Special Semiconductor Devices

Special Semiconductor Devices Tunnel Diode Varactor Diode PIN Diode 

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Symbol

Tunnel Diode 

high impurity concentration • Ordinary diode concentration-1part in 10^8 parts • Tunnel diode concentration-1part in 10^3 parts • negative resistance region that allows very fast switching speeds, up to 5 GHz • Materials used-Ga and GaAs

Tunneling phenomenon 

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Width of the junction barrier varies inversely as the square root of impurity concentration Quantum mechanism-Electron penetrate through the barriertunneling

V-I Characteristics of Tunnel

diode 

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Very good conductor in forward bias Huge current for small applied voltage Negative resistance region Low resistance device

Varactor Diode 

voltage-controlled capacitor

w inversely proportional to C  Forward bias –space charge width decreases and C increases  Reverse bias- space charge width increases and C decreases  Voltage-variable capacitance of p-n junction in reverse bias condition – useful in number of application  Application based on Voltage-variable capacitance-varactors, varicaps or volacaps 

T

T

Characteristics of Varactor Diode

PIN Diode Intrinsic layer sandwitched between P and N regions  Intrinsic layer offers high resistance  Increase voltage- resistance reduces  Intrinsic layer-advantages 

• Decrease in capacitance between p & N region –(w inversely proportional to C) • Allows faster time response P

I

N

Characteristics of PIN Diode Forward

Reverse

Silicon Controlled Rectifier conventional rectifier controlled by a gate signal  The main circuit is a rectifier, however the application of a forward voltage is not enough for conduction  gate signal controls the rectifier conduction 

construction

symbol

SCR working Apply +ve voltage to anode w.r.t cathode  J & J –forward biased  J –reverse biased  reverse Voltage appears across J  Current through the device – reverse saturation current 

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3

2

3

When I <0,Applied voltage increases – J reverse bias voltage increases  current increases slowly until the breakover voltage (V ) increased  At this point diode switches from OFF to ON state  I >0, forward bias voltage applied to gate -V decreased; current increased  SCR Switch ON controlled by I  Once the SCR turned on- gate loses the control 

G

2

BO

G

BO

G

SCR Characteristics

Two Transistor Analogy

The general transistor equations are, I C = β I B + ( 1 + β ) ICBO I C = α I E + I CBO I E = IC + I B

I B = I E ( 1 − α ) − ICBO

C o n s id e rin g P N P tra n s is to r o f th e e q u iv a le n t c irc u it, I E 1 = I, A I C α =I,C1 IC B O = ∴

I B1

I C1 ,B O I B

I( A1 α =1 )

1

α

I = 1 B

I C −B O(1

, =

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C o n s id e r in g N P N t r a n s is t o f th e e q u iv a le n t c ir c u it, IC = 2I,C I B 2I,B = 2I E IK I C2 = α 2

I C2 = α 2 (

Ik IA)

I+C B O2 IG+

I(C2B) O2 +

From the equivalent circuit, we see that ∴ I C2 =I B1 ⇒ IA

α 2 I g + I CBO 1 + I CBO 2 = 1 − ( α 1 + α2)

If α + α =1;then I =∞  Anode suddenly reaches high value  Device triggered from OFF state to ON state  Regenerative characteristics  α + α =1 and proper I  I =I cause a flow of I T -ON state  I = I ; T - switched ON  Due to regenerative action if the gate signal removed the device keep on conducting 

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1

G

C2

2

A

2

G

B2

C2

B1

1

2

Thyristor Ratings Latching current – minimum current required to latch or trigger device from OFF to ON state  Holding current - minimum value of current to hold the device ON state  gate current –current applied to the gate for control purpose  Voltage safety factor=PIV/√ 2 x RMS value of operating voltage 

SCR-Most important type of power semiconductor device.  Highest power handling capability.  Rating of 1200V / 1500A with switching frequencies ranging from 1KHz to 20KHz. 

DIAC Construction

Bidirectional avalanche diode  No control terminal  Identical characteristics for both forward and reverse half  Positive half cycle –MT1 +ve w.r.t MT2  Negative half cycle –MT2 +ve w.r.t MT1 

characteristics of DIAC 

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At voltage less than the breakover voltage- small amount of leakage current flows through the deviceOFF state When voltage level reaches breakover voltage device starts conducting Exhibits –ve resistance characteristics

Unijunction Transistor(UJT)

The unijunction transistor(UJT) is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor.  bar of N type semiconductor material into which P type material has been diffused somewhere along its length 

R is known as the interbase resistance, and is the sum of RB1 and RB2  RBB = RB1 + R (1)  VRB1 is the voltage developed across RB1; this is given by the voltage divider rule:  VRB1 = RB1 / RB1 + R x V (2) 

BB

B2

B2

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BB

VRB1 = RB1 / RBB x V  The ratio RB1 / RBB is referred to as the intrinsic standoff ratio (η ) 

BB

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