse

voltage, you will ultimately reach the breakdown voltage of the diode, at which the component will loose its characteristics and will allow the current to flow in either directions.

• This voltage is termed as breakdown voltage.• Breakdown voltage may go up to 40 - 50V.

Breakdown Region

CUTIN VOLTAGE

• Cutin, offset, breakpoint, knee or threshold voltage is the voltage below which the current is very small (< than 1% of the max.).

• Beyond this voltage current rises very rapidly.• For Ge, cutin Voltage = 0.2 (approximately),• For Si, cutin Voltage = 0.6 (approximately).

CUTIN VOLTAGE

Forward Volt Ampere Characteristics Ge & Si Diode. (1N270) & (1N3605).

KNEE VOLTAGE CUTIN VOLTAGE• Break in Si diode is offset about 0.4 V than Ge diode.• The reason for this difference is that the reverse

saturation current in Ge diode is larger by 1000 times than Si diode.

• Io is in the range of Microampere for Ge & Io is in the range of Nanoampere for Si at room Temperature.

• Also n=2 for small currents in Si diode. This initial dependence of current on voltage causes delay in rise of current.

KNEE VOLTAGE CUTIN VOLTAGE• In the forward region of the graph, the voltage at

which the current starts to increase quickly is called the knee voltage of the diode.

• If the knee voltage is higher, the diode conducts easily.

• If the knee voltage is lower, the diode conducts poorly

• The knee voltage for a Si diode is 0.6 - 0.7 V and for a Ge diode 0.2 - 0.3 V.

Logarithmic Characteristics

V-I Characteristics at three different temperatures for Si diode.The shaded area indicates 25*C limits of controlled conductance.

Log (I) = log (Io)+ 0.434V/nVT